CH 4 FORCES

1. CH 4FORCES Isaac Newton YouTube - Newton's Laws of Motion illustrated with 3D animations and motion graphics

2. Skydiving Intro • What forces are acting on the skydivers? • What acceleration occurs? • How can divers change these forces? change their velocities?

3. Introduction • Changing position is defined as velocity. • Changing velocity is defined as acceleration. • What causes acceleration? • FORCES

4. Force • Force : A push or pull upon an object. • System: The object that experiences the force. • Environment/External World: Everything around the object that is exerting the forces. • Agent: Whatever is causing the force.

5. Types of Forces: • contact forces • field forces

6. Contact forces Physically contacting each other. • frictional forces • normal forces • applied forces • tensional forces • air resistance forces • Spring forces

7. Friction ForceFf • Force exerted by a surface as an object moves across it or makes an effort to move across it. • Opposes the motion of the object. • Friction depends upon the nature of the two surfaces and upon the degree to which they are pressed together.

9. Normal Force FN • The contact force exerted by a surface on an object. • Perpendicular to the surface. Normal Force

10. Applied Force Fa • A force which is applied to an object by a person or another object.

11. Tensional Force Ft • The force transmitted through a string, rope, or wire when it is pulled tight by forces acting from each end. • Force is parallel to the rope and points away from the object.

12. Air Resistance Force Fair • Type of frictional force • Opposes the motion of the object. • Often neglected (won’t make much difference). • Noticeable for objects which travel at high speeds (a skydiver or a downhill skier) or for objects with large surface areas. Fair = 5N FW = 25N

13. Spring Force Fspring • Force exerted by a compressed or stretched spring upon any object which is attached to it. • An object which compresses or stretches a spring is always acted upon by a force which restores the object to its rest or equilibrium position (force is opposite of the displacement of the object).

14. Gravitational Force (Weight) Fg or FW • Force at which the earth, moon, or other massively large object attracts another object towards itself. • FW= m * gwhere g = 9.8 m/s2 (on Earth) and m = mass (in kg) • Do not confuse weight with mass. • “When you have the mass and you want the weight - multiply by 9.8”

15. Mass & Weight • Mass • Constant (even on moon) • Balance • Kilograms • Weight • Gravity dependent • Scale • Newtons

16. Free Body Diagrams • Show the magnitude and direction of all forces acting upon an object • Determines Net Force (Fnet) • Always draw the tail of the vector on the object.

17. Free Body Diagram • An egg is free-falling from a nest in a tree. • Neglect air resistance. • Diagram the forces acting on the egg as it is falling.

18. Free Body Diagram • A girl is suspended motionless from a bar which hangs from the ceiling by two ropes. • Diagram the forces on the bar.

19. Free Body Diagram • A skydiver is descending with a constant velocity. • Consider air resistance. • Diagram the forces.

20. Free Body Diagram • A car is coasting to the right and slowing down. • Diagram the forces.

21. Free Body Diagram • A rightward force is applied in order to move a book across a desk at constant velocity. • Consider frictional forces. • Neglect air resistance. • Diagram the forces.

23. Forces are VECTORS • Pay attention to direction!!! • Remember your rules for adding vectors • Forces in opposite directions need to have opposite signs. • Net Force – the vector sum of all forces on an object

24. Net Force1200 + (-800) = 400N (Up) A construction crane applies a 1200N force to an 81.6 kg sign

25. Net Force600N + (-800N) = -200N (down) An 800N skydiver free falls with 600N air resistance.

26. Net Force = 20 N left

27. Newton’s 2nd Law • Net force causes acceleration • The acceleration depends on • the net force acting on the object (directly related) • the mass of the object (inversely related) • Fnet = m * a

28. FORCE UNITS - Newton • One Newton is the amount of force required to give a 1-kg mass an acceleration of 1 m/s2 • A Newton is abbreviated by a "N."

29. F=ma • What acceleration will result when a 12-N net force is applied to a 3-kg object? • F = m * a • 12N = (3 kg) ( a )  • a = 4 m/s2

30. Important: • If the net force on an object is 0, the object will have a constant velocity or no velocity. • It is in equilibrium • If the net force on an object is not 0, the object will have an acceleration in the same direction as the net force. • Objects do not require force to continue in motion. • Horizontal forces are independent of vertical motion.

31. Try It: The diagram represents a pushed shopping cart.

32. Example: Arnold needs to lift a 35 kg rock. If he exerts an upward force of 502 N on the rock, what is the rock's acceleration?

33. Newton’s 1st Law (Inertia) An object that is at rest will remain at rest, and an object that is moving will continue to move in a straight line with constant speed, if and only if the net force acting on the object is zero. “Objects keep on doing what they're doing"

34. Inertia • Inertia – the tendency of an object to resist change • At rest – it will stay at rest • Moving at a constant velocity – it will keep moving at a constant velocity • Inertia is a property of the object, therefore it is not a force.

35. Inertia and Mass • Some objects have more inertia (harder to change what they are doing) • Inertia depends on mass more mass = more inertia

36. Inertia • http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/newtlaws/cci.html

37. Inertia

38. 1st Law – Coffee What will happen to a coffee cup filled to the rim while starting a car from rest or while bringing a car to rest from a state of motion?

39. 1st Law Application • The head of a hammer can be tightened onto the wooden handle by banging the bottom of the handle against a hard surface. Why?

40. How does the1st Law Apply to: • Ketchup • Skateboards • Seatbelts

41. Check for Understanding • Ben is being chased through the woods by a moose. If Ben makes a zigzag pattern through the woods, he is able to use the large mass of the moose to his own advantage. • Explain this in terms of inertia and Newton's first law of motion. • The large mass of the moose means the moose has a large inertia. Thus, Ben can more easily change his own state of motion (make quick changes in direction) while the moose has extreme difficulty changing its state of motion. Physics for better living!

42. Check for Understanding • The 15th hole at the Putt-Putt Golf Course has a large metal rim which guides the ball towards the hole. • A player hits their golf ball around the metal rim When the ball leaves the rim, which path (1, 2, or 3) will the golf ball follow? • 2. the ball will follow an "inertial path" (i.e., a straight line). • Once leaving the rim, there is no more unbalanced forces to change its state of motion.

43. Gravitational Force/Weight • Fg = mg • g = 9.8 m/s2 • You can use a scale to measure weight. • When you are standing on a bathroom scale, the upward force of the scale on you equals the downward force of your weight. • The reading on the scale is determined by the amount of force the springs inside exert on you. • In a different location the compression of the springs may be different

44. Apparent Weight • Apparent weight – what your weight “appears” to be due to the net force acting on it and giving it an acceleration • EX: If you were riding on a scale in an elevator it may not always read your actual weight. If you are accelerating, the net force has to be in the direction of your acceleration.

45. Weightlessness • Weightlessness – An object's apparent weight of zero that results when there are no contact forces pushing up on an object. • EX: If the cable of the elevator breaks and you and the scale are both in free fall.

46. Example: • Suppose you are standing in an elevator that is accelerating upward. Is the magnitude of the normal force exerted on you by the floor of the elevator the same as, larger than, or smaller than the magnitude of your weight? • You are riding in an elevator when you suspect the cable has broken. What can you do to quickly determine if this is the case or not?

47. Drag Force • Drag force – the force exerted by a fluid on the object moving through the fluid. • Depends on: • Motion of the object • Properties of the object • Properties of the fluid • EX: As the speed of the object increases, so does the drag force.

48. When an object is falling, it experiences an upward drag force due to the air and a downward force due to gravity. • Terminal Velocity - the constant velocity that is reached when the drag force (force due to air resistance) equals the force due to gravity.

49. Terminal Velocity • As an object gains speed, it encounters an increasing amount of upward air resistance force. • Objects will continue to accelerate (gain speed) until the air resistance force balances the downward force • Once the upward force of air resistance upon an object is large enough to balance the downward force of gravity, the object is said to have reached a terminal velocity. The terminal velocity is the final velocity of the object; the object will continue to fall to the ground with this terminal velocity. • Which has a larger terminal velocity – feather or elephant? Explain.

50. Free Fall – NO Air Resistance