Download KEY - Mrs. Wendorf

Physics Fix 16
Name: ______________________________
Critical Thinking Questions
1. We have previously learned about inertia. Give a definition for inertia.
Inertia is the tendency of an object to maintain its current state of
motion—if an object is at rest, it tends to stay at rest; if it is moving, it
tends to keep its motion.
2. Obviously, a boulder is more difficult to move than a bowling ball. Which substance has a greater
rest inertia?
The boulder, because it has a greater mass.
3. A semi truck will be more difficult to stop than a motor cycle if they are both going at the same
speed. Which vehicle has the greater motion inertia?
The semi truck will have the greater motion inertia, again, because of its
greater mass.
4. Now consider two very similar semi trucks. One travels at 40 km/hr and another travels at 75 km/hr.
Which truck will have the greater motion inertia?
Assuming identical masses, the faster truck will have the greater motion inertia.
5. Which two of the following characteristics affect the “motion inertia”?
A) Direction
B) Velocity C) Acceleration
D) Mass
Information: Momentum
“Motion inertia” is the tendency of an object to continue in motion. Momentum is the word most
often used when speaking of an object continuing in motion. There is a mathematical equation
that describes momentum which is based on your answer to question 5. The equation is:
momentum = mass x velocity OR momentum = mv
The units for momentum are simply the mass units times the velocity units. Kg m/s and g km/hr
are examples of momentum units.
Critical Thinking Questions
6. Calculate the momentum of a 25,000 kg truck traveling at a speed of 20 m/s. (Include units.)
500000 kg×m/s
7. Is it possible for a truck and a motorcycle to have the same momentum? Explain.
Yes, since the motorcycle has less mass, it would need to be
moving faster than the truck.
8. What is the momentum of a 1900 kg car that is sitting in the driveway?
Zero. The car is not moving (velocity is zero), therefore, the
momentum is zero.
Information: Changing Momentum
As Newton’s 1st Law states, an object’s velocity will not change unless a force acts on the object.
Since momentum involves velocity, a force will change an object’s momentum. The force that
causes a change in momentum is called an impulse.
An impulse may be a very quick push or a push that lasts for a long time. To calculate how
much the momentum changes you need to know how long the force lasts.
“” means “change in”
Ft = m(v)
Impulse Force
Change in momentum
F = force of impact in Newtons
m = mass of the object in kg
t = the time of impact, how long the impact force acts on the object
v = change in velocity of the object
Before proceeding to the questions, please notice the difference between “force of impact” and
“impulse force”. The impulse force is actually the force of impact multiplied by the time of
impact.
Critical Thinking Questions
9. Some men were trying to push a car that had run out of gas. They applied a total force of 400
N for 220 seconds.
a) Calculate the impulse force. (The units should be N-s.)
88000 Ns
b) If the car’s velocity changed from 0 m/s to 1.75 m/s, calculate the mass of the car.
50,290 kg
10. A plate with a mass of 0.375 kg is dropped and has a speed of 3 m/s when it suddenly hits the
hard kitchen floor. Calculate the change in momentum. (Note: the final speed of the plate is,
of course, zero when it hits the floor.)
1.125 kg×m/s
11. Consider the plate from the previous question. The floor provided an impact force that acted
on the plate. It acted on the plate very suddenly—it was a mere fraction of a second. If the
force acted on the plate for 0.00024 s, calculate the force of impact.
4687.5 N
12. The change in momentum for the plate in this situation is fixed—1.125 kg m/s (your answer
to question 10). The force of impact is very large, thereby breaking the plate. If the time
were larger than 0.00024 s, would the force of impact be larger or smaller than your answer
to question 11.
If the time were larger, the force would have to be smaller.
13. Increasing the time of impact will decrease the force of impact.
14. Consider again the plate in questions 10 and 11. If the plate hits a pillow instead of the floor
it might not break. This question will examine the physics that explains why the plate may
not break.
a) If the plate falls onto a pillow instead of the floor will the time of impact be greater or
smaller? Explain.
The time of impact will be greater because the pillow
causes the plate to slow down over a distance equal to the
thickness of the pillow.
b) Hopefully for part a, you answered that the time would be greater. It will be greater
because the pillow causes a gradual slowing of the plate instead of a sudden slowing as it
hits the floor. Go on to part c.
c) The time of impact when the plate hits the pillow is 0.0085 s. Now calculate the force of
impact. (The change in momentum is the same as it was in question 10. Only the time
has changed; the time is no longer 0.00024 s as it was in question 11.)
132 N
15. Calculate the impulse force with the pillow and then without the pillow for the plate in
questions 10-14.
The impulse force is the same in both situations ≈ 1.125 N
16. Explain the physics of why an air bag works. Your explanation should include the terms
“force of impact”, “time of impact”, and “change in momentum”.
With and without an airbag, the change in momentum will be the
same. The main difference is with the time of impact. An airbag
increases the time of impact just like the pillow did with the plate. If
the time of impact is greater, the driver will feel a decreased force of
impact which should injure the driver less severely.
Skill Practice 16
Name: ______________________________
17. Some men were trying to push a car that had run out of gas. They applied a total force of 570
N for 2 minutes (careful of units).
c) Calculate the impulse force. (The units should be N-s.)
68,400 N-s
d) If the car’s velocity changed from 0 m/s to 1.20 m/s, calculate the mass of the car.
57,000 kg
18. Calculate the momentum of a 4,200 kg car traveling at a speed of 27 m/s.
113,400 kg-m/s
19. If the car in the previous question slowed to a speed of 12 m/s, what is the change in
momentum?
Change in momentum = 63,000 kg-m/s
4. A glass with a mass of 0.472 kg is dropped and has a speed of 2.7 m/s when it suddenly hits
the hard dining room floor.
a) Calculate the impulse force
1.27 N-s
b) If the time of impact is 0.021 seconds, what was the force of impact?
60.7 N