1. Friction and Gravity
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2. Review: Newton’s 1st law
Newton’s 1st law of motion states – an object keeps doing what it is doing (resting or in motion), unless acted upon by an outside force.
Most common outside forces – friction and gravity

3. Friction
The force that one surface exerts on another when the two rub against each other.
Friction acts in the direction opposite to the objects direction of motion.
Ex: ball rolling on ground, hands rubbing together, tires rolling on ground, feet sliding on floor, etc…

4. FRICTION Review Friction A force that opposes motion
(It always goes in the OPPOSITE direction of motion)

5. Force of Friction Dependent on 2 things 1) Type of surface
Ex: more rough the surface, greater the friction, more smooth = less friction

6. Force of Friction 2) How hard surfaces are rubbed together.
Ex: rub hands together with a lot of pressure = hot hands.
More mass of car = more friction between car and ground = more pressure and more friction

7. Causes of Friction
Friction is caused by microscopic hills and valleys of one surface that catch onto the hills and valleys of another surface

8. Types of Friction Static Friction acts on objects that are not moving
Ex. Trying to push box (not moving)
Kinetic friction acts on objects that are moving
Ex. Snowboarding

9. Static Friction
Occurs between 2 surfaces in contact and keeps an object in place

10. Kinetic Friction Sliding
Occurs between 2 objects that rub surfaces (solid surfaces slide over each other)
Hand brakes
Sliding into home

11. Kinetic Friction Rolling
Occurs between the wheels of an object and a surface or when an object rolls (when an object rolls over a surface)
Rolling
Bicycles
Ball bearings

12. Kinetic Friction Fluid
Gas or liquid friction, air resistance (when an object moves through a liquid or gas.)
Fluid
Air resistance
Treading water

13. What type of friction is the rock experiencing in each picture?

14. What type of friction is present ?
On a person’s skateboard and the road
On a sofa resting on the carpet
On a person parachuting from a plane
On a box being pushed along the ground

15. Coefficient of Friction
The level of friction that different materials exhibit is measured by the coefficient of friction.
Coefficient of Friction = Friction Force
Normal Force
The formula is µ = f / N
µ=coefficient of friction (how much friction occurs between 2 surfaces, ratio)
f = force of friction, N = normal force
(N= mg)

16. Calculating Friction
Normal Force
Motion
Friction Force

17. Is Friction Useful?
Walking and all movement on ground not possible without friction
Sand on icy roads
Light a Match
Gymnasts put chalk on hands and wear grips to increase friction

18. Friction Unuseful You want to reduce friction with:
Car Parts- use lubricants to reduce friction in engine parts
Joints in body-synovial fluid in joints and cartilage covers bones
Reduce friction when skiing – wax board or skis
Reduce air resistance for race cars for speed
Swimmers reduced friction between water and body

19. Ways to Overcome or Reduce Friction
Change type of friction – add wheels, lubricant between solid parts, etc…
Reduce pressure or amount of force b/w the objects
Ball bearings – changes from sliding to rolling friction.

20. Mass vs. Weight
Mass – the measure of the amount of matter in an object
Weight – The measure of gravitational force of an object

21. Weight vs. Mass Weight changes, mass does not change
W = m*g, measured in Newtons (4.41 N = 1lb)
Weight (W) is measured in Newtons (N)
Mass (m) is measured in kilograms
Acceleration due to gravity (g) is in meters/second/second
[9.81 m/s2]

22. GRAVITY
The force of attraction between 2 masses (anything with mass pulls on each other)
The force that pulls ALL objects towards each other.
Gravitational force increases as mass increases
Gravitational force decreases as distance between objects increases
F=1/d2

24. Weight vs. Mass What is the difference between weight and mass?
Can the mass of an object change?
Can the weight of an object change?

25. Law of Universal Gravitation
Definition: The force of gravity acts between all objects in the universe.
Ex: you, me, the table, the sun, all things are attracted to each other by gravity.
The force of gravity is dependent on two things: mass of object and distance between objects.

26. Law of Universal Gravitation
Universal Gravitational constant = 6.67 x 10-11
If masses are large the force of gravity is large
If the distances between objects are large the force of gravity weakens

27. Example Problem
Here is a problem which envisions an unfortunate astronaut stranded exactly half way between the earth and moon. What is the gravitational force of the earth acting on him? The data we need are
The mass of the astronaut: m1 = 100 kg
The mass of the earth: m2 = 6 x 1024kg
The distance between earth and moon: r = 3.8 x 108 m
The universal gravitational constant: G = 6.67 x N kg m2/kg2

28. How Does Gravity Work?

29. Mass and Gravity
All objects travel at a rate of 9.81 m/s2, because gravity does not take into account the mass of an object.
Objects that are thrown are called projectiles. Does not interfere with its freefall.

30. Gravity and Motion: Free Fall
When the only force acting on an object is gravity, the object is in free fall
All objects in free fall accelerate at the same rate regardless of their masses
The acceleration of gravity at the surface of Earth is 9.81 m/s2

31. Gravity and Motion: Air Resistance
Air resistance is a fluid friction that acts on objects that fall through air
Because of air resistance things will not fall at the same rate
Objects that are bigger (more mass) will fall at the same rate as small objects due to Newton’s 2nd law- more force = more accelerration

32. Newton’s 2nd Law States Same Thing

33. Forces opposing gravity: Air Resistance
As an object speeds up, air resistance increases, until the air resistance equals the force of gravity.
When forces are balanced there is no acceleration.
Terminal velocity – when force of gravity = force of air resistance, and you no longer accelerate.

34. Terminal Velocity v = g*t
Terminal velocity – As acceleration increases so does air resistance, until both forces are equal and the object is falling at constant velocity
v = g*t

35. V= g x t v = velocity g = 9.81 m/s/s t = time
Example : A boy looking out a window from his apartment on the 4th floor of the building decides to drop a water balloon on an unsuspecting person below. He hears a splat 4 seconds later. How fast was the balloon going when it hit the pavement below?

36. V = g x t g = 9.81 m/s2 v = g x t t = 4s v = 9.81 m/s2 x 4s
v = ? v = m/s

37. Freefall Speed and Graphing
Use the formula v = gt to calculate how fast an object is traveling for each second of freefall.

38. Freefall Chart 1s 9.8 8 s 78.4 15s 2s 19.6 9s 16s 3s 29.4 10s 17s 4s
Time in sec
Velocity m/s 1s
9.8
8 s
78.4
15s 2s
19.6 9s
16s 3s
29.4
10s
17s 4s
39.2
11s
18s 5s 49
12s
19s 6s
58.8
13s
20s 7S
68.6
14s

39. Projectile Motion
Question: Will gravity accelerate 2 balls that are released at the same time at the same rate (9.81 m/s/s) if one is released down and the other is thrown out?

40. Projectile Motion
An object that free falls with horizontal motion is a projectile
A projectile will accelerate down the at the same rate as an object dropped
The projectile moves horizontally at constant speed

41. Projectile Motion
A projectile (anything that is thrown or launched) has 2 motion components that are independent of each other
1. Horizontal motion – constant velocity if no air resistance
2. Vertical motion – gravity pulls on all objects with the same rate of acceleration 9.81 m/s/s

42. Projectile Motion

43. Satellite Motion- Falling Projectile that misses the ground
Satellites in orbit around the earth continuously fall towards Earth, but because the Earth is curved they travel around It
Gravity acts as a centripetal force, keeping the object in a circular path

44. Elastic Forces
Matter is considered elastic if it returns to its original shape after it is squeezed or stretched
Compression is an elastic force that squeezes or pushes matter together
Sitting on a couch
Tension is an elastic force that stretched or pulls matter apart
Pulling a guitar string tight