Transcript Momentum - Denton ISD

Momentum
• It was first introduced by Isaac
Newton.
• Momentum is symbolized by the
letter ‘p’
• It means inertia in motion.
• Momentum = mass * velocity
• (Velocity has direction, therefore
momentum is a vector)
•p
=
m *
• Unit of p =
v
kg * m/s
Therefore, the amount of momentum
is based on the amount of mass of
the object and its speed.
• A moving object has larger
momentum if
•
1. mass is large or
•
2. speed is large
• At rest, an object has No (zero)
momentum. Why?
Question—
•Which has more momentum: a truck moving
downhill or a roller skate moving down hill at
the same speed?
•Will the trucker always have more momentum
than the roller skate?
•Can the truck and the roller skate ever have
the same momentum?
• Therefore, if velocity changes, so
does the momentum and the body
will be accelerated, therefore a net
force is present.
• Impulse
• Impulse = change in momentum
•
I
= mv
•
I
= F*t
• Unit
=
• Therefore
•
Ft=mv
(kg*m/s2) * s
N*s
• --longer time a force is exerted on
something…the more change in
momentum (greater the impulse)
• --If you wish to change the
momentum of an object, exert an
impulse on it. To do this, force and
time are important.
• There are several ways to change
the momentum of an object.
• Case 1 – Increase Momentum by…
•
~Increasing the force on an
object
• ~(or) extend the time of contact
•
Ex: hitting a baseball – swing
hard & follow through
• Case 2 -- Decrease Momentum over a
long time by…
•
~if longer time, yields a smaller force
• Ex: in your car – would you rather hit a
concrete wall or a haystack?
•
(haystack because it extends the time
of contact )
• Ex: padded dashboards, airbags,
catch ball by moving arm backward
with the ball, jumping to the ground
by bending knees, falling on
wooden floor vs. concrete
• Question—If the boxer is able to make
the duration of the impact five times as
long by riding with the punch, by how
much will the impact be reduced?
•
--The force of impact will be 5 times
less than if you didn’t pull back.
• Case 3 – Decreasing Momentum Over a
short time by…
•
•
~If the time is short…Force of impact
is large
• Ex. Car hits wall
• Ex. Karate chop breaks bricks
(Bounce makes force even greater!)
• Bouncing
•
Impulses are greater when bouncing takes
place because…you need an impulse to bring
the object to a stop as well as to send it back
in the opposite direction.
•
• Question? Which would hurt more…
•
if a flower pot fell on your head, or if it hit
and bounced back?
• Ans: Hit and bounced back
Examples:
• If the mass of an object is 8kg and its
velocity is 1.4 m/s, what is its momentum?
• What is the velocity of an object whose
mass is 3.2 kg and momentum is 2.41 N*s?
• What is the mass of an object whose
momentum is 8.21 N*s and whose velocity
is 4.4 m/s?
• What is the impulse on an object when the force
exerted is -22 N and is exerted over a time of
31.1 seconds?
• If the impulse on an object is 85 N*s, and the
force exerted on it is 14 N, how long was the
force exerted on the object?
• What is the force exerted on an object is the
impulse on it is 42 N*s and the force is exerted
over a time of 53 seconds?