Momentum Chapter 7.1 – 7.4

Momentum “Inertia in motion” Momentum= mass x velocity p=mv

If both are traveling at the same speed down the highway, which has the larger momentum? Semi, because it has more mass!

How can the smart car and the semi have the same momentum? The smart car’s velocity must be greater than the semi’s velocity so that: Masssemi x velocitysemi=masscar x velocitycar

Objects at rest, have no momentum. p=mv=m*0=0

Change in Momentum Caused by change in mass or velocity or both Usually it is velocity that changes, so, acceleration occurs Forces cause accelerations Greater the force, the greater the change in velocity, the greater the change in momentum.

Forces and Time How long a force acts is important. If you apply the same force for a longer period of time, you will get a greater change in momentum. Impulse= force x time (FΔt) The greater the impulse, the greater the change in momentum Impulse = change in momentum (FΔt)=mv

Increasing Momentum Apply the greatest force, for the longest period of time Follow through when swinging a bat, increases contact time with ball

Decreasing Momentum If your car was out of control and you needed to stop and had to choose between hitting a brick wall or a haystack, which would you choose?

But, Why? In the case of hitting either the wall or the haystack and coming to a stop, your momentum is decreased by the same impulse. By hitting the haystack instead of the wall, you extend the impact time (time with which your momentum is brought to zero) which greatly reduces the force of impact. Mass x velocity = force x time (momentum) (impulse)

Another example… A glass dish is more likely to survive a drop on a carpet floor than a tile floor because the carpet has more “give” than the tile. Increases the impact time, so decreases the force.

Bouncing Impulses are greater when an object bounces For example: Suppose you catch a falling pot with your hands, you provide the impulse to reduce its momentum to zero. If the object bounces, your body must be providing an additional impulse to move it upward

Law of Conservation of Momentum Conserved- “unchanged” In the absence of an external force, the momentum of a system remains unchanged A system will always have the same momentum before some internal interaction as it has after the interaction occurs.

Example: When balls are lifted and released so they make contact with the others, the momentum of the balls is the same before and after the collision. The same number of balls emerge at the same speed on the other side. The momentum before the collision is seen to be equal to the momentum after the collision.