SAT Physics Subject Test

Chapter 5 Linear Momentum

CONSERVATION OF LINEAR MOMENTUM


The law of conservation of linear momentum states that in an isolated system, the total linear momentum will remain constant.


You may recall that Newton’s third law says that when one object exerts a force on a second object, the second object exerts an equal but opposite force on the first. Since Newton’s second law says that the impulse delivered to an object is equal to the resulting change in its linear momentum, J = ∆p, the two interacting objects experience equal but opposite momentum changes (assuming that there are no external forces), which implies that the total linear momentum of the system will remain constant. In fact, given any number of interacting objects, each pair that comes in contact will undergo equal but opposite momentum changes, so the result described for two interacting objects will actually hold for any number of objects, given that the only forces they feel are from each other.

When is Momentum
Conserved?

Remember that
momentum is conserved
in an isolated system. This
means that if an object
collides with a wall, the
floor, or any permanently
immovable object,
momentum is NOT
conserved.

  7. An astronaut is floating in space near her shuttle when she realizes that the cord that’s supposed to attach her to the ship has become disconnected. Her total mass (body + suit + equipment) is 91 kg. She reaches into her pocket, finds a 1 kg metal tool, and throws it out into space with a velocity of 6 m/s directly away from the ship. If the ship is 10 m away, how long will it take her to reach it?

Here’s How to Crack It

Here, the astronaut + tool are the system. Because of conservation of linear momentum

Using distance = rate × time, we find