Big truck vs. little car, which is harder to
Momentum (P) = mass (m) x velocity (v)
P = mv
Will the big truck always have more inertia?
…. more momentum?
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
To increase the momentum of an object, increase
either the force, the time the force is applied, or
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.
To decrease momentum over a long time, less
force is needed. ∆mv = F t
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.
To decrease momentum over a short time,
increases force. ∆mv = F t
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?
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.
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
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)