Transcript What is Momentum? - Effingham County Schools

An object with a lot of momentum is also hard to
stop
Momentum = p = mv
Units: kg∙m/s
m=mass
v=velocity
Momentum is also a vector (it has direction)
To stop an object, a force must be applied for a
period of time.
This is called Impulse (J)
J = FΔt
Units: N∙s
F = force (N)
Δt = time elapsed (s)

Using Newton’s 2nd Law we get
FΔt= mΔv
Which means
Impulse = change in momentum
•
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 time (Δt) increases, what happens to the
amount of force (F)?
Answer: Force decreases! When dropped on a
pillow, the egg starts to slow down as soon as it
touches it. A pillow increases the time needed
to stop the egg; therefore, less force is applied
to the egg.
• A Thought Experiment:
•
Suppose that you were captured by an
evil physicist who gave you the following choice:
• You must either:
• Stand in front of a 1000 kg. truck moving at
1 m/s, or
• Stand in front of a 1 kg. frozen meatball
moving at 1000 m/s.
• …think…
• Truck:
• Truck's momentum = mv = (1000 kg)(1 m/s) = 1000
kg m/s
• Truck's kinetic energy = 0.5 mv2 = (0.5)(1000 kg)(1
m/s)2 = 500 Joules
• Meatball:
• Meatball's momentum = mv = (1 kg)(1000 m/s) =
1000 kg m/s
• Meatball's kinetic energy = 0.5 mv2 = (0.5)(1kg)
(1000 m/s)2 = 500 000 Joules
Using vectors to
understand
• A 120 kg lineman moving west at 2 m/s tackles an
80 kg fullback moving east at 8 m/s.
• After the collision, both players move east at 2 m/s.
• Draw vector diagrams for the before AND after
situations; draw two diagrams.
Answer to previous slide
question
www.physicsclassroom.com
impulse = change in
momentum
www.physicsclassroom.com
Bouncing increases
impulse explain...
www.physicsclassroom.com
Impulse= change in
momentum
www.physicsclassroom.com
• Elastic
collisions:
momentum is
redistributed or
transferred without
loss.
m1v1o + m2v2o = m1 v1f +m2v2f
• Inelastic
collisions:
when two objects
become entangled
m1v1o + m2 v2o = (m1+m2)vf
vi = 10m/s
vi = 0 (at rest)
M
M
Before Collision
p = mv
vf = 5m/s
M
mredvred
M
+
After Collision
p = 2mv
mbluevblue = m(red +blue) vf
Granny & Little Timmy:
calculate the final velocity.
www.physicsclassroom.com
Law of Conservation
of Momentum
• In the absence of an external
force, the momentum of a
system does not change.
www.physicsclassroom.com
Stopping a Truck
• Suppose a large truck and a
small truck are moving with
equal momentum. Which will
require more work to stop?
Which will require more force
to stop in a given distance?
http://hyperphysics.phy-astr.gsu.edu/hbase/truckc2.html#c1
More Force to Stop the Little
Truck!??
• Since the little truck is
moving substantially
faster it will have more
Kinetic Energy!
Impulse
http://sol.sci.uop.edu/~jfalward/physics17/chapter3/chapter3.html
YIKES!
• If the boulder
and the boy
have the same
momentum,
will the boulder
crush the
boy?
• …do not try this
at home!
http://sol.sci.uop.edu/~jfalward/physics17/chapter3/chapter3.html
Truck Collision
• In a head-on collision:
1.
2.
3.
4.
5.
6.
Which
Which
Which
Which
Which
Which
truck will experience the greatest force?
truck will experience the greatest impulse?
truck will experience the greatest change in momentum?
truck will experience the greatest change in velocity?
truck will experience the greatest acceleration?
truck would you rather be in during the collision?
http://hyperphysics.phy-astr.gsu.edu/hbase/truckc.html#c1
Ride in
the BIG
truck! 
http://hyperphysics.phy-astr.gsu.edu/hbase/truckc.html#c2