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Name: ________________________ Class: ___________________ Date: __________
ID: A
Forces practice
1. What is necessary for an object to accelerate?
2. If two equal forces act on an object in opposite directions, what is the net force? What is the acceleration?
3. Can an object be in motion if the net force acting on it is zero? Explain.
4. A bowling ball loses momentum of 0.5 kg × m/s when it hits some pins. How much momentum did the pins
gain?
5. Explain how mass and weight are different from each other.
6. Why is your weight less on the Moon than on Earth, but your mass is the same?
7. What is weight?
8. The space shuttle moves into a higher orbit farther from Earth. Explain what happens to the gravitational force
on the shuttle.
9. What causes friction?
10. You push on a crate with a force of 10 N to the right, and your friend pushes on the crate with a force of 25 N
to the left. Describe and explain the motion of the crate.
11. Forces of 10 N down, 10 N to the right, and 5 N to the left are acting on a ball. It accelerates horizontally to the
right. What other force, if any, is acting on the ball? Explain.
12. Explain the motions of a hockey puck during a hockey game in terms of Newton's first law.
13. You push a heavy crate. At first it doesn't move. You push harder, and it finally starts to move, but you still
have to exert some force to keep it moving at a constant velocity. Explain.
14. A book and a piece of paper the same size fall at different rates. How can you change the shape of the paper so
that it will fall nearly as fast as the book?
15. Two objects that have the same weight are dropped from a tall building. One object is larger and flatter than the
other. Explain why the larger object hits the ground last.
16. Calculate the force on an object that has a mass of 12 kg and an acceleration of 4 m/s2.
17. A car of mass 1330 kg is traveling at 28.0 m/s. The driver applies the brakes to bring the car to rest in 5.6 s.
Calculate the force of the brakes on the car.
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Name: ________________________
ID: A
18. Two men pull a 31-kg box with forces 9.7 N and 7.6 N in the directions shown below. Find the acceleration
of the box and the direction in which the box moves.
9.7 N
7.6 N
19. Two horizontal forces, 315 N and 145 N are applied to a sled resting on a frictionless skating rink. If they are
applied in the same direction, what is the net horizontal force on the sled?
20. Two horizontal forces, 145 N and 315 N are applied to a sled resting on a frictionless skating rink. If they are
applied in opposite directions, what is the net horizontal force on the sled?
21. An Eskimo pushes a loaded sled with a mass of 300 kg over the frictionless surface of hard-packed snow. He
exerts a constant 170 N force as he does so. What is the sled’s acceleration?
22. A mouse pushes a piece of cheese with a mass of 6.4 g over the frictionless surface of an air hockey table. He
exerts a constant 0.5 N force as he does so. What is the cheese’s acceleration?
23. A moose pushes a barrel with a mass of 6.8 kg over the frictionless surface of a frozen pond. He exerts a
constant 50 N force as he does so. What is the barrel’s acceleration?
24. Calculate the force on an object that has a mass of 12 kg and an acceleration of 4 m/s2.
25. Calculate the weight of a person with a mass of 67 kg that is holding a 12 kg box on Earth.
26. What is the mass of a cannon ball that is shot through the air with an acceleration of 6 m/s2 and a net force of
1,000 N?
27. You are in space on a planet that you have just discovered. You weigh yourself and discover that you weigh 60
N. What is the gravitational strength if you have a mass of 65 kg?
28. If you have a mass of 80 kg and jump out of a plane, what must the force of the air resistance be in order for
you to travel at terminal velocity?
29. Two students push a box across the ground while another student pushes on the box in the opposite direction.
What is the force of friction? Each student pushes with a force of 50 N.
30. Suppose Sally wants to push a couch across the room. Explain how static friction and sliding friction are
involved in this process.
31. When riding in an elevator, why does a person appear to lose weight when accelerating downward?
32. A pitcher releases a fastball that moves toward home plate. Other than the force exerted by the pitcher, what are
two forces that act on the ball as it travels between the pitcher and home plate? How does each of these
forces change the ball’s motion? Classify the forces acting on the ball as balanced or unbalanced.
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ID: A
Forces practice
Answer Section
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an unbalanced force is acting on it
0, 0
Yes, the object could be moving at a constant velocity (not changing its motion).
0.5 kg × m/s
Mass measures the amount of matter; weight measures the force of gravity on the matter.
Your mass is the same because you didn't change the amount of matter, but the gravitational force decreased,
decreasing your weight.
the gravitational force exerted on an object
It decreases because the shuttle is farther from Earth.
microwelds that occur between bumps on two surfaces
The crate accelerates to the left because there is a net force of 15 N acting to the left.
There must be a force of 10 N acting upward to keep the ball from falling down.
A net force is applied to the puck by a player, the puck moves off at a constant velocity until another net force
is applied by another player to change its motion.
At first you had to exert a large force to overcome static friction. After the crate started to move, you still had to
exert a smaller force to overcome sliding friction.
Crumple the paper into a tight ball to reduce air resistance.
The flatter object has a greater air resistance so it will hit the ground last.
48 N
F = ma = 12 kg × 4 m/s2 = 48 kg × m/s2 = 48 N
6650 N
0.068 m/s2 to the left
315 N + 145 N = 460 N
315 N – 145 N = 170 N in the direction of the larger force.
a = F/m
a = 170 N/300 kg
a = 0.57 m/s2
a = F/m
a = 0.5 N/0.0064 kg
a = 78.125 m/s2
a = F/m
a = 50 N/6.8 kg
a = 7.35 m/s2
48 N
[F = ma = 12 kg × 4 m/s2 = 48 N]
N=kg x N/kg= (67 kg+ 12 kg) x 9.8 N/kg= 774.2 N
F= ma; m= F/a= 1,000 N/ 6 m/s2=166.66 kg
F= mg; g= F/m= 60 N/ 65 kg= .92 N/kg
Fg= mg= 80 kg x 9.8 N/kg= 784 N
50+50-50=50 N
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ID: A
30. Before Sally is able to push the couch across the room, she must overcome static friction which is the
frictional force that prevents two surfaces from sliding past each other. In this case, the two surfaces are the
bottom of the couch and the floor. Once she exerts enough force on the couch, Sally is able to push the
couch across the floor, but as soon as she stops pushing the couch stops moving. This is sliding friction or
the force that opposes the motion of two surfaces sliding past each other. In order to oppose sliding friction,
Sally must continually apply enough force to overcome the sliding friction.
Score Description
Student defines static and sliding friction and clearly explains how they act to
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prevent the movement and oppose the movement of the couch across the room.
Student explains how static and sliding friction act to prevent the movement and
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oppose the movement of the couch across the room.
Student correctly explains the actions of static friction, but incorrectly explains the
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actions of sliding friction.
OR
Student correctly explains the actions of sliding friction, but incorrectly explains the
actions of static friction.
Student only explains sliding or static friction involved in pushing the couch across
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the room.
OR
Student reverses the explanation of static and sliding friction in pushing the couch
across the room.
OR
Student defines static and sliding friction, but does not apply them to pushing a
couch across the room.
Student’s response is totally incorrect or irrelevant.
0
Blank No student response.
31. A person’s apparent weight is equal to the upward force of the scale on him or her. This force decreases when
the elevator accelerates downward.
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ID: A
32. Two forces that act on the ball as it moves through the air are gravity and friction. Gravity causes the ball to
move downward toward Earth. Friction causes the ball’s motion to get slower the farther it moves through
the air. Because the ball’s velocity is changing as it moves, it can be determined that the forces acting on the
ball are unbalanced.
Score
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3
2
1
0
Blank
Description
Student’s response includes the following four elements:
identification of friction and gravity as forces acting on the ball;
explanation that gravity causes the ball to move toward Earth;
explanation that friction slows the ball’s motion;
classification of the forces acting on the ball as unbalanced
Student’s response includes three of the following four elements:
identification of friction and gravity as forces acting on the ball;
explanation that gravity causes the ball to move toward Earth;
explanation that friction slows the ball’s motion;
classification of the forces acting on the ball as unbalanced
Student’s response includes two of the following four elements:
identification of friction and gravity as forces acting on the ball;
explanation that gravity causes the ball to move toward Earth;
explanation that friction slows the ball’s motion;
classification of the forces acting on the ball as unbalanced
Student’s response includes one of the following four elements:
identification of friction and gravity as forces acting on the ball;
explanation that gravity causes the ball to move toward Earth;
explanation that friction slows the ball’s motion;
classification of the forces acting on the ball as unbalanced
Student’s response it totally incorrect or irrelevant.
No student response.
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