Chapter 6:3 - Momentum - Physical Science

Chapter 6:3 - Momentum

Physical Science

by Tjamal808

​What is Momentum?

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​Momentum is a property of moving objects.

​It is the product of the mass and velocity of an object.

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The more momentum an object has, the harder it is to stop the object or change its direction.

Calculating Momentum

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You can calculate momentum with the following equation:

​p = m x v

​(p is momentum; m is mass; v is velocity)

The answer is always written as

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​[momentum] kg•m/s [direction]

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M is the mass of the object in kilograms.

​V is the object's velocity in meters per second.

​Momentum has a direction; its direction is always the same as the direction of the object's velocity.

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​Example of Momentum

• ​If a compact car and a large truck are traveling with the same velocity and the drivers of both vehicles hit the brakes at the same time, the compact car will stop first. This is because the truck has more mass and momentum than the car does. Therefore, a larger force is needed to stop the truck.

Multiple Choice

If two vehicles have the same mass and the drivers hit the brakes at the same time, will the fast-moving or slow-moving car be harder to stop?

• A fast-moving car

• A slow-moving car

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The fast-moving car has a greater velocity and thus more momentum than the slow-moving car of the same mass. So, a fast-moving car is harder to stop than a slow-moving car.

The Law of Conservation of Momentum

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States that any time objects collide, the total amount of momentum stays the same.

When a moving object hits another object, some or all of the momentum of the first object is transferred to the object that is hit. If only some of the momentum is transferred, the rest of the momentum stays with the first object.

The Law of Conservation of Momentum

• This law is true for any collision if no forces act on the colliding objects.

• It applies whether the objects stick together or bounce off each other after they collide.

Example of the Law of Conservation of Momentum

A white cue ball hits a red billiard ball so that the billiard ball starts moving and the cue ball stops. The cue ball had a certain amount of momentum before the collision. During the collision, all of the cue ball's momentum was transferred to the red billiard ball. After the collision, the billiard ball moved away with the same amount of momentum the cue ball had.

​As shown in the figure, the momentum before a collision is equal to the momentum after the collision.

Objects Sticking Together

​Sometimes, objects stick together after a collision. After two objects stick together, they move as one object. The mass of the combined objects is equal to the masses of the two objects added together. In a head-on collision, the combined objects move in the direction of the object that had the greater momentum prior to the collision. Together, the objects have a velocity that differs from the velocity of either object before the collision because momentum is conserved and depends on mass and velocity. When mass changes, velocity must change too.

​Objects Sticking Together

​​Sometimes, objects stick together after a collision. After two objects stick together, they move as one object. The mass of the combined objects is equal to the masses of the two objects added together. In a head-on collision, the combined objects move in the direction of the object that had the greater momentum prior to the collision. Together, the objects have a velocity that differs from the velocity of either object before the collision because momentum is conserved and depends on mass and velocity. When mass changes, velocity must change too.

​Objects Bouncing Off Each Other

In some collisions, the objects bounce off each other. During such collisions, momentum is usually transferred from one object to another object. The transfer of momentum causes the objects to move in different directions at different speeds. However, the total momentum of all the objects will remain the same before and after the collision.

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​Examples of such collisions:

• ​bowling ball and bowling pins

• ​billiard balls

• ​bumper cars

​EXAMPLES OF CONSERVATION OF MOMENTUM

Conservation of Momentum and Newton's Third Law

• Newton's third law of motion can explain conservation of momentum.

• ​In the billiard ball example, the cue ball hit the billiard ball with a certain amount of force. This was the action force. The reaction force was the equal but opposite force exerted by the billiard ball on the cue ball.

• The action force made the billiard balls start moving, and the reaction force made the cue ball stop moving.

• Because the action and reaction forces are equal and opposite, momentum is neither gained nor lost.

Section Review​

​Page 169, #1-4

Open Ended

Use the following in a sentence: momentum.

Multiple Choice

Which of the following has the smallest amount of momentum?

• a loaded truck driven at highway speeds

• a track athlete running a race

• a baby crawling on the floor

• a jet airplane being towed toward an airport

Open Ended

Explain the law of conservation of momentum.

Open Ended

How is Newton's third law of motion related to the law of conservation of momentum?

​Section Review Answers

(page 169, #1-4)

1. Momentum is a quantity defined as the product of the mass and velocity of an object.

2. ​c. a baby crawling on the floor

3. ​The law of conservation of momentum states that when two or more objects collide, momentum may be transferred, but the total amount of momentum does not change.

4. ​Newton's third law of motion is related to the law of conservation of momentum because the forces exerted by colliding objects onto each other are equal in magnitude and opposite in direction; thus, momentum is neither gained nor lost.

Poll

After going through this lesson, how well do you understand the concept of momentum?

• I understand it and I am confident.

• I understand it but I need more practice.

• I do not completely understand it.

• I am utterly confused.