## 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 MomentumConserved?**

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