# newtons third law

## calculating momentum

Momentum can be calculated by multiplying an objects mass in kilograms (kg) by its velocity in meters per second (m/s). For example, assume that a golf ball has a mass of 0.05 kg. If the ball is traveling at a velocity of 50 m/s, its momentum is: Momentum = 0.05 kg 50 m/s = 2.5 kg m/s Note that the SI unit for momentum is kgm/s. Problem Solving Problem: What is the momentum of a 40-kg child who is running straight ahead with a velocity of 2 m/s? Solution: The child has momentum of: 40 kg 2 m/s = 80 kgm/s. You Try It! Problem: Which football player has greater momentum? Player A: mass = 60 kg; velocity = 2.5 m/s Player B: mass = 65 kg; velocity = 2.0 m/s

## conservation of momentum

When an action and reaction occur, momentum is transferred from one object to the other. However, the com- bined momentum of the objects remains the same. In other words, momentum is conserved. This is the law of conservation of momentum. Consider the example of a truck colliding with a car, which is illustrated in Figure 14.11. Both vehicles are moving in the same direction before and after the collision, but the truck is moving faster than the car before the collision occurs. During the collision, the truck transfers some of its momentum to the car. After the collision, the truck is moving slower and the car is moving faster than before the collision occurred. Nonetheless, their combined momentum is the same both before and after the collision. You can see an animation showing how momentum is conserved in a head-on collision at this URL: .

## action and reaction

Newtons third law of motion states that every action has an equal and opposite reaction. This means that forces always act in pairs. First an action occurs, such as the skateboarders pushing together. Then a reaction occurs that is equal in strength to the action but in the opposite direction. In the case of the skateboarders, they move apart, and the distance they move depends on how hard they first pushed together. You can see other examples of actions and reactions in Figure 14.9. You can watch a video about actions and reactions at this URL: You might think that actions and reactions would cancel each other out like balanced forces do. Balanced forces, which are also equal and opposite, cancel each other out because they act on the same object. Action and reaction forces, in contrast, act on different objects, so they dont cancel each other out and, in fact, often result in motion. For example, in Figure 14.9, the kangaroos action acts on the ground, but the grounds reaction acts on the kangaroo. As a result, the kangaroo jumps away from the ground. One of the action-reaction examples in the Figure 14.9 does not result in motion. Do you know which one it is?

## momentum

What if a friend asked you to play catch with a bowling ball, like the one pictured in Figure 14.10? Hopefully, you would refuse to play! A bowling ball would be too heavy to catch without risk of injury assuming you could even throw it. Thats because a bowling ball has a lot of mass. This gives it a great deal of momentum. Momentum is a property of a moving object that makes the object hard to stop. It equals the objects mass times its velocity. It can be represented by the equation: Momentum = Mass Velocity This equation shows that momentum is directly related to both mass and velocity. An object has greater momentum if it has greater mass, greater velocity, or both. For example, a bowling ball has greater momentum than a softball when both are moving at the same velocity because the bowling ball has greater mass. However, a softball moving at a very high velocity say, 100 miles an hour would have greater momentum than a slow-rolling bowling ball. If an object isnt moving at all, it has no momentum. Thats because its velocity is zero, and zero times anything is zero.

## kqed out of the park the physics of baseball

At UC Berkeley, a team of undergrads is experimenting with velocity, force, and aerodynamics. But you wont find them in a lab they work on a baseball diamond, throwing fast balls, sliders and curve balls. QUEST discovers how the principles of physics can make the difference between a strike and a home run. For more information on the physics of baseball, see http://science.kqed.org/quest/video/out-of-the-park-the-physics-of-baseball/ . MEDIA Click image to the left or use the URL below. URL:

## kqed newtons laws of motion

Paul Doherty of the Exploratorium performs a "sit-down" lecture on one of Sir Issac Newtons most famous laws. For more information on Newtons laws of motion, see http://science.kqed.org/quest/video/quest-lab-newtons-laws- MEDIA Click image to the left or use the URL below. URL:

## instructional diagrams

No diagram descriptions associated with this lesson

## questions

Forces always act in pairs.

``````-->  a. true

b. false
``````

What happens when a boater pushes against the water with an oar?

``````a. The water pushes back.

b. The boat moves in the opposite direction.

c. The oar moves the boat.

-->  d. two of the above
``````

Action and reaction forces always cancel out.

``````a. true

-->  b. false
``````

Action and reaction forces are not balanced because they

``````a. are unequal in strength.

b. act in the same direction.

-->  c. act on different objects.

d. cancel each other out.
``````

Action and reaction forces always result in motion.

``````a. true

-->  b. false
``````

An object has greater momentum if it has

``````a. smaller size.

b. greater mass.

c. greater velocity.

-->  d. two of the above
``````

Which statement about momentum is false?

``````-->  a. Momentum is a force.

b. Momentum may be transferred.

c. Momentum is always conserved.

d. Momentum is a property only of moving objects.
``````

Only moving objects have momentum.

``````-->  a. true

b. false
``````

A smaller mass cannot have as much momentum as a larger mass.

``````a. true

-->  b. false
``````

The momentum of a 50-kg object moving at a velocity of 2 m/s is

``````-->  a. 100 kg  m/s

b. 50 kg  m/s

c. 25 kg  m/s

d. 2 kg  m/s
``````

Momentum can be transferred from one object to another.

``````-->  a. true

b. false
``````

When an action and reaction occur, momentum is usually lost.

``````a. true

-->  b. false
``````

Momentum is conserved only in head-on collisions.

``````a. true

-->  b. false
``````

Newtons third law of motion is also called the law of conservation of momentum.

``````a. true

-->  b. false
``````

Momentum is another term for acceleration.

``````a. true

-->  b. false
``````

Momentum is a measure of an objects velocity.

``````a. true

-->  b. false
``````

If you double the velocity of an object, its momentum also doubles.

``````-->  a. true

b. false
``````

The law of conservation of momentum applies to actions and reactions.

``````-->  a. true

b. false
``````

After two objects collide, their combined momentum is always zero.

``````a. true

-->  b. false
``````

A bowling ball has greater momentum than a softball if both have the same velocity.

``````-->  a. true

b. false
``````

how to calculate momentum

``````a. momentum

b. Newtons third law of motion

c. balanced forces

d. kg  m/s

e. law of conservation of momentum

f. action-reaction forces

-->  g. mass  velocity
``````

SI unit for momentum

``````a. momentum

b. Newtons third law of motion

c. balanced forces

-->  d. kg  m/s

e. law of conservation of momentum

f. action-reaction forces

g. mass  velocity
``````

equal and opposite forces that act on different objects

``````a. momentum

b. Newtons third law of motion

c. balanced forces

d. kg  m/s

e. law of conservation of momentum

-->  f. action-reaction forces

g. mass  velocity
``````

combined momentum of objects remains the same when an action-reaction occurs

``````a. momentum

b. Newtons third law of motion

c. balanced forces

d. kg  m/s

-->  e. law of conservation of momentum

f. action-reaction forces

g. mass  velocity
``````

property of a moving object that makes it hard to stop

``````-->  a. momentum

b. Newtons third law of motion

c. balanced forces

d. kg  m/s

e. law of conservation of momentum

f. action-reaction forces

g. mass  velocity
``````

equal and opposite forces that act on the same object

``````a. momentum

b. Newtons third law of motion

-->  c. balanced forces

d. kg  m/s

e. law of conservation of momentum

f. action-reaction forces

g. mass  velocity
``````

every action has an equal and opposite reaction

``````a. momentum

-->  b. Newtons third law of motion

c. balanced forces

d. kg  m/s

e. law of conservation of momentum

f. action-reaction forces

g. mass  velocity
``````

When an action force occurs, the reaction force is always

``````a. in the same direction as the action force.

-->  b. equal and opposite to the action force.

c. applied to the same object as the action force.

d. two of the above
``````

When you stand on the floor, the force of your body pushing down on the floor is

``````-->  a. matched by the floor pushing up on your body.

b. less than the reaction force applied by the floor.

c. a reaction to the floor pushing up.

d. none of the above
``````

When a kangaroo jumps, the kangaroos action force acts on the ground and the reaction force

``````a. is exerted by the ground.

b. acts on the kangaroo.

c. is greater than the action force.

-->  d. two of the above
``````

If the following objects are all moving at the same velocity, which of the objects has the greatest momentum?

``````a. pea

b. marble

c. volleyball

-->  d. bowling ball
``````

Momentum is directly related to

``````a. mass.

b. velocity.

c. distance.

-->  d. two of the above
``````

Momentum is a

``````a. force of nature.

b. form of energy.

-->  c. property of an object.

d. measure of an objects motion.
``````

What is the momentum of a 9-kilogram object that has a velocity of 3 m/s?

``````a. 3 kg/m/s

b. 6 kg/s/m

c. 12 kg  s/m

-->  d. 27 kg  m/s
``````

## diagram questions

No diagram questions associated with this lesson