1. Prepared by:
Joemarie E. Solares
BSEDIIIB
Biological Science
Major

2. How hard is it to stop a moving object?
To stop an object, we have to apply a force over
a period of time.
This is called Impulse
Impulse = FΔt Units: N∙s
F = force (N)
Δt = time elapsed (s)

4. Changes in
momentum may
occur when there is
either a change in
the mass of an object,
a change in velocity,
or both.

5. If momentum changed, as is most
often of the case, then the velocity
changes. Acceleration occurs. And
what produces acceleration? The
answer is Force. The greater the
force that acts on an object, the
greater will be the change in
velocity and , hence, the change in
momentum.

6. Impulse = force x time
interval
Or in short hand notation:
Impulse = Ft

7. Units
Impulse is measured in N.s (Ft)
or kg.m.s-1 (mv – mu)

8. Impulse Changes Momentum
Impulse Changes Momentum in much the
same way that force changes velocity.
The relationship of impulse to change of
momentum comes from Newton’s 2ndLaw
(a=F/m). The time interval of impulse is
“buried” in the term for acceleration
(change in velocity/time interval.

9. Rearrangement of Newton’s 2nd Law
gives:
Force x time interval = change in (mass x velocity)
or,
equivalently,
Impulse = change in Momentum

10. Case 1: Increasing Momentum
If you wish to increase
the momentum of
something as much as
possible, you not only
apply the greatest force
you can, you also extend
the time of application as
much as possible.

11. Case 2: Decreasing Momentum over a Long time
A large change in
Momentum over a long
time requires a safely
small average force.

12. Case 3: Decreasing Momentum over a Short Time
Remember that for an object
brought to rest, the impulse is
the same, no how it is stopped.
But if the time is short, the
force will be large.

13. BOUNCING

14. You know that if a flowerpot falls from a shelf
onto your head, you may be in trouble. And
whether you know it or not, if it bounces from
your head, you may be in trouble. Impulses are
greater when bouncing takes place. This is
because the impulse required to bring something
to a stop and then, in effect, “throw it back
again” is greater than the impulse required
merely to bring something to a stop.

15. Why does an egg break or not break?
• An egg dropped on a tile floor breaks, but an egg
dropped on a pillow does not. Why?
FΔt= mΔv
In both cases, m and Δv are the same.
If Δt goes up, what happens to F, the force?
Right! Force goes down. When dropped on a pillow,
the egg starts to slow down as soon as it touches it.
A pillow increases the time the egg takes to stops.

16. Practice Problem
A 57 gram tennis ball falls on a tile floor. The ball
changes velocity from -1.2 m/s to +1.2 m/s in 0.02 s.
What is the average force on the ball?
Identify the variables:
Mass = 57 g = 0.057 kg
Δvelocity = +1.2 – (-1.2) = 2.4 m/s
Time = 0.02 s
using FΔt= mΔv
F x (0.02 s) = (0.057 kg)(2.4 m/s)
F= 6.8 N

17. Car Crash
Would you rather be in a
head on collision with an
identical car, traveling at the
same speed as you, or a
brick wall?
Assume in both situations you
come to a complete stop.
Take a guess

18. Car Crash (cont.)
Everyone should vote now
Raise one finger if you think it is
better to hit another car, two
if it’s better to hit a wall and
three if it doesn’t matter.
And the answer is…..

19. Car Crash (cont.)
The answer is…
It Does Not Matter!
Look at FΔt= mΔv
In both situations, Δt, m, and Δv are
the same! The time it takes you to
stop depends on your car, m is the
mass of your car, and Δv depends
on how fast you were initially
traveling.

20. Egg Drop connection
• How are you going to use this in your egg drop?
Which of these variables can you control?
FΔt= mΔv
Which of them do you want to maximize, which do
you want to minimize
(note: we are looking at the force on the egg.
Therefore, m represents the egg mass, not the
entire mass of the project)