Download Ch. 12 Review Period: Name: ANSWER KEY Physical Science Date

Ch. 12 Review
Physical Science
Period:
Name:
Date:
ANSWER KEY
Remember as you answer these ASK YOURSELF WHY IT WORKS AND WHAT THAT MEANS.
1. A force is a
push or pull on one object by another object.
2. The SI unit of force is the
Newton (N)
3. What are the base metric units of the unit from #2?
kg·m/s2
4. When an unbalanced force acts on an object (when there is a net force), what happens to the
object? it will accelerate
5. When an object has no net force acting on it, what happens to the object?
it doesn’t accelerate
6. What kind of friction occurs: as a fish swims through water? fluid ; as a book is slid across a
tabletop? sliding (kinetic); and as a cabinet is pushed but doesn’t move? starting (static)
7. Which is bigger, starting friction or sliding friction?
starting (static)
8. How does surface area affect sliding friction? surface area has essentially no affect on friction
9. As you push a cereal box at constant velocity across a tabletop, the sliding friction acting on the
cereal box is in the
opposite direction of the motion and is equal to
the
magnitude of the pushing force.
10. The property of matter that resists changes in motion is called
inertia
11. Two parachutists, Bob and Murray, jump out of an airplane at the same time. Murray has a mass
of 80kg, while Bob has a mass of 40kg, who will hit the ground first? Murray
12. The two aspects that affect terminal velocity are surface area and
mass (or weight)
13. What happens when an object reaches terminal velocity while in falling through an atmosphere in
terms of forces and accelerations: in terminal velocity, the upwards force of air resistance matches
the downwards force of the weight, so the forces are balanced and the object doesn’t accelerate.
14. If a 1kg object has a terminal velocity of 10m/s, what would the terminal velocity of a 4kg object
if it was the same size and shape? Show your work!
Mass is proportional to (term. vel)2, v1 = 10m/s so v12 = 100m2/s2. The mass was quadrupled, so
the terminal velocity squared must be quadrupled, so v22 = 400m2/s2, so v2 = √
15. A 20kg object has a terminal velocity of 50m/s, if another object of the same size and shape has a
terminal velocity of 100m/s, what is the new object’s mass? Show your work!
Mass is proportional to (term. vel)2, v1 = 50m/s so v12 = 2500m2/s2. v2 = 100m/s so v22 =
10,000m2/s2, so the term. vel squared was increased by 4 times, so the mass must also have been
increased 4 time, so the new mass must be 20kg⋅
8 kg
16. A 1000 kg car accelerates at 2 m/s2, what is the net force on the car? 2,000N
17. According to Newton’s second law of motion, the acceleration of an object equals the net force
acting on the object divided by the object’s
mass
18. The formula for finding your weight is: mass*acceleration due to gravity
19. Newton’s third law of motion describes what forces? action and reaction forces
20. What type of forces have reaction forces?
ALL forces have a reaction force!!!
21. What really dictates who will win a game of tug-of-war (don’t say whoever pulls harder)? Since
both people pull on the rope with the same force (Newton’s 3rd Law), it’s not who pulls harder.
It’s how hard the team can push off the ground and thus have the ground push back on them, in
essence, it’s who has more friction.
22. The product of an object’s mass and velocity is its
momentum
and is defined as
inertia in motion
23. What is the momentum of a 50-kg ice skater gliding across the ice at a speed of 2 m/s? 100kg·m/s
24. When you jump in the air, how does the force the earth pulls on you compare to the force you pull
on the earth?
exactly the same magnitude, opposite direction.
25. Two of the same type of cannons fire two cannonballs of the same size at the same launch
velocity. One cannonball is solid steel, while the other cannonball is also steel but hollow in the
middle. Which will have the greater momentum?
solid steel (has more mass)
26. How do the direction of net force compare to the direction of acceleration? They point in the
same direction
27. What happens when the net force points in the same direction as the object’s velocity?
The
object’s speed increases
28. What happens when the net force points in the opposite direction as the object’s velocity? The
object’s speed decreases
29. What happens when the net force points 90˚ to the object’s velocity? The object’s direction
changes but the speed stays the same.
30. What are the 4 universal forces: electromagnetic, strong nuclear, weak nuclear, and gravitational
forces
31. Which of the universal forces is the weakest? gravitational (it takes huge masses to see the effect)
32. Which of the universal forces is the strongest? Strong nuclear force
33. Which of the universal forces is most effective over long distances? Gravitational (acts across
universe)
34. Which of the universal forces acts over the shortest distance? Weak nuclear force (only 10-18m)
35. What universal forces can both attract and repel?
electromagnetic
36. What evidence do we have that nuclear forces are stronger than electromagnetic forces at very
small distances? The electric repulsion of protons in the nucleus in an atom must be smaller than
the force drawing them together, or else the nucleus could fly apart.
37. Draw and label (BOTH value and name) the free-body diagrams for each of the following
scenarios:
a. A 5N block on a flat surface being pushed
b. A 500N person standing motionless on a flat
with 10N of force to the left at constant
surface pushing with a 100N force to the
velocity (hint, what does constant velocity
right against a wall.
mean).
You push on the wall
Ffloor
Since you’re moving
to the right, so the
500N
Ffloor
at constant velocity
wall pushes you to
5N
there must be 0N net
Fsurface
FWall
Fperson
the left, since you’re
Fsurface
force, so if you push
100N
100N
10N
not moving you
10N
the block with 10N,
FEarth
know there must be
FEarth
friction must hold it
500N
no net force, thus,
5N
back with 10N
friction from the
floor prevents you
from moving away
from the wall.
38. Complete the following force diagrams (arrows not to scale):
a. A 20N object is being pushed and is sliding
b. A 10N object is being pushed and is
to the right against 40N of friction, with a
increasing speed to the right, there is a 20N
net force of 30N to the left.
net force, and 20N of friction.
Fground: 20
Fsurface 40N
N
Fperson: 10N (40 – 30)
FEarth: 20
N
Is the object increasing speed, decreasing
speed, changing direction or a
combination?
Net force and velocity are pointing in opposite
directions, so the object is decreasing speed.
Fground: 10
Fsurface 20N
N
Fperson 40N (40 – 20
= 20 N net force)
FEarth: 10
N
Is the object increasing speed, decreasing
speed, changing direction or a
combination?
Since net force and velocity are pointing in the
same direction, the speed is increasing.
39. A person argues that when you jump in the air, you cannot be pulling the earth with the same force
as it pulls you because the earth is not drawn towards you. What is wrong with this argument?
Make sure you use a = Fnet/m in your answer.
They are assuming that different massed object behave the same with the same force applied.
However, because the mass of the Earth is SOOOOO much larger than the mass of a person, the
same size force divided by a MUUUUCH large mass makes a MUUUUCH smaller acceleration.
The acceleration of the Earth is simply too small to detect.
40. A 5kg gun, suspended in the air by strings, fires a 0.01kg bullet (initially at rest). The bullet
leaves the barrel with a velocity of 200m/s after 0.001s. Assume constant acceleration while in
the barrel:
a. How much net force was applied to the bullet? (Hint, find acceleration first) Show your work!
a = Δv/Δt = (200m/s-0m/s)/0.001s = 200,000m/s/s. Fnet = m·a = 0.01kg·200,000m/s2 = 2,000N
b. How much acceleration would the gun recoil with? (hint: use 3rd law first then the 2nd Law).
Show your work!
Fnet on gun will be the same as on the bullet. So Fnet = 2,000N, so a = Fnet/m = 2,000N/5kg =
400m/s2
41. A 15kg cannonball is fired perfectly horizontally from 500kg cannon on wheels (assume no
friction). The cannonball leaves the cannon with a speed of 300m/s.
a. What is the momentum of the cannonball after firing, and what must the momentum of the
cannon be after firing the cannonball?
pcb = mcb·vcb = 15kg·300m/s = 4,500 kg·m/s. Since the initial momentum was 0kg·m/s
(515kg·0m/s), the final momentum must also be 0kg·m/s, so the momentum of the cannon must
be -4,500kg·m/s to cancel out the 4,500kg·m/s of the cannon.
b. What will the cannon’s recoil velocity be after firing the cannonball?
Since the momentum of the cannon was -4,500kg·m/s and is also mc·vc, so 500kg·vc = 4,500kg·m/s so vc = -9m/s (9m/s backwards)
42. 70kg Gertrude and her 30kg granddaughter Sally are ice skating. Gertrude is initially going 5m/s
to the right while Sally is going 2m/s to the left. Gertrude runs into and picks up Sally. What will
their combined velocity be the instant after the collision?
pi_tot = pf_tot; pi_tot = pi_g + pi_s = 70kg·5m/s + 30kg·(-2m/s) = 350kg·m/s – 60kg·m/s = 290kg·m/s
pf_tot = 290kg·m/s = mtot·vf = 100kg·vf  vf = 290kg·m/s / 100kg = 2.9kg·m/s
43. A 20kg object is on a flat surface pushed to the right with 100N, there is 20N of sliding friction.
Draw the force diagram for this situation, then find the net force on the block, and then the block’s
acceleration.
W = m⋅g
kg⋅1 m/s2 = 200N
If there’s
N going down there must be
N going up
Fground: 200N
because it doesn’t accelerate vertically
Fsurface: 20N
Fperson: 100N
Fnet = 100N – 20N = 80N
a = Fnet/m = 80N/20kg = 4m/s2
FEarth: 200N
44. A 40kg object is being pushed such that it accelerates at 2m/s2 on a flat surface with 20N of
friction. Complete the force diagram for this situation. (Note that arrows may not be to scale)
Fground: 400
Fsurface: 20N
N
Fperson: 100
FEarth: 400
N
N
W = m⋅g 4 kg⋅1 m/s2 = 400N
If there’s
N going down there must be
N going up
because it doesn’t accelerate vertically
Fnet = m⋅a
kg⋅ m/s2 = 80N
Since Fnet = Fperson – Fsurface we have 80N = Fperson – 20N
So, Fperson = 100N
45. A large German Shepard pulls on the leash of a small Chihuahua, and the Chihuahua strains to get
away. Which dog pulls harder on the leash? How do you know?
Newton’s 3rd law says that the force the Chihuahua puts on the German Shepard must be
equal and opposite to the force the German Shepard puts on the Chihuahua.