Draft BCPhysics11-4 Edvantage Interactive.pdf - BC Science ...

Quick Check
1. What is the momentum of a 100 kg motorbike travelling at 10 m/s?
2. What is the mass of plane that is travelling at 200 km/h and has a momentum of 1.1 × 10 6 kg•m/s?
3. How fast does a 0.01 kg bug have to fly to have a momentum of 0.25 kg•m/s? Is this possible?
Impulse
According to Newton’s second law, in its original form F = mv
t
∆p = F∆t = m∆v
. This can be rearranged to:
The product of the force and the time interval during which it acts is called the
impulse. It is given the symbol I. The last equation shows that the impulse is equal to the
change in momentum it produces.
This is an important relationship when considering an object that is undergoing a
change in momentum. An object’s momentum changes because an impulse has been
placed on the object. For example, if you are driving down a road at a constant velocity,
you and your vehicle have a momentum. When you press on the gas pedal, the velocity
of the vehicle increases. The momentum has also increased. This increased momentum
was caused by a force being exerted on the vehicle over a period of time. Or put another
way, the vehicle experienced an impulse. The amount of impulse equals the change in
momentum in the vehicle.
Units for Momentum and Impulse
Momentum is measured in kg•m/s because these units have the dimensions of mass and
velocity. Impulse is measured in N•s because these units have the dimensions of force and
time. Since impulse is equal to change in momentum, these units must be equivalent. It
can easily be shown that this is true:
[N•s] = [kg•m/s 2 ] • [s] = [kg•m/s]
Momentum and impulse may be expressed in either unit.
182 Chapter 4 Newton’s Laws of Motion — DRAFT © Edvantage Interactive 2011 ISBN 978-0-9864778-3-6