# LINEAR MOMENTUM

• Center of mass is that point where all of the mass of an object, or group of objects, can be considered to be located.
• In one dimension we can set up a coordinate system for two masses

• Xcm = (1/5m)[mx + (4m)(1.5x)]
• In general, we have to deal with vectors:
• The center of mass of a system of particles moves as though all the mass of the system were concentrated at that point and all the external forces were applied at that point.
• Linear Momentum: p = mv
• Using the second law we can show that the net force is the rate of change of the momentum: FNET = dp/dt
• This relationship leads us to a new quantity, the IMPULSE.

• What has impulse to do with automotive air bags?
• How do we find the linear momentum of a system that contains many particles?
• Conservation of Linear Momentum: If we have a system of particles, the total momentum will not change (we say that it is conserved) if the vector sum of the external forces is zero.

EXAMPLE

A penguin of mass 10 kg is standing on an ice floe so that he is 20 meters from the shore. The mass of the floe is 40 kg. He walks 8.0 meters on the floe toward the shore. How far is he from the shore?