1. Inertia in Motion

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.  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.  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.  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.  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.  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.  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.  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)