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Chapter 3 Reading Guide:
Newton’s Second Law of Motion – Force and Acceleration
Complete the Explore! Activity (p.37)
1. Compare the rate at which the book and paper fell when they were side-by-side: Paper
fell more slowly.
2. Compare the rate at which the book and paper fell with the paper on top of the book:
They fell at the same rate.
3. What effect does air resistance have on a falling object? Air resistance slows a falling
object.
3.1)
Galileo
Developed the Concept of
Acceleration
(p 38)
1. Explain what this diagram shows about acceleration (see right):
The ball gains a constant amount of velocity every second
as it rolls downhill.
2. Write the formula for acceleration
Change in velocity
Acceleration =
Time interval
a =
List three ways that velocity can change:
3.
v
t
1- Speed can
increase
2- Speed can decrease
3- change direction
4. What does the Greek letter delta () mean:
Change in.
5. Explain what it means to have an acceleration of 5 km/hr/s:
It means every second your speed increases by another 5 km/hr.
6. Why are there two units of time in an acceleration unit?
Acceleration is a change in velocity over time. Velocity is
distance over time.
7. Define Free Fall:
When air resistance is not a factor, an object is in free fall
8. What is very significant about the number 10 m/s/s (same as 10 m/s 2)?
That is the acceleration due to gravity on Earth. It is the acceleration
of any free-falling object in the Earth’s gravity. The exact number is
9/8 m/s/s
9. See the picture at right. Sketch in the missing speedometer needle at t=3 s,
4 s, and 5 s. Then explain what the picture shows.
The picture shows that the falling rock gains 10 m/s of velocity each second.
10. Describe the acceleration experienced by a ball thrown upwards from the moment it
leaves the throwers hand to the moment it impacts the ground. (A sketch can help):
As the ball is moving up it is slowing down at a rate of 10 m/s/s. Eventually this will slow the
ball to a stop and then the ball will fall back to earth gaining 10 m/s of velocity every second.
3.2)
Force
Causes Acceleration (p 40)
1. Acceleration is caused by
applying
force
. For example: a push or a pull
2. Draw a diagram to explain the Net Force on a box if you push on it with a 25 N force and
someone else pushes on it in the opposite direction with a 15 N force.
3. What does it mean to say that the acceleration of an object is proportional to the net
force acting on the object?
If mass remains constant and you double the force you will double the acceleration. If you
triple the force, acceleration triples. Etc.
4. What does the symbol () mean in the expression Acceleration  net force?
The symbol “” means “is proportional to.”
If acceleration is proportional to net force (a  Fnet) then if one of the
variables is changed the other variable changes by the same magnitude.
For example, if force on an object is doubled then the acceleration will
also double. Similarly, it would take triple the force to get a threefold
increase in acceleration.
5. Explain what the diagram at right is showing:
If force is doubled, acceleration increases (doubled). Notice two
hands are pushing instead of just one)
If mass is doubled, acceleration decreases (by half).
6. Complete the Concept Check questions on p 40. (attach your own paper)
3.3) MASS is a Measure of INERTIA
1. A more
massive can has more inertia than an
more resistant to a change in motion .
empty
can. In other words it is
2. Why do tractor trailer trucks require powerful engines?
Because they have a lot of mass.
3. Explain the difference between mass and volume.
Mass is a measure of how much matter an object contains. Its units are kg. Volume is a
measure of how much space the object occupies. Its units are ml, cc, etc.
4. What is the main difference between mass and weight?
Mass is unaffected by gravity and weight depends on gravity.
5. How can inertia help you sense an object’s mass?
The more mass an object has the more it will resist a change in its motion. In other words, the
more INERTIA it has.
6. Define:
Mass: The amount of matter an object contains. It is unaffected by gravity.
Weight: The force of gravity on an object.
7. What does it mean to say that Mass  Weight?
It means that mass and weight are proportional to each other. As one changes the other one
changes in proportion … meaning … if we double the mass, the weight also doubles, etc…
8. Complete the Concept Check questions on p 42. (on your own attached paper).
9. The standard unit for mass is: kilograms (kg)
10. On Earth; 1 kg =
10
N,
1 kg =
2.2
pounds.
11. What happens to the weight of a 1 kg mass as it is moved away from the earth where gravity is less?
The weight decreases. The mass remains constant.
3.4) Mass Resists Acceleration (p 43)
12. Explain what this picture shows about the
relationship between mass and acceleration?
If the force remains constant, then adding mass will decrease acceleration. Thus, one brick
has the greatest acceleration. Three bricks has the least acceleration.
13. Now explain what it means to say, “Acceleration is inversely proportional to mass.”
It means that if force is constant, as mass is increased, acceleration decreases.
14. More mass means
Less
acceleration because more mass means more
resistance to changes in motion
(also known as INERTIA.)
15. Complete the Concept Check questions on p 44. (on your own attached paper).
3.5) Newton’s Second Law Links Force, Acceleration and Mass (p 44)
16. Write Newton’s Second Law (in words) in the box below:
Newton’s Second Law
The acceleration produced by a net force on an object is directly
proportional to the net force, is in the same direction as the net force, and
is inversely proportional to the mass of the object.
FORCE = MASS X ACCELERATION
17. Write the second law as an equation in the box below:
Newton’s Second Law
Acceleration =
Net Force
m/s/s
Acceleration units must be: __________
Mass
Newtons (N)
Force units must be: __________
Kg
Mass units must be: __________
18. A. If the engine in a 4000 kg truck exerts a 6,000 N force, what is the acceleration of the truck?
(show all work)
b. If we double the engine force to 12,000 N, what will be the acceleration of the truck? ______
c.
If we load the truck so that it now has a mass of 8000 kg, what will be the acceleration of the
truck (with the original 6000 N force)? ______
19. If the net force on an object is doubled, its acceleration will
of an object is doubled, the acceleration will be
halved
double
If the mass
.
20. What will happen to the acceleration if we double both the mass and the net force acting
on an object? Explain.
Acceleration will be unchanged because although the mass is doubled, which will cut the
acceleration in half, the fore is also doubled which will double the acceleration.
21. Complete the Concept Check questions on p 45. (on your own attached paper).
22. Calculate the acceleration on each of the objects
shown here. Show your work completely:
A.
B.
C.
D.
3.6) Friction is a Force that Affects Motion (p 45)
23. Friction always acts in a direction
.
against, or opposite, the forward motion
24. Study the Practicing Physical Science on page 46. Then complete the worksheet on the
last page in this packet.
3.7) Objects in Free Fall have Equal Acceleration (p 47)
25. Free Fall means falling under the influence of gravity (and no other forces such as air
ressitance).
26. What did Galileo demonstrate at the Leaning Tower of Pisa?
That objects in free fall will fall with the same acceleration regardless of their mass.
3.8) Newton’s Second Law Explains Why Objects in free Fall have Equal Acceleration
27. Show in a labeled sketch and with equations how Newton’s Second Law explains why the
acceleration of all falling objects is the same (on earth).
Weight is the force due to gravity on an object. The ratio of weight to mass is the same for
any object. Although a larger mass has more weight and thus more force pulling it downward, it
also has more mass.
3.9) Acceleration of Fall is Less When Air Drag Acts
28. Use the terms terminal speed and frontal surface to
explain how a flying squirrel manages to land safely
jumping from a high tree.
Air drag (or air resistance)depends on two things; speed and surface area.
The flying squirrel increases its frontal surface by spreading its limbs. This increases the
upward force of air resistance and thus reduces the terminal velocity.