1. Chapter 11 Force

2. Newton’s First Law
An object at rest will remain at rest and an object in motion will continue in motion at a constant velocity unless acted upon by a net force

3. Newton’s First Law of Motion “Law of Inertia”
tendency of an object to resist any change in its motion
increases as mass increases

4. Inertia Inertia is the tendency to resist change in motion
Change in motion is always caused by a force
An object will keep doing what it’s doing until a force acts on it

5. Mass and Inertia Massive objects resist change in motion
Objects with more mass need larger forces to change their motion

6. Newton’s 1st Law
An object at rest (still) will stay at rest until a force acts on it.
An object in motion will stay in motion until a force acts on it.

7. Newton’s 1st Law
You aren’t wearing a seatbelt while driving (NOT a good idea). You run your car into a wall. Your car stops, but you remain in motion, so you fly through the windshield.
If you’re wearing your seatbelt, you slow down with the car. Airbags can reduce the force of the impact.

8. Force, Mass, and Acceleration
Exerting a larger force on an object causes a greater change in velocity.
A greater change in velocity means there is greater acceleration.

9. F = ma Newton’s Second Law of Motion
The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.
AKA-When mass goes up, acceleration goes down, and when force goes up, acceleration goes up
F = ma

10. Force, Mass, and Acceleration
If the same force is exerted on a more massive and a less massive object, the more massive object can’t accelerate as much.
The more massive object will have a lower velocity.

11. F = ma F m a F m F: force (N) m: mass (kg) a: accel (m/s2)
Newton’s Second Law m F a F m
F = ma
F: force (N)
m: mass (kg)
a: accel (m/s2)
1 N = 1 kg ·m/s2

12. force of attraction between any two objects in the universe
Gravity
Gravity
force of attraction between any two objects in the universe
increases as...
mass increases (one or both objects)
distance decreases

13. Who experiences more gravity - the astronaut or the politician?
Which exerts more gravity - the Earth or the moon?
more mass
less distance

14. W = mg Weight the force of gravity on an object MASS WEIGHT Gravity
W: weight (N)
m: mass (kg)
g: acceleration due to gravity (m/s2)
MASS
always the same
(kg)
WEIGHT
depends on gravity
(N)

15. Would you weigh more on Earth or Jupiter?
Gravity
Would you weigh more on Earth or Jupiter?
Jupiter because...
greater mass
greater gravity
greater weight

16. Animation from “Multimedia Physics Studios.”
Gravity
Accel. due to gravity (g)
In the absence of air resistance, all falling objects have the same acceleration!
On Earth: g = 9.8 m/s2
elephant
feather
Animation from “Multimedia Physics Studios.”

17. a F m F = ? F = ma m = 40 kg F = (40 kg)(4 m/s2) a = 4 m/s2 F = 160 N
Calculations
What force would be required to accelerate a 40 kg mass by 4 m/s2?
GIVEN:
F = ?
m = 40 kg
a = 4 m/s2
WORK:
F = ma
F = (40 kg)(4 m/s2)
F = 160 N m F a

18. a F m m = 4.0 kg a = F ÷ m F = 30 N a = (30 N) ÷ (4.0 kg) a = ?
Calculations
A 4.0 kg shotput is thrown with 30 N of force. What is its acceleration?
GIVEN:
m = 4.0 kg
F = 30 N
a = ?
WORK:
a = F ÷ m
a = (30 N) ÷ (4.0 kg)
a = 7.5 m/s2 m F a

19. a F m F(W) = 572 N m = F ÷ a m = ? m = (572 N) ÷ (9.8 m/s2)
Calculations
Mrs. Ellis weighs 572 N. What is her mass?
GIVEN:
F(W) = 572 N
m = ?
a(g) = 9.8 m/s2
WORK:
m = F ÷ a
m = (572 N) ÷ (9.8 m/s2)
m = 58.4 kg m F a

20. ConcepTest
Is the following statement true or false?
An astronaut has less mass on the moon since the moon exerts a weaker gravitational force.
False! Mass does not depend on gravity, weight does. The astronaut has less weight on the moon.

21. when an object is influenced only by the force of gravity
Free-Fall
Free-Fall
when an object is influenced only by the force of gravity
Weightlessness
sensation produced when an object and its surroundings are in free-fall
object is not weightless!
CUP DEMO

22. Free-Fall
Weightlessness
surroundings are falling at the same rate so they don’t exert a force on the object

23. Space Shuttle Missions NASA’s KC-135 - “The Vomit Comet”
Free-Fall
Space Shuttle Missions
Go to Space Settlement Video Library.
NASA’s KC “The Vomit Comet”
Go to CNN.com.
Go to NASA.

24. ConcepTest 1
TRUE or FALSE:
An astronaut on the Space Shuttle feels weightless because there is no gravity in space.
FALSE!
There is gravity which is causing the Shuttle to free-fall towards the Earth. She feels weightless because she’s free-falling at the same rate.

25. Newton’s Third Law Newton’s Third Law of Motion
When one object exerts a force on a second object, the second object exerts an equal but opposite force on the first.

26. Newton’s Third Law One object exerts a force on the second object
The second exerts a force back that is equal in strength, but opposite in direction

27. Aren’t these “balanced forces” resulting in no acceleration?
Newton’s Third Law
Problem:
How can a horse pull a cart if the cart is pulling back on the horse with an equal but opposite force?
Aren’t these “balanced forces” resulting in no acceleration?
NO!!!

28. forces are equal and opposite but act on different objects
Newton’s Third Law
Explanation:
forces are equal and opposite but act on different objects
they are not “balanced forces”
the movement of the horse depends on the forces acting on the horse

29. Newton’s Third Law
Action and Reaction forces do not cancel – they can act on different objects or spread in different directions

30. Action-Reaction Pairs
Newton’s Third Law
Action-Reaction Pairs
The hammer exerts a force on the nail to the right.
The nail exerts an equal but opposite force on the hammer to the left.

31. Action-Reaction Pairs
Newton’s Third Law
Action-Reaction Pairs
The rocket exerts a downward force on the exhaust gases.
The gases exert an equal but opposite upward force on the rocket. FG FR

32. F m Action-Reaction Pairs Both objects accelerate.
Newton’s Third Law
Action-Reaction Pairs
Both objects accelerate.
The amount of acceleration depends on the mass of the object. F m
Small mass  more acceleration
Large mass  less acceleration

33. p = mv v p m Momentum quantity of motion p: momentum (kg ·m/s)
m: mass (kg)
v: velocity (m/s)

34. Momentum: inertia in motion
when you’re moving, momentum keeps you moving
an object with lots of momentum is hard to stop

35. v p m Momentum: p = momentum (in kg m/s) m = mass (in kg)
p = mv
p = momentum (in kg m/s)
m = mass (in kg)
v = velocity (in m/s)
momentum is a vector quantity, so a complete answer will include magnitude and direction

36. Is it possible for a motorcycle and a large truck to have the same momentum?
m = 400 kg
m = 40,000 kg

37. v p m p = ? p = mv m = 280 kg p = (280 kg)(3.2 m/s) v = 3.2 m/s
Momentum
Find the momentum of a bumper car if it has a total mass of 280 kg and a velocity of 3.2 m/s.
GIVEN:
p = ?
m = 280 kg
v = 3.2 m/s
WORK:
p = mv
p = (280 kg)(3.2 m/s)
p = 896 kg·m/s m p v

38. v p m p = 675 kg·m/s v = p ÷ m m = 300 kg v = (675 kg·m/s)÷(300 kg)
Momentum
The momentum of a second bumper car is 675 kg·m/s. What is its velocity if its total mass is 300 kg?
GIVEN:
p = 675 kg·m/s
m = 300 kg
v = ?
WORK:
v = p ÷ m
v = (675 kg·m/s)÷(300 kg)
v = 2.25 m/s m p v

39. Example: What is the momentum of a 150 kg defensive tackle at 5 m/s north towards the end zone?

40. Example: What is the momentum of a 325 pound defensive tackle at 5 m/s north towards the end zone?
m = 150 kg)
v = 5 m/s north
p = mv
momentum
p = (150 kg) (5 m/s)
p = 750 kg m/s north

41. To change the momentum of an object, you must change…
mass and/or velocity
usually velocity changes
change in velocity is acceleration
to make an object accelerate, a force is required
So a force causes a change in the momentum of an object.
Size of force and the time the force acts both affect change in momentum.

42. The Importance of “Following Through” in Sports
The follow-through increases the time of collision and thus increases the change in momentum, making the ball have a higher velocity!

43. t Δv I F I m Impulse: what changes the momentum Impulse = Ft
Depends on force and time of collision
Impulse = Ft
Impulse = change of momentum
units for impulse are same as units for momentum
Impulse = Δp OR
Ft = m(Δv) m I Δv

44. Example: A hockey player applies an average force of 80. 0 N to a 0
Example: A hockey player applies an average force of 80.0 N to a 0.25 kg hockey puck for a time of 0.10 seconds. Determine the impulse experienced by the hockey puck.

45. Example: A hockey player applies an average force of 80. 0 N to a 0
Example: A hockey player applies an average force of 80.0 N to a 0.25 kg hockey puck for a time of 0.10 seconds. Determine the impulse experienced by the hockey puck.
F = 80.0 N
t = 0.10 s
Impulse = Ft
impulse
Impulse = (80.0 N) (0.1 s)
Impulse = 8 N s
Impulse = 8 kg m/s F I t
kg m/s2 N

46. Δv I m I = ? I = m Δ v m = 950 kg I = (950 kg)(9m/s) Δv = 40-31 m/s
Example #2
What is the change in momentum (impulse) of a 950 kg car that travels from 40 m/s to 31 m/s?
GIVEN:
I = ?
m = 950 kg
Δv = m/s
= 9 m/s
WORK:
I = m Δ v
I = (950 kg)(9m/s)
I = 8550 kg·m/s m I Δv

47. t I F I = 8550 kg* m/s F= I÷ t F = ? N F = (8550)÷ (30) t= 30 s
Example #3
If the from the last problem had an impulse of kg*m/s, and it took 30 seconds to change speeds, then what force caused the change?
GIVEN:
I = 8550 kg* m/s
F = ? N
t= 30 s
WORK:
F= I÷ t
F = (8550)÷ (30)
F = 285 N F I t

48. The Effect of Collision Time upon the Force
Impulse (kg m/s)
Time (s)
Force (N)
100 1 2 50 4 25 10
1000
0.1 F I t

49. Time and force are ________ proportional.
inversely
For an object in a collision,
to decrease the effect of the force, the time must be increased
To Increase the effect of the force, the time must be decreased.

50. Air Bags
Air bags increase the time allowed to stop the momentum of the driver and passenger, decreasing the force of impact.
Time increases 100 times!
Force decreases 100 times!

51. Use of Mats in Sports
Mats increase the time allowed to stop the momentum of the athlete, decreasing the force of impact.

52. “Riding the Punch”
When a boxer recognizes that he will be hit in the head by his opponent, the boxer often relaxes his neck and allows his head to move backwards upon impact.
“Riding the punch” increases the time the force is applied, decreasing the force of impact.

53. Conservation of Momentum
Law of Conservation of Momentum
The total momentum in a group of objects doesn’t change unless outside forces act on the objects.
pbefore = pafter

54. Conservation of Momentum
Elastic Collision
KE is conserved
Inelastic Collision
KE is not conserved

55. Conservation of Momentum
A 5-kg cart traveling at 1.2 m/s strikes a stationary 2-kg cart and they connect. Find their speed after the collision.
BEFORE
Cart 1:
m = 5 kg
v = 4.2 m/s
Cart 2 :
m = 2 kg
v = 0 m/s
AFTER
Cart 1 + 2:
m = 7 kg
v = ?
p = 21 kg·m/s m p v
p = 0
v = p ÷ m
v = (21 kg·m/s) ÷ (7 kg)
v = 3 m/s
pbefore = 21 kg·m/s
pafter = 21 kg·m/s

56. Conservation of Momentum
A 50-kg clown is shot out of a 250-kg cannon at a speed of 20 m/s. What is the recoil speed of the cannon?
BEFORE
Clown:
m = 50 kg
v = 0 m/s
Cannon:
m = 250 kg
AFTER
Clown:
m = 50 kg
v = 20 m/s
Cannon:
m = 250 kg
v = ? m/s
p = 0
p = 1000 kg·m/s
p = 0
p = kg·m/s
pbefore = 0
pafter = 0

57. Conservation of Momentum
So…now we can solve for velocity.
GIVEN:
p = kg·m/s
m = 250 kg
v = ?
WORK:
v = p ÷ m
v = (-1000 kg·m/s)÷(250 kg)
v = - 4 m/s (4 m/s backwards) m p v