Transcript Impulse

Impulse
A change in momentum
How to change momentum
• Any object that has momentum is going to be
hard to stop
• To stop an object a force needs to be applied
to the object, against the object’s motion and
for a certain period of time
• It takes a greater force or amount of time to
stop an object with a large momentum
• The momentum of the object is changed since
the velocity is changed
Changes in momentum in real life
• In a football game, the defensive players stop
the momentum of the offensive players so the
offense does not score
• When driving, we apply the brakes which is a
force to stop the car and change the velocity
of the car
Impulse
• Impulse = change in momentum
• The formula for impulse is:
• Impulse = Δp = F * t
• Δp = impulse in N*s (change in momentum)
• F = force in Newtons (N)
t = time in seconds (s)
• reminder p = mv so Δp = m *Δv
Collision
• In a collision, the object experiences an
impulse
• The impulse is equal to the change in
momentum
• The object will speed up or slow down or
change direction due to the force for a given
amount of time
Elastic Collisions
• When a ball rebounds off a wall there will be a
change in velocity and a change in
momentum, that is the impulse
• The greater the rebound effect the greater the
acceleration, momentum change and impulse
• A rebound has direction and speed change
Elastic Collisions
• If a rebound has the same speed (and
momentum) before and after the collision, it is
called an elastic collision
• Elastic collisions usually have a large velocity
change, a large momentum change and a
large force
Practice – fill in the blanks
p = mv
Δp = F * Δt
Δp = m * Δv
Force
F
Time
t
Impulse Momentum
Δp
Change Δp
Mass
m
Velocity
Change Δv
N
s
N*s
kg
m/s
10
-4
1
0.010
2
0.100
3
0.010
4
-20000
5
-200
kg*m/s
-40
10
-200
50
-200
1.0
-8
50
Sample Problem
• A .50kg cart is pulled with a 1N force for 1 second and a
.50kg cart is pulled with a 2N force for .50seconds.
Which cart has the greatest acceleration and impulse?
Cart 1 a = F/m so a = 1N/.50kg = 2 m/s2
impulse = F * t = 1N * 1s = 1N*s
cart 2 a = F/m so a = 2N/.5kg = 4m/s2
impulse = F * t = 2N * .5s = 1N*s
• Cart 2 has a greater acceleration but both carts have
the same change in momentum or impulse
Sample Problem 2
• A hockey player applies an average force of
80N to a .25kg hockey puck for a time of .10
seconds, what is the impulse of the hockey
puck?
• Δp = F * t
• Δp = 80N x .10s = 8N*s