#### Transcript 28 Momentum and Energy Review of 37s and 38s - lindsey

```Physics ρ , Work, Power, and
Energy Review
March 12, 2008
Please take out the 37s and 38s,
Thank you!
37s
1st #1) A girl hangs from a bar. The force of
gravity, her weight, is pulling her down. What is
the name of the force in the ropes that holds her
up?
Tension
1. On April 15, 1912, Titanic sank after running into an iceberg. What
was its velocity when it collided with the ice berg if it had a mass
of 4.23x108 kg and a momentum of 4.9x109 kg•m/s?
  mv
4.9 x10 kgm / s  4.23x10 kg  v
9
8
v  11.6 m / s
2. Wayne hits a stationary 0.12 kg hockey puck with a force that lasts
for 0.010 seconds and makes the puck shoot across the ice with
a speed of 20.0 m/s. With what force did Wayne hit the puck?
Ft  mv
F  0.010s  0.12kg (20.0m/s)
F  240 N
3. Paul has a mass of 60.0 kg and is riding in his sports car at 25.0 m/s when he slams
into parked car. A seatbelt would bring his body to a stop in 0.400 seconds, but
Paul was not wearing his seatbelt and was therefore stopped by the windshield
in 0.001 seconds. What force would the windshield have exerted on him? How
many times greater is the stopping force of the windshield than the seatbelt?
mv  Ft
60kg  (25  0m / s )  F (0.001s)
1500 kgm / s  F ( 0.001 s )
6
F  1.5 x 10
N
3. Paul has a mass of 60.0 kg and is riding in his sports car at 25.0 m/s when he slams
into parked car. A seatbelt would bring his body to a stop in 0.400 seconds, but
Paul was not wearing his seatbelt and was therefore stopped by the windshield
in 0.001 seconds. What force would the windshield have exerted on him? How
many times greater is the stopping force of the windshield than the seatbelt?
mv  Ft
1500 kgm / s  F ( 0.400 s )
F  3750 N
6
1.5 x10 N
 400 x
3750 N
0.400 s
 400 x
0.001s
4. Granny whizzes around the rink and is suddenly confronted with Adam at rest directly in
her path. Rather than knock him over, she picks him up and continues in motion
without braking. Find the velocity of Granny and Adam after their collision.
•
Granny's mass = 50 kg
•
Granny's initial speed = 3 m/s
•
•
Adam's initial speed = 0 m/s
mg vg  ma va  (mg  ma )vga
(50 x3)  (25 x0)  (50  25)vga
150  75 v
v  2.0 m / s
5. When there is a car accident, some people slow down to check out what is
happening, which can cause more accidents. If an 800 kg car slows to 13
m/s to check out an accident scene, but a 1200 kg truck behind the car
continues traveling at 25 m/s, with what velocity will the two move if they
lock bumpers after a rear-end collision.
m
1
v
1
 m
2
v
2
 ( m
1
 m
2
) v
( 1200 x 25 )  ( 800 x 13 )  ( 1200  800 ) v
30000  10400  2000 v
v
3
3
 20.2 m / s
3
3
6. Lee is rolling along on his 4.0 kg skateboard with a constant velocity of 3.0 m/s
when he jumps off the back and continues forward with a velocity 2.0 m/s
relative to the ground. This causes the skateboard to go flying forward with
a speed of 15.5 m/s relative to the ground. What is Lee’s mass?
( m
( m
L
L
 m
) v  m
S
 4.0 ) 3.0  m
3m
L
L
L
v
L
 m
S
v
S
2.0  ( 4.0 x 15.5 )
 12  2 m
m
L
L
 50 kg
 62
7. Luke Skywalker must swing Princess Leia across a large chasm in order to
escape the Storm Troopers. If Luke and Leia’s combined mass is 145.0
kg, calculate the tension in the rope just before Luke and Leia start their
swing, when the pair makes an angle of 30.0 degrees with the vertical.
F
F
T
F

T
T

F
g
cos 
145 kg ( 9.8 m / s
2
cos 30
 1640.8 N
)
8. A tow truck is pulling a car off the road at a
constant speed. If the tow line makes a
10° with the vertical as shown and the
weight on the line is 10290 N, what is the
tension in the line?
F
F
T
F
T
T


F
g
cos 
10290 N
cos 10
 10448.7 N
38’s
1. After firing a gun, the momentum of the gun is equal and opposite to
the momentum of a bullet because of the law of conservation of
momentum. A rifle recoils from firing a bullet. Why is the velocity
of the gun so much smaller than velocity of the bullet?
Mv = MV
The gun has a larger mass.
2. A moving car has 200 J of kinetic energy.
If the car’s speed doubles, what is its
kinetic energy?
2v
KE = ½ m v2
KE = ½ m (2v)2
KE = 4 x = 800J
3. If a skater’s speed increases so she has twice
the momentum, then how much has her
kinetic energy increased?
  mv
KE 
1
2
mv
2   m( 2v )
2
KE 
1
m ( 2 v )
2
KE  4 x
2
4. How does an air bag and seat belt
change the impulse in a car
crash?
decreases the force of
impact by increases
the time of impact
t
Δρ=F
5. A person can lift barrels a vertical distance of 1
meter or can roll them up a 2 meter long ramp
to the same elevation. If they use the ramp, will
they do more or less work?
Same amount of work
6. What is the purpose of using a simple
machine such as a pulley or lever?
decrease the force by
increasing the distance
7. What has more momentum a large truck with a mass of
1000 kg moving at 30 mph or small car with a mass of
600 kg moving at 60 mph?
  mv
Small Car
  mv
Large Truck
  mv
  600 kg ( 60 mph )
  ( 1000 )( 30 )
  36000 kgmph
  30000 kgmph
8. When you jump off something high, you usually bend your knees
as you reach the ground instead of landing stiff-legged. By
doing this, the time of impact is increased by 10 times. How
does the average force on you body change?
decreased by 10 times
  Ft
  ? F (  10 t )
F 
1
10
9. Legend has it that Isaac Newton “discovered” gravity when an
apple fell from a tree and hit him on the head. If a 200 g apple
fell 7.0 m before hitting Newton, what was the apples change in
potential energy?
PE  mgh
m
PE  (.200 kg )( 9.8
s
)( 7 m )
2
PE  13.72 J
10. Bill shoots a pool ball with a mass of 1 kg off a pool table that is 0.68 m
high and the ball hits the floor with a speed of 6.0 m/s. How fast was
the ball moving when it left the table? (Use the law of conservation of
energy)
•
before after
•
PE + KE = PE + KE
1
1
2
2
(1)9.8(.68)  (1)v  (1)(9.8)(0)  (1)6
2
2
6.664  .5 v
.5 v
2
2
 0  18
 11.336
v  4.76 m / s
11. How much power does it require for Phil to vertical lift
a 20 kg weight 3 m off the ground in 1.5 seconds?
W  Fd
P 
m
W  ( 20 kg )( 9.8
s
W
t
)( 3 m )
2
W  588 J
P 
588 J
1.5 s
P  392 W
12. Brian has a mass of 30.0 kg. He climbs a tree 8.0 m to his tree
house. How much work has Brain done? When he is in the tree
house, what is his potential energy relative to the ground?
W  Fd
PE  mgh
W  ( 30 )( 9.8 )( 8 ) PE  ( 30 )( 9.8 )( 8 )
W  2352 J
PE  2352J
13. Which requires more work: lifting a 50 kg sack vertically
2 meters in 2 seconds or lifting a 25 kg sack a
vertically 4 meters in 4 seconds?
W  Fd
W  ( 50 kg )( 9.8 m / s
2
)( 2 m )
W  980 J
W  ( 25 kg )( 9.8 m / s
W  980 J
2
)( 4 m )
14. A girl can easily lift a 5000 N car with the help of a lever. When she
pushes down 25 cm with 50 N of force, how far does the car
move?
W
in
W
out
Fd  Fd
50 N ( 0.25 m )  ( 5000 N ) d
12.5  5000 d
d  0.0025 m
or 2.5 mm
15. Ben, a 90 kg tackle traveling at 7.0 m/s, collides with
Dan, a 45 kg quarterback running at -2.0 m/s in the
other direction. After the collision Ben continues to
travel forward at 1.0 m/s. How fast is Dan knocked
backwards?
m
v
m
90 ( 7 )  45 ( 2 )  90 ( 1 )  45 v
f
D
D
B
v
m
f
B
v
 m
v
B
f
630  90  90  45 v
450  45 v
v
f
f
 10 m / s
f
D
16. A grey hound at a race track can run at a speed of 16.0
m/s. What is the kinetic energy of a 20.0 kg
greyhound?
KE 
1
mv
2
2
KE 
1
( 20 kg )( 16 m / s )
2
KE  2560 J
2
Reminders
Next Tuesday
– Test
– Packet
• 16s, 17s, 18s, 37s, 38s, EC
– Great America Field Trip Form and \$ due
– If you will be gone for DECA – make
arrangements for taking the test
Work on notecard for test, POTW, extra credit
```