1. Friction, Gravity, and Elastic Forces

2. Previously, in physical science…
You learned about forces and combining forces. Now, you continue to combine forces and identify two or more forces acting on an object.

3. Std 2d Students know how to…
Identify separately the two or more forces that are acting on a single static object, including gravity, elastic forces due to tension or compression in matter, and friction.

5. 1. What is friction?
The force that two surfaces exert on each other when they rub against each other.

6. Byproduct of friction…Heat!

9. 2. What direction does friction act?
Acts in direction opposite to the objects in motion.

10. 3. How do you show friction as a force?
An arrow opposite of the object’s motion
                                                                   
Friction force
Motion

11. 4. What determines the strength of friction?
The types of surfaces (smooth or rough)
and how hard the surfaces push together

12. 5. What are 4 types of friction?
a. Static friction: acts on object NOT moving:

13. b. Sliding Friction
Two solid surfaces slide over each other
ex: book on desk

18. c. Rolling Friction
Object rolls across a surface
ex: ball bearings

22. Solid object moves through a fluid ex: oil, water, air
d. Fluid Friction
Solid object moves through a fluid
ex: oil, water, air
(what do you use in your car to reduce friction?)

31. What are two ways in which friction can be useful?
allows objects to remain in place!
allows moving objects to stop to avoid colliding with other objects!

32. 6. What is gravity?
The force that pulls objects toward each other
                       

33. 7. How do you show gravity as a force on Earth?
An arrow toward the center of Earth
Gravity

34. 8. What is weight? The gravitational force exerted on an object
8. What is weight?
The gravitational force exerted on an object

35. 9. What is the acceleration due to Earth’s gravity?
9.8 m/s2

36. What is your weight (on Earth)?
Weight = Mass × 9.8 m/s2
Weight = ?

37. 10. What is free fall?
When the force of gravity alone causes an object to accelerate downward.
Causes an object to accelerate downward at 9.8 m/s2

38. 11. What is air resistance?
Type of fluid friction as an object moves through air.
Depends on speed and surface area

40. Air resistance

41. Air resistance

42. Free fall:

43. Free fall:
Gravity
Net Force
accelerate downward
Air resistance

44. Terminal velocity:

45. Terminal velocity:
Gravity
0 net force
Move at constant speed
Air resistance

47. Gravity
Net Force
Air resistance

48. Let’s drop coffee filters (again)!
Leave one coffee filter open
Crumple the other coffee filter
Drop them at the same time, what happens?
Draw a diagram of your observations
Draw in your force arrows, including gravity and air resistance.

50. Fluid friction
Gravity

52. Air resistance
Rolling friction
Gravity

53. 12. When is matter elastic?
When it returns to its original shape after it is squeezed or stretched.

54. …all matter has some degree of elasticity…

55. 13. What are 2 types of elastic forces?
Compression: when an elastic force squeezes or pushes matter together.
ex: couch cushion
Tension: when an elastic force stretches or pulls matter apart
ex: guitar string

56. Sometimes the tension forces can be great…like when catching a marlin…

57. Compression forces squeeze together…like the shocks on a car

58. Think of compression on a sponge…or a Twinkie