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Section 12.1 Forces
1. A force is defined as a(n) push or a(n) pull that acts on an object.
2. True or false? A force can act to cause an object at rest to move or it can accelerate an object that is already
moving.True
3. How can a force change the motion of an object that is already moving?
Add force in same direction as motion: speed up motion.
Add force in opposite direction of motion: slow down or stop motion.
4. Circle the letter of the best answer. What force causes a 1-kg mass to accelerate at a rate of 1 meter per second
each second?
a. 1 kg/m • s2
b. 1 kg/s
c. 1 kg • m
d. 1 newton
5. The overall force acting on an object after all the forces are combined is the net force.
6. How do balanced and unbalanced forces affect the motion of an object?
Balanced: zero net force so no change in motion
Unbalanced: nonzero net force which results in a change in motion/direction
7. True or false? Friction is a force that helps objects that are touching move past each other more easily.
False, motion is opposite to object motion
8. Circle the letters that identify types of friction.
a. rolling
b. gravity
c. static
d. sliding
9. The friction force that acts on objects that are at rest is static.
10. Why is less force needed to keep an object moving than to start the object in motion?
There needs to be enough force to overcome static friction which is more than sliding friction.
11. Complete the table below about friction forces.
Types of Friction Forces
Friction Force
Example
Static
A couch on a rug
Sliding
Pushing a book along your desk
Rolling
A ball rolling downhill
12. True or false? Fluid friction is a force that opposes the motion of an object through a fluid such as water.
True
13. Gravity is a(n) attractive force that pulls objects together.
14. True or false? Earth’s gravity acts downward toward the center of Earth. True
15. Describe how gravity and air resistance affect the motion of a falling object. Gravity: downward force on
an object toward the center of Earth. Air resistance: upward force opposite to gravity on the object.
16. True or false? Terminal velocity is the constant velocity of a falling object when the force of air resistance
equals the force of gravity. True
17. The curved path caused by the combination of an initial forward velocity and the downward force of gravity
is known as projectile motion.
18. In the Venn diagram, write the phrases listed below to describe unbalanced forces and balanced forces.
change an object’s motion
push or pull
net force = 0 N
do not change an object’s motion
have direction
net force does not equal 0 N
Balanced Forces
Unbalanced Forces
change in an object’s motion
net force does not equal 0 N
have direction
push or pull
do not change object’s motion
net force = 0 N
Match each term with its definition by writing the letter of the correct definition in the right column on the line
beside the term in the left column
19. a Newton
a.
the SI unit for force
20. c force
b.
sum of all forces acting on an object
21. e balanced forces
c.
push or pull
22. d unbalanced forces
d.
can change an object’s motion
23. b net force
e.
will not change an object’s motion
In the next 4 questions, if the statement is false, change the underlined word(s) to make the statement true.
24. True Balanced forces do not change the motion of an object.
25. False – all Forces cause some changes in motion.
26. False – a A net force causes no change in an object’s motion.
27. True If Manuel exerts a force of 10 N to push a desk to the right at the same time Lynn exerts a force of
15N to push the desk to the left, the desk will move to the left.
28. When you pull on a window shade, you exert a(n) force.
29. A(n) arrow can be used to represent the direction and strength of a force.
30. The strength of a force is measured in newtons.
31. The net force determines how and if an object will change motion.
32. When two forces act in opposite directions, the object will accelerate in the same direction as the greater force.
33. A force is described by its strength and by the direction in which it acts.
Net Force, Mass, and Change in Motion
Unbalanced forces cause a chance in an object’s motion. The net force
acting on the object causes it to speed up, slow down, or change direction.
Changes in motion, that is, speeding up, slowing down, or changing
direction, are called acceleration. When an object of a certain mass is acted
upon by a net force, the amount of change in the object’s motion (its
acceleration) is proportional to the size of the net force.
When two values are proportional, an increase in one causes the other value
to increase or decrease. These relationships between proportional values are
often in the natural world. For this reason, they have specific names.
Directly proportional: When the net force increases, the change in motion
for an object of certain mass increases. Any two values that increase or
decrease in the same way are directly proportional. Figure 1 shows a graph
of this relationship.
Inversely proportional: Whenever an increase in one value results in
a decrease in another value (and vice versa), the two values are inversely
proportional. Figure 2 shows an inverse relationship for the amount of
change in motion and mass.
Look at the table below that shows how net force, mass, and the change in
motion are related.
If acceleration is:
and mass is:
force must be:
1 m/s2
0.1kg
0.1N
1 m/s2
0.2kg
0.2N
1 m/s2
0.5kg
0.5N
1 m/s2
0.7kg
0.7N
1 m/s2
1.0kg
1.0N
34. On graph paper, plot each pair of values for mass and force from the table. Let the horizontal axis represent
mass, and the vertical axis represent force. Connect the points with lines. Straight line, positive slope
35. When acceleration is held constant and objects of different mass are observed, are mass and force directly
proportional or inversely proportional? Explain.
Directly. The slope of the line on a force vs. mass graph is a linear sloping upward line.
36. If you pull on one end of the skateboard with a force of 10 N, while your partner pulls on the other end with a
force of 10 N, what do you think will happen to the skateboard?
It will not move
37. If you pull on one end of the skateboard with a force of 3 N, while your partner pulls on the other end with a force
of 6 N, what do you think will happen to the skateboard?
The skateboard will move toward your friend with 3 N force
38. What can you say about an object at rest pulled in opposite directions by equal forces?
The object will remain at rest
39. What can you say about an object at rest pulled in opposite directions by unequal forces?
The object will move in the direction of the greater force
40. What are the two factors that affect the frictional force between two surfaces?
Type of surface and how hard the surfaces are pushed together
41. What two factors affect the gravitational force between two objects?
Mass: more mass means more gravitational force
Distance between objects
42.How does mass differ from weight?
Mass: amount of substance an object has and is not affected by gravity
Weight: measure of the force of gravity on an object and is affected by gravity
Match each term with its definition by writing the letter of the correct definition in the right column on the line
beside the term in the left column.
43. c friction
a.
the force that pulls objects toward each other
44. e rolling friction b.
the type of friction that exists between oil and a door hinge
45. d sliding friction c.
the force that one surface exerts on another when two surfaces
rub against each other
46. b fluid friction
d.
the type of friction that occurs when you rub sandpaper against wood
47. g static friction
e.
the type of friction that occurs when a wheel turns on a surface
48. f weight
f.
a measure of the force of gravity on an object
49. a gravity
g.
the type of friction that occurs between objects that aren’t moving
The Great Pyramids
The ancient Egyptians built pyramids out of large blocks of limestone. No one knows for sure how they managed to
move the blocks across land.
One idea about how the Egyptians moved the stone blocks without modern machines is a simple one. Set a heavy book
on a table. If you try pushing it with your little finger, it will be hard to do. Next, place five round pencils, parallel to
each other, under the book and try again. This time you can move the book easily. You have replaced sliding friction
with rolling friction. The force needed to overcome rolling friction is much less than the force needed to overcome
sliding friction. Some people believe that the ancient Egyptians used this understanding of friction and moved the
heavy stone blocks by placing a layer of wooden logs under the blocks. As the stone was pulled forward, the logs in
back were picked up and placed in front of the block again, to provide a kind of friction-reducing track along which to
roll the blocks of stone.
50. List two reasons why the limestone blocks of the pyramids were so difficult to push across land.
The blocks are large and the blocks are heavy
51. How might the Egyptians have been able to move the heavy stone blocks?
They placed round logs under the blocks and replaced them as the blocks moved forward
52. Can you think of another way the Egyptians might have tried to reduce friction to move the heavy blocks?
They replaced static friction/sliding friction with rolling friction (easier to overcome)
53. Historians know that large stone blocks can be dragged if logs are placed under them. Is the idea that the
Egyptians built the pyramids by rolling stone blocks on logs a fact or a hypothesis?
Hypothesis: the reading lists this as an “idea” instead of a statement of fact or refer to a written record
54. The Law of Universal Gravitation states that the force of gravity acts between all objects in the universe that
have mass.
55. As distance increases, gravitational force decreases.
56. When you stand on a bathroom scale, it displays the force of gravity that Earth is exerting on you.
57. Friction acts in a direction opposite to the direction of the object’s motion.
58. When the irregularities of one surface come into contact with those of another surface, friction occurs.
59. The applied force required to push something across a surface increases as friction
60. ___ When you swim in a pool,
A sliding friction occurs.
B static friction occurs.
C rolling friction occurs.
D fluid friction occurs.
61.
A
B
C
D
When you rub your palms together,
sliding friction occurs.
static friction occurs.
rolling friction occurs
fluid friction occurs.
A
B
C
D
When you skateboard on a ramp,
sliding friction occurs.
static friction occurs.
rolling friction occurs.
fluid friction occurs.
A
B
C
D
When you push a desk that doesn’t move,
sliding friction occurs.
static friction occurs.
rolling friction occurs.
fluid friction occurs.
62.
63.