Momentum and Systems

Which of the following are considered isolated systems? Select all the apply.

• Earth (including the atmosphere)

• A sealed glass container

• The universe as a whole

• A tightly sealed opaque container that is a good insulator

Answer explanation

The only "true" isolated system is the universe as a whole, but a sealed, opaque container that is well-insulated is an excellent approximation (for short time frames). Both the Earth and a sealed glass container exchange energy with the surroundings. To a small degree, the Earth also exchanges matter - it is an open system that can be treated as a closed system with small errors in most cases.

Which of the following statements about the momentum of an isolated system are FACTS? Select all that apply.

• The momentum is constant

• The momentum of the system is the sum of the momenta of the particles

• The particles of the system exchange momentum through collisions

• A single particle in the system can have zero momentum at a given point in time

What is the momentum of a single 1.25 kg particle that is moving at 1.00 m/s in the negative direction?

Answer explanation

When two or more units are multiplied by each other, use * between them for clarity, .e.g. kg*m/s, not kgm/s.

Note the negative direction of motion! Momentum is a vector; thus it can be a positive OR negative quanitity.

What is the momentum of a 2-particle system if the particles are moving colinearly, one with momentum 10.0 kg*m/s and the second with a momentum of -5.0 kg*m/s?

Answer explanation

The total momentum of ANY system is equal to the sum of the momenta of the particles of the system. If the particles' motions are colinear, a simple sum gives the momentum of the system.

Two identical particles, each with a mass of 1.00 kg travel toward each other (in linear motion) with speeds of 1.00 m/s. What is the total momentum of the 2-particle system?

Two identical particles, each with a mass of 1.00 kg travel toward each other (in linear motion) with speeds of 1.00 m/s. If one particle has a velocity of 1.00 m/s after the collision, what is the speed of the other particle after the collision?

Answer explanation

Note that the question asks for the SPEED, not the VELOCITY. The velocity of the second particle would have to be negative if the first particle has a positive velocity, but speeds will ALWAYS be positive values.