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Topic 2: Mechanics 2.2 – Forces

Essential idea: Classical physics requires a force to change a state of motion, as suggested by Newton in his laws of motion.

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Topic 2: Mechanics 2.2 – Forces

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  1. Essential idea: Classical physics requires a force to change a state of motion, as suggested by Newton in his laws of motion. Nature of science: (1) Using mathematics: Isaac Newton provided the basis for much of our understanding of forces and motion by formalizing the previous work of scientists through the application of mathematics by inventing calculus to assist with this. (2) Intuition: The tale of the falling apple describes simply one of the many flashes of intuition that went into the publication of Philosophiæ Naturalis Principia Mathematica in 1687. Topic 2: Mechanics2.2 – Forces

  2. Topic 2: Mechanics2.2 – Forces Understandings: • Objects as point particles • Free-body diagrams • Translational equilibrium • Newton’s laws of motion • Solid friction

  3. Topic 2: Mechanics2.2 – Forces Applications and skills: • Representing forces as vectors • Sketching and interpreting free-body diagrams • Describing the consequences of Newton’s first law for translational equilibrium • Using Newton’s second law quantitatively and qualitatively • Identifying force pairs in the context of Newton’s third law • Solving problems involving forces and determining resultant force • Describing solid friction (static and dynamic) by coefficients of friction

  4. Topic 2: Mechanics2.2 – Forces Guidance: • Students should label forces using commonly accepted names or symbols (for example: weight or force of gravity or mg) • Free-body diagrams should show scaled vector lengths acting from the point of application • Examples and questions will be limited to constant mass • mg should be identified as weight • Calculations relating to the determination of resultant forces will be restricted to one- and two-dimensional situations

  5. Topic 2: Mechanics2.2 – Forces Data booklet reference: • F = ma • Ff ≤ µsR • Ff = µdR Theory of knowledge: • Classical physics believed that the whole of the future of the universe could be predicted from knowledge of the present state. To what extent can knowledge of the present give us knowledge of the future?

  6. Topic 2: Mechanics2.2 – Forces Utilization: • Motion of charged particles in fields (see Physics sub-topics 5.4, 6.1, 11.1, 12.2) • Application of friction in circular motion (see Physics sub-topic 6.1) • Construction (considering ancient and modern approaches to safety, longevity and consideration of local weather and geological influences) • Biomechanics (see Sports, exercise and health science SL sub-topic 4.3)

  7. Topic 2: Mechanics2.2 – Forces Aims: • Aims 2 and 3: Newton’s work is often described by the quote from a letter he wrote to his rival, Robert Hooke, which states: “What Descartes did was a good step. You have added much [in] several ways. If I have seen a little further it is by standing on the shoulders of Giants.” This quote is also inspired, this time by writers who had been using versions of it for at least 500 years before Newton’s time. • Aim 6: experiments could include (but are not limited to): verification of Newton’s second law; investigating forces in equilibrium; determination of the effects of friction.

  8. Newton’s laws of motion Mechanics is the branch of physics which concerns itself with forces, and how they affect a body's motion. Kinematics is the sub-branch of mechanics which studies only a body's motion without regard to causes. Dynamics is the sub-branch of mechanics which studies the forces which cause a body's motion. Topic 2: Mechanics2.2 – Forces The two pillars of mechanics Dynamics Kinematics Topic 2.2 Topic 2.1 Galileo Newton

  9. where g =10 ms -2 weight W = mg and m is the mass in kg Representing forces as vectors A force is a push or a pull measured in Newtons. One force we are very familiar with is the force of gravity, AKA the weight. The very concepts of push and pull imply direction. Thus forces are vectors. The direction of the weight is down toward the center of the earth. If you have a weight of 90 Newtons (or 90 N), your weight can be expressed as a vector: 90 N, down. We will show later that weight has the formula Topic 2: Mechanics2.2 – Forces

  10. Free-body diagram mass force where g =10 ms -2 weight W = mg W and m is the mass in kg Objects as point particles and Free-body diagrams Topic 2: Mechanics2.2 – Forces • EXAMPLE: Calculate the weight of a 25-kg object. • SOLUTION: • Since m = 25 kg and g = 10 ms-2, • W = mg = (25)(10) = 250 N (or 250 n). • Note that W inherits its direction from the fact that g points downward. • We sketch the mass as a point particle (dot), and the weight as a vector in a free-body diagram:

  11. R W Objects as point particles and Free-body diagrams Certainly there are other forces besides weight that you are familiar with. For example, when you set a mass on a tabletop, even though it stops moving, it still has a weight. The implication is that the tabletop applies a counterforce to the weight, called a normal force. Note that the weight and the normal forces are the same length – they balance. The normal force is called a surface contact force. Topic 2: Mechanics2.2 – Forces FYIThe normal force is often called (unwisely) the reaction force – thus the R designation.

  12. Ff R T the tension W Objects as point particles and Free-body diagrams TensionT can only be a pull and never a push. FrictionFf tries to oppose the motion. Friction Ff is parallel to the contact surface. NormalR is perpendicular to the contact surface. Friction and normal are mutually perpendicular. Ff R. Friction and normal are surface contact forces. WeightW is an action-at-a-distance force. Topic 2: Mechanics2.2 – Forces Contact surface

  13. Ff R T W Sketching and interpreting free-body diagrams Weight is sketched from the center of an object. Normal is always sketched perpendicular to the contact surface. Friction is sketched parallel to the contact surface. Tension is sketched at whatever angle is given. Topic 2: Mechanics2.2 – Forces

  14. Ff Ff 30° Free-body diagram T R R T 30° W W Sketching and interpreting free-body diagrams • EXAMPLE: An object has a tension acting on it at 30° as shown. Sketch in the forces, and draw a free-body diagram. • SOLUTION: • Weight is drawn from the center, down. • Normal is drawn perpendicular to the surface from the surface. • Friction is drawn par- allel to the surface. Topic 2: Mechanics2.2 – Forces

  15. Ff R T W Solving problems involving forces and resultant force The resultant (or net) force is just the vector sum of all of the forces acting on a body. Topic 2: Mechanics2.2 – Forces EXAMPLE: An object has mass of 25 kg. A tension of 50 n and a friction force of 30 n are acting on it as shown. What is the resultant force? • SOLUTION: • Since the weight and the normal forces cancel out in the y-direction, we only need to worry about the forces in the x-direction. • The net force is thus 50 – 30 = 20 n (+x-dir). 50 n 30 n

  16. F2 30. n Fnet 50. n F1 Solving problems involving forces and resultant force The resultant (or net) force is just the vector sum of all of the forces acting on a body. Topic 2: Mechanics2.2 – Forces net force Fx,net = Fx Fnet = F Fy,net = Fy EXAMPLE: An object has exactly two forces F1 = 50. n and F2 = 30. n applied simultaneously to it. What is the resultant force’s magnitude? • SOLUTION: • Fnet = F= F1+ F2so we simply graphically add the two vectors: • The magnitude is given by • Fnet2 = 502 + 302 • Fnet = 58 n.

  17. F2 30. n Fnet 50. n F1 Solving problems involving forces and resultant force The resultant (or net) force is just the vector sum of all of the forces acting on a body. Topic 2: Mechanics2.2 – Forces net force Fx,net = Fx Fnet = F Fy,net = Fy EXAMPLE: An object has exactly two forces F1 = 50. n and F2 = 30. n applied simultaneously to it as shown. What is the resultant force’s direction? • SOLUTION: • Direction is measured from the (+) x-axis. • Opposite and adjacent are given directly, so use tangent. • tan = opp / adj = 30 / 50 = 0.6 •  = tan-1(0.6) = 31°. 

  18. F2 30. n Solving problems involving forces and resultant force EXAMPLE: An object has exactly two forces F1 = 50. n and F2 = 30. n applied simultaneously to it. What is the resultant force’s magnitude? Topic 2: Mechanics2.2 – Forces • SOLUTION: • Begin by resolving F1 into its x- and y-components. • Then Fnet,x = 44 n and • Fnet,y = 23 + 30 = 53 n. • Fnet2 = Fnet,x2 + Fnet,y2 • Fnet2 = 442 + 532 • Fnet= 69 n. F1 50. n 50sin28 23 n 28° 44 n 50cos28

  19. tension T T T T friction f f f f Force f T Time Topic 2: Mechanics2.2 – Forces Solid friction Recall that friction acts opposite to the intended direction of motion, and parallel to the contact surface. Suppose we begin to pull a crate to the right, with gradually increasing force. We plot the applied force, and the friction force, as functions of time: dynamic friction static friction static dynamic

  20. Fd tension friction Force Time static dynamic Topic 2: Mechanics2.2 – Forces Fs,max Solid friction During the static phase, the static friction forceFs exactly matches the applied (tension) force. Fs increases linearly until it reaches a maximum value Fs,max. The friction force then almost instantaneously decreases to a constant value Fd, called the dynamic friction force. Take note of the following general properties of the friction force: Fd= a constant Fd< Fs,max 0≤ Fs≤ Fs,max

  21. Topic 2: Mechanics2.2 – Forces Solid friction So, what exactly causes friction? People in the manufacturing sector who work with metals know that the more you smoothen and polish two metal surfaces, the more strongly they stick together if brought in contact. In fact, if suitably polished in a vacuum, they will stick so hard that they cannot be separated. We say that the two pieces of metal have been cold-welded.

  22. surface 1 surface 1 surface 1 surface 2 surface 2 surface 2 cold welds Topic 2: Mechanics2.2 – Forces Solid friction At the atomic level, when two surfaces come into contact, small peaks on one surface cold weld with small peaks on the other surface. Applying the initial sideways force, all of the cold welds oppose the motion. If the force is sufficiently large, the cold welds break, and new peaks contact each other and cold weld. If the surfaces remain in relative sliding motion, fewer welds have a chance to form. We define the unitless constant, called the coefficient of frictionμ, which depends on the composition of the two surfaces, as the ratio of Ff / R.

  23. Topic 2: Mechanics2.2 – Forces Describing solid friction by coefficients of friction Since there are two types of friction, static and dynamic, every pair of materials will have two coefficients of friction, μs and μd. In addition to the "roughness" or "smoothness" of the materials, the friction force depends, not surprisingly, on the normal force R. The harder the two surfaces are squished together (this is what the normal force measures) the more cold welds can form. Here are the relationships between the friction force Ff, the coefficients of friction μ, and the normal force R: friction Ff = μdR Ff ≤ μs R dynamic static

  24. y R x Ff mg mg sin 15° mg cos 15° μs= FBD, coin Topic 2: Mechanics2.2 – Forces Describing solid friction by coefficients of friction 15° EXAMPLE: A piece of wood with a coin on it is raised on one end until the coin just begins to slip. The angle the wood makes with the horizontal is θ = 15°. What is the coefficient of static friction? Thus the coefficient of static friction between the metal of the coin and the wood of the plank is 0.268. θ = 15° ∑Fy= 0 ∑Fx= 0 R – mgcos15° = 0 Ff – mgsin15° = 0 Ff = mgsin15° R = mgcos15° Ff = μs N = tan15° mgsin15° = μsmgcos15° = 0.268

  25. y R x Ff mg FBD, coin Topic 2: Mechanics2.2 – Forces Describing solid friction by coefficients of friction 12° EXAMPLE: Now suppose the plank of wood is long enough so that you can lower it to the point that the coin keeps slipping, but no longer accelerates (v = 0). If this new angle is 12°, what is the coefficient of dynamic friction? Thus the coefficient of dynamic friction between the metal of the coin and the wood of the plank is 0.213. θ = 12° ∑Fx= 0 ∑Fy= 0 Ff – mgsin12° = 0 R – mgcos12° = 0 Ff = mgsin12° R = mgcos12° Fd = μd R μd= tan12° = 0.213 mgsin12° = μd mgcos12°

  26. Newton’s laws of motion – The first law Newton’s first law is related to certain studies made by Galileo Galilee which contradicted Aristotelian tenets. Aristotle basically said “The natural state of motion of all objects (but the heavenly ones) is one of rest.” A child will learn that if you stop pushing a wagon, the wagon will eventually stop moving. This simple observation will lead the child to come up with a force law that looks something like this: “In order for a body to be in motion, there must be a force acting on it.” As we will show on the next slide, both of these observations are false! Topic 2: Mechanics2.2 – Forces FALSE FALSE

  27. Inertia will only change if there is a force. Newton’s laws of motion – The first law Here’s how Galileo (1564-1642) thought: If I give a cart a push on a smooth, level surface, it will eventually stop. What can I do to increase the distance it rolls without pushing it harder or changing the slope? If I can minimize the friction, it’ll go farther. In fact, he reasoned, if I eliminate the friction altogether the cart will roll forever! Galileo called the tendency of an object to not change its state of motion inertia. Topic 2: Mechanics2.2 – Forces

  28. A body’s velocity will only change if there is a net force acting on it. Describing the consequences of Newton’s first law for translational equilibrium Newton’s first law is drawn from his concept of net force and Galileo’s concept of inertia. Newton’s first law says that the velocity of an object will not change if there is no net force acting on it. In his words...“Every body continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed thereon.” In symbols... F= 0 is the condition for translational equilibrium. Topic 2: Mechanics2.2 – Forces v = 0 v = CONST F Newton’s first law IfF= 0, then v = CONST.

  29. Translational equilibrium As a memorable demonstration of inertia – matter’s tendency to not change its state of motion (or its state of rest) - consider this: A water balloon is cut very rapidly with a knife. For an instant the water remains at rest! Don’t try this at home, kids. Topic 2: Mechanics2.2 – Forces

  30. T3 T2 T1 45° 30° T3 mg FBD, m FBD, knot 45° 30° T2 T1 T3 knot m Translational equilibrium • EXAMPLE: An object of mass m is hanging via three cords as shown. Find the tension in each of the three cords, in terms of m. • SOLUTION: • Give each tension a name to organize your effort. • Draw a free body diagram of the mass and the knot. • T3is the easiest force to find. Why? • Since m is not moving, its FBD tells us that • Fy = 0 or T3– mg= 0 or T3= mg . Topic 2: Mechanics2.2 – Forces

  31. T3 T2 T1 45° 30° T3 mg FBD, m FBD, knot 45° 30° T2 T1 T3 knot m Translational equilibrium • EXAMPLE: An object of mass m is hanging via three cords as shown. Find the tension in each of the three cords, in terms of m. • SOLUTION: T3= mg • Now we break T1 and T2 down to components. • Looking at the FBD of the knot we see that • T1x= T1 cos30° = 0.866T1 • T1y= T1 sin30° = 0.500T1 • T2x= T2 cos45° = 0.707T2 • T2y= T2 sin45° = 0.707T2 Topic 2: Mechanics2.2 – Forces

  32. T3 T2 T1 45° 30° T3 mg FBD, m FBD, knot 45° 30° T2 T1 T3 knot m Translational equilibrium EXAMPLE: An object of mass m is hanging via three cords as shown. Find the tension in each of the three cords, in terms of m. SOLUTION: T3= mg Topic 2: Mechanics2.2 – Forces ∑Fx= 0 0.707T2 -0.866T1 = 0 T2 =1.225T1 ∑Fy= 0 0.707T2 +0.500T1 - T3 = 0 0.707(1.225T1)+0.500T1 = T3 T1 = mg / 1.366 T2 =1.225(mg / 1.366) T2 =0.897mg

  33. 45° 30° T2 T1 T3 knot m Solving problems involving forces and resultant force PRACTICE: A 25-kg mass is hanging via three cords as shown. Find the tension in each of the three cords, in Newtons. SOLUTION: • Since all of the angles are the same use the formulas we just derived: • T3= mg = 25(10) = 250 n • T1 = mg / 1.366 = 25(10) / 1.366 = 180 n • T2 =0.897mg = 0.897(25)(10) = 220 n Topic 2: Mechanics2.2 – Forces FYIThis was an example of using Newton’s first law with v = 0. The next example shows how to use Newton’s first law when v is constant, but not zero.

  34. L F D W Solving problems involving forces and resultant force EXAMPLE: A 1000-kg airplane is flying at a constant velocity of 125 ms-1. Label and determine the value of the weight W, the lift L, the drag D and the thrust F if the drag is 25000 N. SOLUTION: • Since the velocity is constant, Newton’s first law applies. Thus Fx = 0 and Fy = 0. • W = mg = 1000(10) = 10000 N (down). SinceFy = 0, L - W = 0, so L = W = 10000 N (up). D = 25000 N tries to impede the aircraft (left). SinceFx = 0, F - D = 0, so F= D = 25000 N (right). Topic 2: Mechanics2.2 – Forces

  35. a = Fnet/m Newton’s laws of motion – The second law Newton reasoned: “If the sum of the forces is not zero, the velocity will change.” Newton knew (as we also know) that a change in velocity is an acceleration. So Newton then asked himself: “How is the sum of the forces related to the acceleration?” Here is what Newton said: “The acceleration of an object is proportional to the net force acting on it, and inversely proportional to its mass.” The bigger the force the bigger the acceleration, and the bigger the mass the smaller the acceleration. Topic 2: Mechanics2.2 – Forces Newton’s second law Fnet=ma (orF= ma )

  36. Newton’s laws of motion – The second law Looking at the form F= manote that if a = 0, then F= 0. But if a = 0, then v = CONST. Thus Newton’s first law is just a special case of his second – namely, when the acceleration is zero. Topic 2: Mechanics2.2 – Forces FYI The condition a = 0 can is thus the condition for translational equilibrium, just as F= 0 is. Finally, if you take a physics course and you can’t use notes, memorize the more general formulas. Newton’s second law Fnet=ma (orF= ma )

  37. Ff 50 n R 30 n T W Newton’s laws of motion – The second law Topic 2: Mechanics2.2 – Forces EXAMPLE: An object has a mass of 25 kg. A tension of 50 n and a friction force of 30 n are acting on it as shown. What is its acceleration? SOLUTION: • The vertical forces W and R cancel out. • The net force is thus Fnet=50 – 30 = 20 n (+x-dir). • From Fnet=ma we get 20 = 25 a so that a = 20 / 25 = 0.8 ms-2(+x-dir). Newton’s second law Fnet=ma (orF= ma )

  38. Newton’s laws of motion – The second law Topic 2: Mechanics2.2 – Forces PRACTICE: Use F = ma to show that the formula for weight is correct. SOLUTION: • F = ma. • But F is the weight W. • And a is the freefall acceleration g. • Thus F = ma becomes W = mg. Newton’s second law Fnet=ma (orF= ma )

  39. L F D W Newton’s laws of motion – The second law Topic 2: Mechanics2.2 – Forces EXAMPLE: A 1000-kg airplane is flying in perfectly level flight. The drag Dis 25000 n and the thrust F is 40000 n. Find its acceleration. SOLUTION: • Since the flight is level, Fy = 0. Fx = F– D = 40000 – 25000 = 15000 n = Fnet. From Fnet=ma we get 15000 = 1000a, or a = 15000 / 1000 = 15 ms-2. Newton’s second law Fnet=ma (orF= ma )

  40. F2 30 n Solving problems involving forces and resultant force EXAMPLE: A 25-kg object has exactly two forces F1 = 40. n and F2 = 30. n applied simultaneously to it. What is the object’s acceleration? Topic 2: Mechanics2.2 – Forces SOLUTION: • Resolve F1into its components: • Then Fnet,x = 36 n and • Fnet,y = 17 + 30 = 47 n. Then Fnet2 = Fnet,x2 + Fnet,y2 Fnet2 = 362 + 472 and Fnet= 59 n. • Then from Fnet = ma we get 59 = 25a, or a = 59 / 25 = 2.4 ms-2. F1 40 n 40sin25 17 n 25° 36 n 40cos25

  41. R 6.0 m 30° mg Solving problems involving forces and resultant force EXAMPLE: A 25-kg object resting on a frictionless incline is released, as shown. What is its acceleration? SOLUTION: Begin with a FBD. Break down the weight into its components. Since R and mgcos30°are perpendicular to the path of the crate they do NOT contribute to its acceleration. Thus Fnet=ma mgsin30° = ma a = 10 sin30° = 5.0 ms-2. Topic 2: Mechanics2.2 – Forces 60 30 mgcos30 mgsin30

  42. 6.0 m s 30° Solving problems involving forces and resultant force u = 0 EXAMPLE: A 25-kg object resting on a frictionless incline is released, as shown. What is its speed at the bottom? SOLUTION: We found that its acceleration is 5.0 ms-2. We will use v2 = u2 + 2asto find v,so we need s. We have opposite and we want hypotenuses so from trigonometry, we use sin = opp / hyp. Thus s = hyp = opp / sin = 6 / sin 30° = 12 m and v2 = u2 + 2as = 02 + 2(5)(12) = 120 so thatv = 11 ms-1. Topic 2: Mechanics2.2 – Forces a = 5 ms-2 v = ?

  43. y 30° x F F N FBD, crate R 30° a a Ff Ff mg mg Solving problems involving forces and resultant force EXAMPLE: A 100.-n crate is to be dragged across the floor by an applied force F = 60 n, as shown. The coefficients of static and dynamic friction are 0.75 and 0.60, respectively. What is the acceleration of the crate? SOLUTION: Static friction will oppose the applied force until it is overcome. Topic 2: Mechanics2.2 – Forces FYI Since friction is proportional to the normal force, be aware of problems where an applied force changes the normal force.

  44. y 30° x F FBD, crate R a Ff mg Solving problems involving forces and resultant force SOLUTION: Determine if the crate even moves. Thus, find the maximum value of the static friction, and compare it to the horizontal applied force: Topic 2: Mechanics2.2 – Forces = 60cos30° = 51.96 n. FH=Fcos30° The maximum static friction force is Fs,max=μs R =0.75R The normal force is found from... R + Fsin30°- mg =0 R =70 R + 60sin30°- 100=0 Fs,max=0.75(70) = 52.5 N Thus the crate will not even begin to move!

  45. y 30° x F FBD, crate R a Ff mg Solving problems involving forces and resultant force EXAMPLE: If someone gives the crate a small push (of how much?) it will “break” loose. What will its acceleration be then? SOLUTION: Topic 2: Mechanics2.2 – Forces The horizontal applied force is still = 51.96 n. = 60cos30° Fcos30° The dynamic friction force is Fd=μd R =0.60R. The reaction force is still R =70. n. Thus Fd=0.60(70) = 42 n. The crate will accelerate. Fcos30°- Fd=ma 51.96- 42=(100 / 10)a a = 0.996 m/s2

  46. Newton’s laws of motion – The third law In words “For every action force there is an equal and opposite reaction force.” In symbols In the big picture, if every force in the universe has a reaction force that is equal and opposite, the sum of all the forces in the whole universe is zero! Topic 2: Mechanics2.2 – Forces Newton’s third law FAB = -FBA FAB is the force on body A by body B. FBA is the force on body B by body A. FYI So why are there accelerations all around us? Because each force of the action-reaction pair acts on a differentmass.

  47. FAB FBA B A A Identifying force pairs in context of Newton’s third law EXAMPLE: When you push on a door with 10 N, the door pushes you back with exactly the same 10 N, but in the opposite direction. Why does the door move, and not you? SOLUTION: Even though the forces are equal and opposite, they are acting on different bodies. Topic 2: Mechanics2.2 – Forces your action the door’s reaction • Each body acts in response only to the force acting on it. • The door CAN’T resist FAB, but you CAN resist FBA.

  48. FBE Identifying force pairs in context of Newton’s third law NBT EXAMPLE: Consider a baseball resting on a tabletop. Discuss each of the forces acting on the baseball, and the associated reaction force. SOLUTION: • Acting on the ball is its weight FBE prior to contact with the table. Topic 2: Mechanics2.2 – Forces NTB FEB Note that FBE (the weight force) and NBT (the normal force) are acting on the ball. • NTB (the normal force) acts on the table. • FEB (the weight force) acts on the earth.

  49. Identifying force pairs in context of Newton’s third law We define a system as a collection of more than one body, mutually interacting with each other. Topic 2: Mechanics2.2 – Forces EXAMPLE: Three billiard balls interacting on a pool table constitute a system. The action-reaction force pairs between the balls are called internal forces. For any system, all internal forces always cancel!

  50. Identifying force pairs in context of Newton’s third law We define a system as a collection of more than one body, mutually interacting with each other. Topic 2: Mechanics2.2 – Forces EXAMPLE: Three colliding billiard balls constitute a system. Discuss all of the internal forces. The internal force pairs only exist while the balls are in contact with one another. Note that a blue force and a red forceact on the white ball. The white ball responds only to those two forces. Note that a single white force acts on the red ball. The red ball responds only to that single force. Note that a single white force acts on the blue ball. The blue ball responds only to that single force.

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