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Momentum

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Momentum

1. 1. Inertia in Motion
2. 2.  Big truck vs. little car, which is harder to stop?  Momentum (P) = mass (m) x velocity (v)  P = mv  Will the big truck always have more inertia?  …. more momentum?
3. 3.  How can an object’s momentum change? (∆mv)  If the mass doesn’t change, the velocity must change. How? Apply a net FORCE.  How big a force? Applied for how long?  Force (F) x time (t) = impulse  To change momentum of an object, exert an impulse (force x time) on it.  Ft = ∆mv
4. 4.  To increase the momentum of an object, increase either the force, the time the force is applied, or both.  Ex:  Pulling an arrow back all the way creates more tension force and increases the time the bow pushes on the arrow.  “Follow through” in golf, tennis, baseball, etc. increases the time the force is applied.  A rifle with a long barrel increases the time the exploding gunpowder acts on the bullet.
5. 5.  To decrease momentum over a long time, less force is needed. ∆mv = F t  Ex:  Pull your hand back when catching a ball  Drive into a haystack instead of a wall  Bend your knees when you jump from a height  Running on dirt has more “give” than asphalt.
6. 6.  To decrease momentum over a short time, increases force. ∆mv = F t  Ex:  Move into a punch instead of away (ouch!!)  Karate expert’s quick chop to break cement bricks.  Want to try bungee jumping with a cord that’s not stretchy? Why/Why not?
7. 7.  The impulse required to bring an object to a stop and then to “throw it back again” is greater than the impulse required merely to bring it to a stop.  Ex: Your head has to provide impulse to stop a falling rock and another impulse to send it back!  Momentum, like force and velocity, is a vector quantity; it has a magnitude and direction.
8. 8.  In accordance with Newton’s 3rd Law, two objects interacting are part of a “system” in which the action and reaction forces cancel.  Ex: Two ice-skaters pushing away from each other, have equal and opposite forces.  When a change in momentum occurs for an object within a system, it is also equal and opposite to the change in momentum of the other object.  The Law of Conservation of Momentum: The total momentum within a system before an interaction is equal to the total momentum after; the total change, or net momentum, is zero; and momentum is said to be conserved, neither gained nor lost.
9. 9.  Momentum is conserved in collisions because the forces that act are internal forces – acting and reacting within the system.  When colliding objects bounce away from each other, we say it is an elastic collision.  When objects collide and stick together, it is called an inelastic collision.  In all cases:  Net Momentum (before) = Net Momentum (after)