SAT Physics Subject Test

Chapter 5 Linear Momentum

IMPULSE

The product of force and the time during which it acts is known as impulse, a term you should be familiar for this test.

That Hurts!

Since concrete is hard and
has no cushion, this makes
the impact time of any
object striking concrete
very short. Forces that
exist over a short period of
time are called impulsive
forces. A large change in
momentum divided by a
short time interval makes
for a painful landing on a
concrete floor.


Impulse is a vector quantity that’s symbolized by J.

J =  ∆t

Newton’s second law can be written in yet another form, in terms of impulse.

J = ∆p


Sometimes this is referred to as the impulse–momentum theorem, but it’s just another way of writing Newton’s second law.

  2. A football team’s kicker punts the ball (mass = 0.42 kg) and gives it a launch speed of 30 m/s. Find the impulse delivered to the football by the kicker’s foot and the average force exerted by the kicker on the ball, given that the impact time is 0.0020 s.

Here’s How to Crack It

Impulse is equal to the change in linear momentum, so

J = ∆p = pf – pi = pf = mv = (0.42 kg)(30 m/s) = 13 kg × m/s

Using the equation , we find that the average force exerted by the kicker is

Questions 3-5

An 80 kg stuntman jumps out of a window that’s 45 m above the ground.

  3. How fast is he falling when he reaches ground level?

  4. He lands on a large, air-filled target, coming to rest in 1.5 s. What average force does he feel while coming to rest?

  5. What if he had instead landed on the ground (impact time = 10 ms) ?

Here’s How to Crack It

3.    His gravitational potential energy turns into kinetic energy: mgh = mv2, so

(You could also have answered this question using Big Five #5.)

4.    Using  = ∆p/∆t, we find that

5.    In this case

Time

Seconds are represented
in questions as s. If you
see a quantity that refers
to time labeled with ms,
it is using milliseconds.
Because impact happens
so quickly, a smaller
quantity of time is
necessary.

This force is equivalent to about 27 tons(!), more than enough force to break bones and cause fatal brain damage. Notice how crucial the impact time is: Increasing the slowing-down time reduces the acceleration and the force, ideally enough to prevent injury. This is the purpose of safety devices such as air bags in cars.

  6. A small block is struck by a force F whose strength varies with time according to the following graph:

What is the impulse delivered to the block?

Here’s How to Crack It

The impulse delivered to the block is equal to the area under the curve. The region is a triangle, so its area,  base × height, can be calculated as follows: