1. Newton’s 3rd Law and Momentum
Section 12-3
Newton’s 3rd Law and Momentum

2. 12-3 Objectives: On page 118, write and Answer the Following:
1.What is Newton’s 3rd Law?
Before:
After:
2.What is needed for an object to have a large momentum?
3.How is momentum conserved?

3. Newton’s Third Law Forces Always Occur in Pairs.
Normally there are action and reaction type forces that act upon every object.
Newton’s third Law is the called the law of action and reaction.
For every action force, there is an equal and opposite reaction force.

4. Newton’s third law implies that forces always occur in pairs
Newton’s third law implies that forces always occur in pairs. But the action and reaction force of a force pair act on different objects.
The action and reaction forces also occur at the same time, but they do not cancel out because they are acting on different objects.
When you kick a soccer ball, the ball and the foot exert equal and opposite forces on one another.

5. III. Momentum A. Moving Objects have Momentum.
: Lighter objects have less momentum than heavier objects.
: A car is easier to stop than a train because the car has less momentum.
: Momentum is calculated by simply multiplying an object’s mass by its velocity.
Momentum = mass x velocity
p = mv
Units: mass  kg
velocity  m/s
momentum  kg m/s

6. III. Momentum : Like velocity, momentum also has direction.
: An object’s momentum is in the same direction as its velocity.
: The more mass an object has, the greater the momentum.
: The faster an object is moving, the greater the momentum.
Greater Momentum Less Momentum
1. High Mass Small Mass
2. High Velocity 2. Small Velocity

7. Examples
1. Calculate the momentum of a 6.0 kg bowling ball moving at 10m/s down the alley.
p = ?
m = 6.0kg
v = 10m/s
p = mv p = (6.0)(10)
p = 60 kg m/s down the alley

8. Examples
2. Calculate the momentum of a 75 kg speed skater moving forward at 16m/s.
p = ?
m = 75kg
v = 16m/s
p = mv p = (75)(16)
p = 1200 kg m/s moving forward

9. Examples
3. Calculate the momentum of a 135 kg ostrich running north at 16m/s.
p = ?
m = 135kg
v = 16m/s
p = mv p = (135)(16)
p = 2160 kg m/s running north

10. Examples
4. Calculate the momentum of a 5 kg baby on a train moving eastward at 72 m/s.
p = ?
m = 5kg
v = 72m/s
p = mv p = (5)(72)
p = 360 kg m/s moving east

11. Examples
5. Calculate the momentum of an 80g kitten running to the left at 6.5m/s.
p = ?
m = 80g = 0.080kg
v = 6.5m/s
p = mv p = (0.080)(6.5)
p = 0.52 kg m/s moving left

12. Examples
6. Calculate the momentum of a 48,500g passenger on a train stopped on the tracks.
p = ?
m = 48,500g = 48.5kg
v = 0m/s
p = mv p = (48.5)(0)
p = 0 kg m/s

13. III. Momentum B. The Law of Conservation of Momentum
: Imagine that 2 cars with different masses and traveling with different velocities collide head on.
: Can you predict what will happen after the collision?
: Momentum can be used to predict the motion of the cars after the collision.
: This is because in the absence of outside influences, the momentum is conserved.

14. III. Momentum
: In other words, the total momentum of the two cars before a collision is the same as the total momentum after the collision.
: Cars can bounce off of each other to move in opposite directions.
: If the cars stick together after a head-on collision, the cars will continue in the direction of the car that originally had the greater momentum.

15. Examples
1. A 6.0 kg bowling ball moving at 10m/s down the alley hits a 1.0kg pin at rest. After the collision, the 6.0kg bowling ball is traveling at 5m/s. How fast is the pin moving?
ptotal before = ptotal after
pball before + ppin before = pball after + ppin after
(6.0)(10) + (1.0)(0) = (6.0)(5) + (1.0)(v)
= v
60 = 30 + v
30 = v
v = 30 m/s

16. Examples
2. A 75kg speed skater moving forward at 16m/s hits another 65kg skater moving forward at 10m/s. If they collide and hold on to each other, how fast are they moving now? ptotal before = ptotal after
psketer before + pskater before = pskaters after
(75)(16) + (65)(10) = ( )(v)
= 140v
1850 = 140v
13.2 = v
v = 13.2 m/s

17. Examples
3. A person fires a 90kg cannonball from a 2000kg cannon (both start at rest). The cannonball reaches a speed of 30m/s. How fast is the cannon traveling?
ptotal before = ptotal after
Pcannonball + Pcannon before = pcannonball after + pcannon after
( )(0) = (90)(30) + (2000)(v)
0 = v
-2700 = 2000v
-1.35 = v v = m/s