Transcript Chapter-07

Collisions
A
B
Conservation of Momentum
A
B
During a collision, if the net external force on the system
is zero, then the momentum of the system is constant.
The momentum of each object in the system will
change due to internal forces, but the total momentum
of the system remains constant.
Example
A
B
mB=1.0 kg and mA=0.5 kg. Initially, cart B is at rest and cart
A moves with an x-velocity of 0.25 m/s. After the collision,
cart A moves in the -x direction with a speed of 0.04 m/s.
What is the velocity of cart B after the collision?
Types of collisions
elastic collision -- kinetic energy is conserved
inelastic collision -- kinetic energy is not conserved
Note: “sticky” collisions are always inelastic. They are
called completely inelastic collision.
Sticky Collision
A
B
mB=1.0 kg and mA=0.5 kg. Initially, cart B is at rest and cart
A moves with an x-velocity of 0.25 m/s. After the collision,
the carts stick together. What is their final velocity?
Sticky Collision--Loss of energy
A
B
For the previous question, how much kinetic energy is lost
due to the collision?
Example
Footall collisions
A 250-lb linebacker running with a speed of 0.8 m/s
tackles a 200-lb quarterback who is initially at rest.
After the collision, they have the same velocity. What
is their velocity?
Impulse
Impulse is defined as
If an object comes to a stop in a short time interval,
then the force on the object is LARGE.
If an object comes to a stop in a long time interval,
then the force on the object is small.
Longer time interval, smaller force
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Shorter time interval, larger force
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Air bags
smart car crash test
Volvo crash test
Poll
Two carts on a track collide and stick together. Which
quantities are conserved during the collision?
1. momentum
2. kinetic energy
3. both momentum and kinetic energy
4. neither momentum nor kinetic energy
Poll
A 300-lb lineman who is running full speed collides with
and tackles a 200-lb quarterback who is initially at rest.
The magnitude of the change in momentum of the
quarterback is
1. less than the magnitude of the change in
momentum of the lineman
2. greater than the magnitude of the change in
momentum of the lineman
3. equal to the magnitude of the change in
momentum of the lineman
Poll
A 300-lb lineman who is running full speed collides with
and tackles a 200-lb quarterback who is initially at rest.
The magnitude of the change in velocity of the
quarterback is
1. less than the magnitude of the change in
velocity of the lineman
2. greater than the magnitude of the change in
velocity of the lineman
3. equal to the magnitude of the change in
velocity of the lineman
Poll
In one case, a superball is thrown at a wall with an initial
speed v in the +x direction and rebounds backward with
a speed v in the -x direction. In a second case, a piece
of clay of the same mass is thrown at the wall and sticks
to the wall. On which object is the change in
momentum the greatest?
1. superball
2. clay
3. neither, because the impulse on each object is the
same
Poll
A rifle lying on the ground at rest discharges and shoots a
bullet. The rifle’s mass is of course greater than the
bullet’s mass. Which object will have a greater magnitude
final momentum?
1. rifle
2. bullet
3. neither, they will have the same momentum
Poll
A rifle lying on the ground at rest discharges and shoots a
bullet. The rifle’s mass is of course greater than the
bullet’s mass. Which object will have a greater magnitude
final velocity?
1. rifle
2. bullet
3. neither, they will have the same momentum
In a crash test, identical vehicles carry identical test
dummy “drivers.” Both vehicles are going at 20 m/s (over
40 mi/hr) when they crash head-on into a brick wall and
are brought to a stop. Dummy 1’s vehicle has no safety
devices. Dummy 2’s vehicle has an air bag that inflates on
impact. If we compare the total impulses exerted on the
two drivers as each is brought to a stop, the impulse on
dummy 2 is ________________ the impulse on dummy 1.
A. greater than
B. equal to
C. less than
In a crash test, identical vehicles carry identical test dummy
“drivers.” Both vehicles are going at 20 m/s (over 40 mi/hr)
when they crash head-on into a brick wall and are brought to a
stop. Dummy 1’s vehicle has no safety devices. Dummy 2’s
vehicle has an air bag that inflates on impact. If F is the average
force exerted on a dummy during its collision, and Dt is the
time interval for which it is exerted, then________________.
A. F is greater on dummy 1 but Dt is the same for 1 and 2
B. F is greater on dummy 1 but Dt is greater for dummy 2
C. F is the same on both but Dt is greater for dummy 2
D. F and Dt are both greater for dummy 1