Physical Science
Forces and Motion Study Guide
** YOU MUST ALSO USE THE NOTES PROVIDED IN CLASS TO PREPARE FOR
THE TEST **
1. What is a “force”?
A push or a pull on an object. Forces have size and direction.
2. What is the SI unit for force?
The SI unit of measurement for force is the newton, abbreviated N.
3. What do we mean by the term “net force”?
The net force is the force that results from combining all the forces
acting on an object.
4. How do you find net force if the forces are going in the same direction? Draw a
force diagram.
If forces acting on an object are going in the same direction, add the
measurements of the forces together. The object will move in the
direction of the forces.
35N
25N
The nurse is PUSHING with 35 N of
force. The orderly is PULLING with 25
N of force. Together, they are putting
60 N of force on the bed. The bed
moves in the direction of the forces.
5. How do you find net force if the forces are going in the opposite direction?
Draw a force diagram.
If forces acting on an object are going in the opposite directions,
subtract the smaller value from the larger value. The object will move in
the direction of the larger force.
3N
4N
The baby chicks are fighting over the worm. One
chick on the left is pulling with 3N of force and the
chick on the right is pulling with 4N.
Since the chick on the right is pulling with more
force, he will get the worm. The net force on the
worm is 1 N to the right.
6. What are “balanced forces”? Draw a force diagram.
Balanced forces occur when the net force
on an object is zero. Balanced forces
5N
produce NO change in motion.
5N
5N
5N
5N
7. What are “unbalanced forces”?
Unbalanced forces occur when the net force on
an object is not zero. The object moves,
because one or more of the forces is (are)
greater than the others. Unbalanced forces
produce a change in motion.
8. Which produces motion – balanced forces, or unbalanced forces?
Unbalanced forces produce motion.
9. Two teams are playing tug of war. If Team A is winning, describe what must
be happening, using the terms net force and either “balanced forces” or
“unbalanced forces”. Draw a force diagram.
If one of the teams is winning, there must be unbalanced forces at work,
because otherwise, there would be no movement at all. Team A must be
exerting more force than Team B.
Team A
Team B
For instance, if the girl on Team A
was pulling with 40 N of force, and
the boy with 35N of force, there
would be 75 N of force altogether on
the rope, in that direction. If they are
winning, it means that Team B is
putting less than 75 N of force on the
rope in the opposite direction.
10. List five other directional terms for force (examples: forward, north).
to the right, backward, to the left, upward or downward, south,
northeast, west
11. What is gravity?
A force of attraction between two objects that is there just because
there is matter there.
12. What is the “Law of Universal Gravitation”? Who came up with it?
Sir Issac Newton is said to have determined that gravity is caused by
the masses of objects. The story goes that he was sitting under an apple
tree when an apple fell and conked him on the head. He started
wondering why apples always fell “down” and never “up”.
The Law of Universal Gravitation says that the larger the masses of
the objects, and the closer together they are, the stronger the
gravitational force between them.
13. Which of these sets of objects have the greater gravitational attraction?
a.
and
b.
and
OR
These do! They have more mass!
14. Which of these sets of objects have the greater gravitational attraction?
and
OR
and
These do! They are
closer together!
15. What is weight?
Weight is the measure of the force on an object due to gravity.
16. Are weight and mass the same? Explain.
No. Mass is the amount of matter in an object.
Weight is a measure of how much gravity is pulling
on an object.
In this picture, there are two astronauts and a
flag. The astronauts and the flag are each made
up of a certain amount of matter, and that
doesn’t change just because they are on the moon.
Their weights, however, will be different when they are on
the moon than when they are on earth. Because the moon
is smaller than earth, it doesn’t have as strong of a
gravitational pull. So their weight will be less than it would
be on earth.
17. WRITE the following terms in the correct boxes, to tell if they are true about
weight, or about mass:
18. What is friction?
Friction is a force that opposes (works against) motion between two
surfaces that are touching.
19. What causes friction? Draw a picture of what causes friction.
Friction is caused by the hills and valleys of one surface sticking against
the hills and valleys of the other surface.
20. Label each picture as illustrating sliding friction, rolling friction, fluid friction
and static friction: (Make sure you understand what each means!)
rolling friction
fluid friction
sliding friction
static friction
21. Name three ways to reduce friction.
 Using lubricant (a slippery liquid that goes between the
surfaces, like grease on wheels)
 Making the surfaces smoother
 Use wheels that create rolling friction instead of sliding
friction.
22. Name two ways to increase friction.
All surfaces are rough—there are no
surfaces that are completely smooth.
But, some surfaces are smoother than
others. Ice, for instance, is smoother
than asphalt, so when it snows, there
is less friction on the roads, making it
harder for cars to stop. Putting sand
on icy roads makes the surface less
smooth, and increases friction so that
cars can drive more safely.
You can also increase friction by switching from rolling friction to sliding
friction.
23. What is air resistance?
Air resistance is a form of fluid friction. It opposes motion. It is caused
by the invisible air particles that exist all around us in the atmosphere
pushing against the falling object. The amount of air resistance acting on
an object depends on the size and shape of the object. Air resistance
would affect a flat sheet of paper more than a crumpled one.
24. What characteristics cause some things to be more affected by air resistance than
others?
Objects that are flat, thin, and have a small mass are most affected by
air resistance. Things like leaves and tissue paper are highly affected by
air resistance. You can see this when you watch them fall. They fall less
quickly than other things, because the air particles push up and oppose
the force of gravity.
25. Look at these two parachutes. Which would make the parachute come down
more slowly? Why?
The first parachute will come down more slowly because it has more air
resistance. There are more air particles underneath the wide swath of
material, pushing up against the motion caused by gravity.
26.What is motion?
Motion is when an object changes its position (location) over time.
27.What two things does speed depend upon?
the amount of distance the object covered, and the amount of time it
took for the object to cover that distance
28.What is the formula for average speed? Show two other ways to arrange this
formula, so that you can find distance, or time, by using the formula.
d
S= t
speed = distance divided by the amount of time it
took to cover that distance
distance = speed
times times
time = distance
divided by speed
29. A car travels 300 km in 6 hours. What is its average speed? Show your
work.
d
S= t
S=
The car’s average speed was 50
kilometers per hour.
300 km
6 hr
S = 50 km/hr
30. A bicyclist has an average speed of 10 km per hour. She rode for 4 hours.
How far did she ride? Show your work.
d = 10 X 4
d = 40 km
She rode 40 kilometers over the 4
hours, at 10 km/hr.
31. An airplane travelled an average of 275 km per hour. It went from
Washington, DC to Orlando, Florida -- a distance of about 1300 km. How long
did it take the plane to get to Orlando? Show your work. Round to the nearest
hundredth.
1300 km
t = 275 km/hr
It took the plane 4.72 hours to go
from Washington DC to Orlando
at that speed.
t = 4.7272727 repeating or 4.73 hours rounded to the nearest
hundredth
32. What is “velocity”?
the speed of an object in a particular direction
You can say the cheetah’s speed is 15 m/s. If you want to talk about the
cheetah’s velocity, you’d say “15 m/s north”.
33. If I say “The plane has a velocity of 265 mph”, is the statement correct or not?
Why or why not?
It is not correct. Velocity is not the same as speed. What the
statement above tells us is the speed. In order to be correct, you would
have to tell the direction the plane was moving. In other words, put on
the direction the plane in moving (“the plane has a velocity of 265 mph
west”.
34. Calculate the velocity of a car that travelled 60 miles to the south, in 1.5 hours.
Speed = 60 miles
1.5 hours
Speed = 40 miles/hour
Velocity = 40 miles/hour south
Remember, velocity is speed in a certain direction.
35. If an airplane makes a left turn in the air, while maintaining its speed, has it
changed its velocity? Why or why not?
Yes. Velocity is speed in a particular direction. So, if you change the
speed or the direction, you’ve made a change in velocity.
36. What is acceleration?
Acceleration is a change in velocity – either a speed change, or a
direction change, or both.
37. Is the following statement true or false: “My mother’s car accelerated as it
turned the corner at 30 mph.”? Explain your answer.
This is a true statement. To accelerate, the object needs to change
speed, change direction, or both. In this example, the car changed
direction as it went around the corner, so it did accelerate.
38. Draw a picture of something that is negatively accelerating and explain what is
happening.
It means the object slowed down. Negative acceleration is slowing down.
When an airplane comes in for a landing, it is
slowing down, or having negative acceleration.
39. What is another word for “negative acceleration”?
Negative acceleration is also known as deceleration. It means slowing
down
40.
Albert, because he ran fastest: 100 meters in 12 second = 8.3 m/s
Charlie. You can tell by the flat line between 8 and 13 seconds.
5 seconds
14 seconds; the race was 100 meters in distance, and the graph
shows it took him 14 seconds to go that distance
As stated above, Albert’s average speed was 8.3 m/s.
Bob’s average speed was 7.1 m/s. Charlie’s average speed was 5.9 m/s
41. The graph below shows the TOTAL distance two cars drove over time (a
time/distance graph). One of them had a constant speed and the other is
accelerating. Which one (A or B) shows the car with the constant speed and
which one shows the car accelerating? How can you tell?
Car A is going at a constant speed. You can tell because the line showing
its speed over time is straight. Car B is not going at a constant speed, it
is accelerating. At first it was going slow, then, at about 10 seconds, it
sped up very quickly.