Transcript momentum

MOMENTUM
Momentum is a commonly used
term in sports. A team that
has the momentum is on the
move and is going to take some
effort to stop.
Momentum is a physics term; it refers to
the quantity of motion that an object has.
A sports team which is "on the move" has
the momentum. If an object is in motion
(on the move) then it has momentum.
Let’s look at some examples of what momentum is.
Which will have a greater momentum, a tennis
ball served at you by me, or by Pete Sampras?
Why ?
More force is needed to stop a
tennis ball served at 135 mph
than one at 75 mph, even
though they both have the same
mass.
Which has more momentum, a car at 45 mph
or a train at 45 mph?
Train
Why ?
More force is needed to stop a train than a
car traveling at the same speed.
So…what factors are important for
determining the momentum of an object?
Both mass and velocity are important
factors when considering the force
needed to change the motion of an object.
This relationship was
originally developed
by Newton in his 2nd Law.
the product of
mass and velocity
of an object
momentum = mass x velocity
ρ = mv
So you got the Mo?
1. Determine the momentum of a ...
A. 60-kg halfback moving eastward at 9 m/s.
B. 1000-kg car moving northward at 20 m/s.
C. 40-kg freshman moving southward at 2 m/s.
2. A car possesses 20 000 units of momentum. What would be the car's
new momentum if ...
A. its velocity were doubled.
B. its velocity were tripled.
C. its mass were doubled (by adding more passengers and a greater
load)
D. both its velocity were doubled and its mass were doubled.
An object’s momentum will change
if its mass and/or velocity changes.
Most common… a change in velocity.
What is a change in velocity called?
Acceleration
a = Vf-Vo
t
According to Newton’s laws,
a net force causes an object to accelerate,
or change its velocity.
Newton’s Second Law
F = ma
= m (Vf-Vo) = m
F t = m v
t
=
p
V
t
Impulse = change in momentum
FΔt = m(Vf-Vo)
So what does this mean?
A 1000 kg car moving at 30 m/s (p = 30,000 kg m/s)
can be stopped by
30,000 N of force acting for 1.0 s (a crash!)
or
by 3000 N of force acting for 10.0 s (normal stop)
Possible?
A 0.50-kg cart (#1) is pulled with a 1.0-N force for 1 sec;
another 0.50 kg cart (#2) is pulled with a 2.0 N-force
for 0.50 seconds.
A. Which cart (#1 or #2) has the greatest acceleration?
Explain.
B. Which cart (#1 or #2) has the greatest impulse?
Explain.
C. Which cart (#1 or #2) has the greatest change in
momentum? Explain.
Rhonda, who has a mass of 60.0 kg, is riding at 25.0 m/s in
her sports car when she must suddenly slam on the brakes
to avoid hitting a dog crossing the road. She strikes the air
bag, which brings her body to a stop in 0.400 s. What
average force does the seat belt exert on her?
If Rhonda had not been wearing her seat belt and not
had an air bag, then the windshield would have
stopped her head in 0.001 s. What average force
would the windshield have exerted on her?
Some applications of impulse
Contact time is reduced if arm's
deceleration is kept as small as
possible.
This is done by using "followthrough", which means to
continue to push during the
contact period.
Spreading impulse out over a longer time means that the
force will be less; either way, the change in momentum of
the boxing glove, fist, and arm will be the same.
Observe that the greater the time over
which the collision occurs, the smaller the
force acting upon the object. Thus, to
minimize the effect of the force on an object
involved in a collision, the time must be
increased; and to maximize the effect of the
force on an object involved in a collision, the
time must be decreased.
For the remainder of class...
PSE Chapter 4 pg 53 #1-6 due BOC 1/6