Conservation of Momentum in Explosions

So far you have looked at momentum and impulse in terms of a single object at a time. You have also taken a look at Newton’s Third Law of Motion and have seen how interacting objects exert equal and opposite forces on each other.

Now, you will see how interacting objects provide impulse to each other, resulting in changes in their momentum.

Suppose you find yourself at rest in the middle of a frozen, frictionless pond. No matter how you try to stand up and walk, the frictionless surface simply does not allow you to push off to get moving.  What can you do so that you begin to move towards the shore?

What if you take off your shoe and throw it to the left?  As you apply a force to the shoe, pushing it to the left, Newton’s Third Law tells us that the shoe would be applying a force to you, pushing you to the right.  This push would be enough, on the frictionless pond, to slowly slide to the right, saving you from your predicament.

As you throw your shoe, there is a force acting for the amount of time of the throw. This provides an impulse to the shoe. The impulse on you would be the same magnitude as the impulse on the shoe, but since the forces are in opposite directions, the impulses would also be in opposite directions. Since impulse changes momentum, both you and the shoe would undergo the same change in momentum, in opposite directions.

 

This type of interaction, where two objects start in contact and at rest with respect to each other, then experience a force pushing them apart, is called an explosion. An explosion can be a person throwing a ball, a gun shooting a bullet, a girl jumping off a skateboard, a man jumping from a boat or a rocket firing its engines.  In each of these, one object is propelled one way, while the other is propelled in the opposite direction.

Explosion

When two objects start at rest with respect to each other then experience a force pushing them apart.

Conservation of Momentum

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Conservation Practice

Now, you will learn how to analyze explosions using conservation of momentum. Try to answer the practice problems on your own prior to viewing the solutions. Make sure to click submit after answering each question. Click the player to get started.

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