Set 4 Problem number 7

• Problem

• Solution

• Generalized Solution

Problem

An object of unknown mass moving at 4 m/s collides with another object. 

• By observing the behavior of the second object we conclude that the first object must have had momentum 40 kg m/s.

What is the mass of the first object?

Solution

An object with velocity 4 m/s has momentum 40 kg m/s, and we are to find its mass.

• Since momentum is the product of mass and velocity, the mass is the momentum divided by the the velocity.
• The mass is thus m = p / v = ( 40 kg m/s)/( 4 m/s) = 10 kg.

Generalized Solution

Momentum p is the quantity that changes with impulse, the product mv of mass and velocity:

• p = m * v

Thus we easily find the mass of an object of known velocity if we know its momentum:

• v = p / m.