1. Momentum
CHAPTER # 5
Momentum

2. St. 11- Momentum So What’s Momentum ?
Momentum is inertia in motion.
Momentum = mass x velocity
p= mv
So, A really slow moving truck and an extremely fast roller skate can have the same momentum.

3. 1 kg
10 m/sec
1000 kg
.01 m/sec
Question :
What is the momentum of an 8Okg runner moving at a speed of 2.5 m/s?

4. Impulse and Momentum Ft = mv
If momentum changes, it’s because mass or velocity change.
Most often mass doesn’t change so velocity changes and that is acceleration.
And mass x acceleration = force
Applying a force over a time interval to an object changes the momentum
Force x time interval = Impulse
Impulse = F t or Ft = mv
Ft = mv

5. MOMENTUM
To INCREASE MOMENTUM, apply the greatest force possible for as long as possible.
Examples :
pulling a sling shot
drawing an arrow in a bow all the way back
FORCE
TIME

6. MOMENTUM Ft Decreasing Momentum
Which would it be more safe to hit in a car ?
Knowing the physics helps us understand why hitting a soft object is better than hitting a hard one. Ft mv mv Ft

7. MOMENTUM
In each case, the momentum is decreased by the same amount or impulse (force x time)
Hitting the haystack extends the impact time
The longer impact time reduces the force of impact and decreases the deceleration.

8. EXAMPLES OF DECREASING MOMENTUM
Bruiser Bruno on boxing …
Increased impact time reduces force of impact
Barney Jervais on bungee Jumping …
Ft = change in
momentum
Ft = change in
momentum
POOF !
CRUNCH !
Ft = Δmv applies here.
mv = the momentum gained before the cord
begins to stretch that we wish to change.
Ft = the impulse the cord supplies to
reduce the momentum to zero.
Because the rubber cord stretches for
a long time the average force on the
jumper is small.

9. Decrease Momentum over short time, increase force
Karate experts sever bricks

10. Questions :
When a dish falls, will the impulse be less if it lands on a carpet than if it lands on a hard ceramic tile floor ?
The impulse would be the same for either surface because there is the same momentum change for each. It is the force that is less for the impulse on the carpet because of the greater time of momentum change. There is a difference between impulse and impact.
If a boxer is able to increase the impact time by 5 times by “riding” with a punch, by how much will the force of impact be reduced?
Since the time of impact increases by 5 times, the force of impact will be reduced by 5 times.

11. St. 12- The Law of Conservation of Momentum
In the absence of an external force, the momentum of a system remains unchanged.
This means that,
The momentum you start with is equal to the momentum you end with.
Pi = Pf

12. COLLISIONS ELASTIC COLLISIONS
Elastic and Inelastic Collisions
ELASTIC COLLISIONS
INELASTIC COLLISIONS– objects stick together and move as one
Momentum transfer from one
Object to another .
Is a Newton’s cradle like the one
Pictured here, an example of an
elastic or inelastic collision?

13. Problem Solving #1
A 6 kg fish swimming at 1 m/sec swallows a 2 kg fish that is at rest. Find the velocity of the fish immediately after “lunch”.
net momentum before = net momentu)m after
(net mv)before = (net mv)after
(m₁v₁ + m₂v₂) = (m₁ + m₂)v
(6x x0) = ( )v
( ) = 8v
6 = 8v
v = .75 m/s

14. Problem Solving #2
An astronaut in space has a mass of 50kg. The astronaut throws a ball that has a mass of 10kg. The ball moves at a velocity of 20m/s. How fast is the astronaut moving?
Answer = - 4 m/s (It’s negative to show that the astronaut is moving back and the ball is moving forward)