Honors Physics - Chapter 9: Lesson 1

Chapter 9: Lesson 1

Momentum and Impulse

Chapter 9: Lesson 1 - Momentum and Impulse

Momentum

Momentum is the product of the mass and the velocity of an object, mass in motion. We abbreviate momentum with a 'p' and the units are kg*m/s. There are no special units for momentum. Since the equation for momentum is p=mv, you just have to bring down the units that you use for mass and the units that you used for velocity.

Momentum is a vector quantity, it has both magnitude and direction. The direction is the same as the velocity of the object.

To increase the momentum, you either have to increase the mass of the object or increase the velocity of the object.

Impulse

Impulse is the product of the net force and the time interval over which it acts. There is no special abbreviation for impulse, since it is force times time, we abbreviate impulse with F∆t. F∆t reminds me of the word FAT, so sometimes you may hear me refer to it as fat.

The units for impulse are also easy to remember. Since impulse is force * time, the units are N*sec. There is no special unit for impulse.

Impulse Momentum Theorem

The impulse given to an object is equal to the change in momentum. The equation F∆t=m∆v is called the impulse momentum theorem.

Impulse is the product of the average force and the time interval during which the force is exerted. A large change in momentum occurs when there is a large impulse. This makes sense if you are looking at the equation, if one side of the equation increases, the other side of the equation will also increase!

Often, when we are exerting a force on something, the force is not constant during the time when when the force is being exerted. When using this equation, we will be using the average force exerted over the time period.

Increasing the Momentum:

To increase the momentum that an object has, you need to apply the greatest force for as long as you possibly can. One example would be a golfer and a golf ball. To get the golf ball to have a large momentum, you need to apply the greatest force that you can over the longest time period. Golfers swing their golf clubs way back to be able to apply a large force over the entire swing to be able to increase the momentum, and therefore increasing the velocity, of their golf ball.

Decreasing the Force:

To decrease the force on an object, you have to increase the time that the force is acting on the object.

Lets look at some examples:

Catching a water balloon: When you catch a water balloon, you move your hands back in the direction that the water balloon is moving. You are increasing the time that the water balloon is slowing down to decrease the force on the water balloon. When you throw a water balloon at a brick wall, the time that it takes the water balloon is slow down is incredibly small, so the force on the water balloon is very large, larger than the balloon can withstand, so it pops!

Air Bags in Cars: Air bags are meant to explode when you have a front end collision in your car. The air bag allows you more time to slow down to decrease the force on your body.

Football helmets and padding: Football helmets have padding so when a players head hits something hard, it increases the time that the head is slowing down to decrease the force on his head. That is the same reason they wear pads when they play!

Check Your Understanding:

Click on the down arrow when you have your answer to check to see if you are correct.

Would you rather stand in one place while getting hit or "roll back" with the punches?

Remember, if you can increase the time that the other person's fist is in contact with your body, then the force can be decreased. If the time is decreased, then the force will increase!

It is much safer for you if you roll back with the punches!

Collisions

When two objects collide, they can either bounce off each other or stick together. When objects bounce off each other, it is called an elastic collision. When two objects bounce off each other, it can also be called rebounding. When two objects stick together, it is an inelastic collision.

Rebounding objects have a large velocity change, because they have to change directions, and therefore have a large momentum change. When getting into a car accident, it is always safer to crumple your car up with the other object that you are getting into the accident with. Crumpling your car increases the time of the collision and also increases the mass, therefore decreasing the velocity of the two cars.

When we are talking about collisions, it is always better to increase the time of the collision to decrease the force. Hitting a chain-link fence or something with give, will always be better than hitting a brick wall. The more you can increase the time that the two objects are colliding, the less force you will have on the object.

Small Time, Big Force

A small time of collision leads to a large force.

Large Time, Small Force

Increasing the time of the collision will decrease the force.

World's Fastest Car Crash

What happens when a car hits a concrete wall at 120 miles per hour? Let just say even your seat belt will not save you with this crash. The time that the car is slowing down is so INCREDIBLY small, that the force is INCREDIBLY large.

Watch the video to see what happens!

Check Your Understanding:

Click on the down arrow when you have your answer to check to see if you are correct.

Is the momentum of a car traveling south different from that of the same car moving north at the same speed?

YES! Momentum is a vector quantity, and the momenta (plural of momentum) of the two cars are in opposite directions.

2. You jump off a table. As your feet hit the floor, you let you legs bend at the knees, explain why.

You reduce the force by increasing the length of time it takes to stop the motion of your body.

3. Which has more momentum, a huge cargo ship tied to a dock or a falling raindrop?

The raindrop, because the cargo ship at rest has zero momentum.

Understanding Car Crashes: It is Basic Physics

What happens to vehicles and their occupants in crashes is determined by science. "You can't argue with the laws of physics," says Griff Jones, award-winning high school physics teacher who goes behind the scenes at the Institute's Vehicle Research Center to explore the basic science behind car crashes. Using a series of vehicle maneuvers on a test track plus filmed results of vehicle crash tests, Jones explains in anything but lecture style the concept of inertia, the relationship between crash forces and inertia, momentum and impulse, and a lot more.

This is a great video that ties this entire lesson together. It is one of the few videos that I show in class because it is so wonderful! Take the time to watch it, it is worth it!

Understanding Car Crashes: Physics Meets Biology

Why do some car crashes produce only minor injuries? How can a single crash of a car into a wall involve three separate collisions? Griff Jones, award-winning science teacher, returns to the Insurance Institute for Highway Safety's Vehicle Research Center to answer these questions and to examine the laws of nature that determine what happens to the human body in a crash.

Jones reviews levels of organization in the body and explains how body cavities house and protect major internal organs. Through creative experiments, he explores how the third collision can cause injuries to organs. He introduces the concepts of stress and strain. He demonstrates how shockwaves can damage tissue and what happens at the cellular level.

Are you interested in a field in medicine? If so, this is a great video that ties what we have been learning about in physics to the medical field. I highly recommend watching this to be able to see the real world applications of what we have been studying in class.

You have finished Lesson 1!

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