Forces

1. Forces

2. No Push – No Go • If there is nothing pushing on an object, it will not move. • If there is nothing pushing on an object, it will not stop. • The List: • No Push – No Go • No Push – No Stop • No Push – No Speed Up • No Push – No Slow Down • No Push – No Turn

3. In Short: No Push – No Accelerate

4. Force – push or pull • a force always acts in a certain direction • ex. if you push something, the force is in the direction of the push

5. Types of Forces • What different types of forces are there? • Force of Gravity • Force of Friction • Air Resistance • Normal Force

6. Who Discovered this? The first person to realize the relationship was Sir Isaac Newton

7. 1st Law of Motion (Law of Inertia) An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.

8. 1st Law • Inertia is the tendency of an object to resist changes in its velocity: whether in motion or motionless. These pumpkins will not move unless acted on by an unbalanced force.

9. 1st Law • Once airborne, unless acted on by an unbalanced force (gravity and air – fluid friction), it would never stop!

10. 1st Law • Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever.

11. NFL • Another way to think about it: • If you are in a car at a stop light, and the driver speeds up really fast, where do you tend to go? • If you are driving and come to a quick stop, which way do you tend to go? • If you spin a bucket on a rope, in what direction will the bucket go if the line is cut?

12. Some Clarification • What is a force? • This is the push or pull on an object that either gets the object moving, makes it stop, or makes it turn. • It is measured in the units of Newtons (N) • What does unbalanced mean? • This means that, if you add all the pushes and pulls together, there is a overall push/pull in one direction

13. Balanced Forces A) 10 N • When we look at forces, we will break them into forces working in the same plane. • This means all vertical forces can be added and all horizontal forces can be added • Which forces can be added together in the diagram? D) 10 N B) 10 N C) 10 N

14. Unbalanced Forces A) 10 N • What happens if the forces do not add to 0? • These are unbalanced forces, and this creates an acceleration in the object. • What is the size of the unbalanced force? D) 10 N B) 40 N C) 10 N

15. REMEMBER!!!!!!!! Forces are vectors and need a direction!!!

16. Free Body Diagrams • When you are performing calculations, you should always look at a drawing of the situation. • We do this using an FBD or Free body diagram • An FBD uses arrows to describe the forces acting on the body and in which direction.

17. FBD’s 10 N • This is an FBD of a situation. • You need to resolve the forces into their x and y components • If there are forces in the same plane, they can be added and subtracted from each other • In this example, Fy = 0, and Fx = -5N or 5N [W] 5 N 10 N 10 N

18. FBD’s Cont’d • What happens if the forces are not in just the x or y? • Break each force into an x and a y component, then find the sum. • This is done using trigonometry (sine and cosine).

19. Net Forces • Net force (resultant force): a vector produced when 2 or more forces act on a single object. • Calculated by adding the force vectors together. • Also described as the overall force acting on an object

20. What affects acceleration? • What different things can you think of that will affect how fast an object will accelerate? • How hard the object is pushed (force) – the bigger the force the bigger the acceleration • How much mass the object has – NOT ITS WEIGHT

21. Mass vs. Weight • Mass • The amount of matter that is in an object. • This amount never changes, no matter where you are. • It is measured in kilograms (kg) • Weight • This value changes depending on where you are. • It is a force, so it is measured in Newtons (N)

22. Newtons’s 1st Law and You Don’t let this be you. Wear seat belts. Because of inertia, objects (including you) resist changes in their motion. When the car going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 m/hour.

23. Is this all Newton could do? • No, he also came up with the following • The rate of change in an objects velocity (acceleration) in directly proportional to the force applied, and inversely proportional to the mass of the object. • This is called Newton’s Second Law.

24. 2nd Law The net force of an object is equal to the product of its mass and acceleration, or F=ma.

25. Newton’s Second Law • Newton also gave us a formula that allows us to calculate the force being applied to an object • F = ma • Where: • F is the force • m is the mass of the object • a is the acceleration of the object

26. 2nd Law • When mass is in kilograms and acceleration is in m/s/s, the unit of force is in newtons (N). • One newton is equal to the force required to accelerate one kilogram of mass at one meter/second/second.

27. 2nd Law (F = m x a) • How much force is needed to accelerate a 1400 kilogram car 2 meters per second/per second? • Write the formula • F = m x a • Fill in given numbers and units • F = 1400 kg x 2 meters per second/second • Solve for the unknown • 2800 kg-meters/second/second or2800 N

28. If mass remains constant, doubling the acceleration, doubles the force. If force remains constant, doubling the mass, halves the acceleration.

29. Newton’s 2nd Lawproves that different masses accelerate to the earth at the same rate, but with different forces. • We know that objects with different masses accelerate to the ground at the same rate. • However, because of the 2nd Law we know that they don’t hit the ground with the same force. F = ma 98 N = 10 kg x 9.8 m/s/s F = ma 9.8 N = 1 kg x 9.8 m/s/s

30. Check Your Understanding • 1. What acceleration will result when a 12 N net force applied to a 3 kg object? A 6 kg object? • 2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the mass. • 3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec? • 4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec?

31. Check Your Understanding • 1. What acceleration will result when a 12 N net force applied to a 3 kg object? 12 N = 3 kg x 4 m/s/s • 2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the mass. 16 N = 3.2 kg x 5 m/s/s • 3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec? 66 kg-m/sec/sec or 66 N • 4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec? • 9800 kg-m/sec/sec or 9800 N

33. Examples • Calculate the force on a 12 kg object if it accelerates at 3.4 m/s2? • F = m a • F = (12 kg)(3.4 m/s2) • F = 40.8 N • F = 41 N (sig figs) • Calculate the acceleration of a 1.79x103 N force acting on a 743 kg object. • F = m a • a = F/m • a = 1.79x103 N/743 kg • A = 2.41 m/s2

34. 3rd Law • For every action, there is an equal and opposite reaction.

35. 3rd Law According to Newton, whenever objects A and B interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body.

36. 3rd Law There are two forces resulting from this interaction - a force on the chair and a force on your body. These two forces are called action and reaction forces.

37. Newton’s 3rd Law in Nature • Consider the propulsion of a fish through the water. A fish uses its fins to push water backwards. In turn, the water reacts by pushing the fish forwards, propelling the fish through the water. • The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards).

38. 3rd Law Flying gracefully through the air, birds depend on Newton’s third law of motion. As the birds push down on the air with their wings, the air pushes their wings up and gives them lift.

39. Consider the flying motion of birds. A bird flies by use of its wings. The wings of a bird push air downwards. In turn, the air reacts by pushing the bird upwards. • The size of the force on the air equals the size of the force on the bird; the direction of the force on the air (downwards) is opposite the direction of the force on the bird (upwards). • Action-reaction force pairs make it possible for birds to fly.

40. Other examples of Newton’s Third Law • The baseball forces the bat to the left (an action); the bat forces the ball to the right (the reaction).

41. 3rd Law • Consider the motion of a car on the way to school. A car is equipped with wheels which spin backwards. As the wheels spin backwards, they grip the road and push the road backwards.

42. 3rd Law The reaction of a rocket is an application of the third law of motion. Various fuels are burned in the engine, producing hot gases. The hot gases push against the inside tube of the rocket and escape out the bottom of the tube. As the gases move downward, the rocket moves in the opposite direction.

43. The Misconception • So long as the forces are balanced on an object, is there motion? • The answer is YES, but why is this true? • N2L says that there needs to be an unbalanced force to have an acceleration, NOT motion • Be careful how you interpret the law!!

44. Gravity • force of attraction between all objects in the universe • the larger an object’s mass, the greater the gravitational force

45. Gravity • the greater the distance between two objects, the smaller the gravitational force • the force of gravity decreases by the amount equal to one divided by the distance (d) squared • 1/d2

46. Newton’s Law of Gravitation • Using his own laws, Newton created a Universal Law of Gravitation that applies to everything in the Universe. • Every particle attracts every other particle with a force that is proportional to the square of the distance between them.

47. NULG • If you perform unit analysis on the previous formula, you will find that they do not match, something is missing, they are not the units of force. • It wasn’t until Cavendish worked on it, that the constant was found, and the equation can be used. • The constant was called the Gravitational Constant, G.

48. NULG • This lead the formula to change to the following: • Example: • Calculate the force of gravitational attraction between a 2.00 kg mass and a 5.00 kg mass separated by a distance of 3.00 m.

49. Elevators • Ever had butterflies in your stomach if you moved really fast up or down on an elevator? • What do you think happens to the weight the floor of the elevator sees as you move up or down? • Does the weight stay the same, get bigger or smaller?

50. Elevators Cont’d • If you stand on an scale that measures Newtons in an elevator the following will happen • Stopped or constant velocity: you will read your weight • Moving up with an acceleration: the scale will read a larger weight (you feel heavier) • Moving down with an acceleration: the scale will read a smaller weight (you feel lighter)