Download 1-2 - Renton School District

1-2
-
ses
m inute
si
G
50-
on
40
to
80
s
Newton’s Laws of Motion
ACTIVITY OVERVIEW
READI
N
Students complete a reading about forces and Newton’s three laws of motion. The
reading is supported by a literacy strategy designed to reveal and address common
misconceptions about force and motion. A second literacy strategy supports comprehension during the reading. Students then discuss aspects of Newton’s laws that seem
to contradict daily experience.
KEY CONCEPTS AND PROCESS SKILLS
(with correlation to NSE 5–8 Content Standards)
1.
An object that is not being subject to a force will continue to move at a constant
speed in a straight line. (PhysSci: 2)
2.
Friction is a force that causes changes in the speed of an object’s motion. (PhysSci: 2)
3.
The magnitude of the change in motion can be calculated using the relationship
F = ma, which is independent of the nature of the force. (NSES Grade 9–12
PhysSci: 4; many state standards)
4.
Whenever one object exerts a force on a second object, the second object exerts
an equal and opposite force on the first. (NSES Grade 9–12 PhysSci: 4; many state
standards)
KEY VOCABULARY
acceleration
balanced forces
inertia
force
friction
net force
unbalanced forces
E-67
Activity 80 • Newton’s Laws of Motion
MATERIALS AND ADVANCE PREPARATION
For the teacher
1
Scoring Guide: UNDERSTANDING CONCEPTS (UC)
For each student
1
Student Sheet 80.1, “Anticipation Guide: Newton’s Laws of Motion”
1
Scoring Guide: UNDERSTANDING CONCEPTS (UC) (optional)
*Not supplied in kit
Masters for Scoring Guides are in Teacher Resources III: Assessment.
TEACHING SUMMARY
Getting Started
1.
(LITERACY) Elicit students’ preconceptions about motion.
Doing the Activity
2.
(LITERACY) Students read about force and motion.
3.
Review key vocabulary from the reading.
Follow-Up
4.
(UC ASSESSMENT, MATHEMATICS) Review the implications of Newton’s Laws.
5.
(LITERACY) Students revisit their ideas about force. if this works)
BACKGROUND INFORMATION
Newton’s Three Laws
First law (Law of Inertia)
Unless acted on by an unbalanced force, an object will maintain a constant velocity.
Constant velocity is defined as a constant speed in an unchanging direction, i.e., a
linear path.
Second law (the relationship of force, mass, and acceleration)
F = ma
Third law (action–reaction)
All forces occur in pairs, and these two forces are equal in magnitude and opposite in
direction.
The forces described in the third law are consistent with the second law in that they
can be determined by F = ma. Forces in the third law are unique, however, in that the
law describes the interaction of forces on two separate objects, which is not specifically
stated in the second law.
E-68
Newton’s Laws of Motion • Activity 80
The pair of forces Newton refers to in the third law are equal in magnitude and opposite in direction. Although the forces are equal, the accelerations are not: the less massive object has a greater acceleration (due to Newton’s second law). For example, if a
basketball hits the ground, the basketball's force on the ground is the same as Earth’s
force on the basketball. Due to the ball’s much smaller mass, however, its acceleration
will be much greater.
Friction
Friction is a force that exists between a piece of matter that is either moving or has a
force applied to it and any other matter it is in contact with. Friction always acts in a
direction opposite to that of the motion or the applied force. It is friction that causes a
moving object that appears to have no outside forces acting on it to slow down. This
everyday observation caused early scientists, and still causes many people, to think
that moving objects must slow down and stop. It was Galileo’s understanding of friction that led him to correctly assert that an object’s “natural tendency” is to keep
going at the same speed. Isaac Newton then related this “natural tendency,” otherwise known as inertia, to mass and force.
Balanced and Unbalanced Forces
If the forces on an object or system add up (taking direction into account) to zero,
there is no net force on the object or system. Another way of describing its motion is to
say the forces are balanced. This object has no change in motion and could be sitting
still or moving with constant speed and direction as stated in Newton’s first law.
If the forces on an object don’t add up to zero (taking direction into account), the
object has a net force on it. Another way of describing its motion is to say the forces are
unbalanced. Objects that have an unbalanced force are accelerating and have a
change in motion as described by Newton’s second law.
E-69
Activity 80 • Newton’s Laws of Motion
TEACHING SUGGESTIONS
The heavy ball has more inertia because it
would take more force to change its motion than
the light one.
GETTING STARTED
1.
(LITERACY) Elicit students’ preconceptions
about motion.
Have students read the introduction, and respond
to any of their questions. Briefly review the definition of force as a push or a pull. Explain that Isaac
Newton is famous for thinking about the motion of
objects in terms of force. Some of his ideas were
truly revolutionary because they do not match
everyday observations and common sense.
This activity is supported by two literacy strategies.
First, an Anticipation Guide elicits students’ ideas
before and after the reading. During the reading,
Stopping To Think Questions, a Directed Activities
Related to Text (DART) strategy, enhances students’s comprehension as they complete the reading. For more information on these and other
literacy strategies, see the literacy section of Teacher
Resources II: Diverse Learners.
Distribute Student Sheet 80.1, “Anticipation Guide: Laws of Motion,” before students begin the reading, and have them
complete the “Before” column. Or you can ask for a
shows of hands, and take a quick tally of students’
responses. You may want to record the tally for each
question on the board or overhead transparency to
save it for review at the end of the activity.
DOING THE ACTIVIT Y
2.
2. What would happen to a baseball if you could throw
it in outer space? Explain in terms of inertia and friction.
Without friction, a thrown baseball would continue in a straight line at a constant speed forever, or until it encountered another force that
changes its motion.
3. A car travels along a straight road at a steady 40
MPH. Are the forces on the car balanced or unbalanced? Explain.
The forces are balanced. The frictional forces
within the car and from the road are balanced
by the equal but opposite force applied by the
engine. The net force is zero, and the car travels
with constant speed and direction.
4. Can a light object that is hit with a small force accelerate as rapidly as a heavier object hit with a big
force? Why or why not?
Yes, because the ratio of force to mass could be
the same for the two situations. For example, the
acceleration of 400 newtons/100 kg is the same
as 4 newtons/1 kg.
5. If you hold a backpack in your hand, the force of
gravity pulls it downward. What force keeps it from
falling to the ground?
The “reaction” force is your hand that applies an
equal and opposite force on the backpack. However, it is impossible to say which of the two
forces is the “action” and which is the “reaction”
since they are applied simultaneously.
(LITERACY) Students read about force and
motion.
As students complete the reading, they should stop
briefly to answer the Stopping To Think questions in
the reading. It is not necessary for students to create
written responses to the Stopping To Think questions, but you may want the whole class to discuss
them, depending on your students’ needs.
Answers to Stopping to Think Questions
1. Which has more inertia, a heavy ball or a light ball
rolling at the same speed in the same direction?
Think about which one is more resistant to a change
in motion.
E-70
3.
Review key vocabulary from the reading.
When students have completed the reading, review
the key vocabulary presented. Some terms, such as
“inertia,” “force,” and “change in motion” have
been used previously in this unit. Friction, the force
that exists at the boundary between any two pieces
of matter, has been discussed previously but is formally introduced here. Review the term net force,
as the combined force acting on an object or system.
Newton’s Laws of Motion • Activity 80
Highlight the relationship between net force, balanced forces, and unbalanced forces. The concept
of net force will be reinforced in the next activity.
FOLLOW-UP
4.
(UC ASSESSMENT, MATHEMATICS) Review the
implications of Newton’s Laws.
Review students’ responses to the Analysis Questions. Expect that their responses to Question 1 will
ignore the force of gravity from other planets,
which does have a significant affect on space
probes. If a discussion about gravity arises from
Analysis Question 1, explain that although gravity
exerts a force for an infinite distance, the affects are
significant only when an object is relatively close to
a very massive object, such as a star, planet, or
moon.
Analysis Question 2 is an opportunity to assess students’ understanding of the concept of inertia and
the relationship between inertia and mass, which is
a main idea in this unit. Use the UNDERSTANDING
CONCEPTS (UC) Scoring Guide to score the responses.
If appropriate, provide each student with a copy of
the UC Scoring Guide.
✓In Analysis Question 3, students review the topics
presented in Activity 78, “Force, Acceleration, and
Mass,” by using the equation F = ma to calculate
forces. Review the mathematics involved in calculating force and acceleration, and the measurement units for those quantities.
5.
✓
(LITERACY) Students revisit their ideas about
force.
Ask students to look again at the questions posed
on Student Sheet 80.1, “Anticipation Guide: Laws of
Motion” and complete the “After” column. Before
reviewing with students the correct responses,
which are provided below, check their work to
determine the accuracy of their understanding.
At the conclusion of this activity, review with students the aspects of Newton’s laws that are difficult
to understand because they seem to contradict our
daily experiences. In particular, students should
mention that the presence of friction makes it difficult to understand the natural motion of objects as
described in the first law. Students may also find the
relationship between the third law and the second
law to be confusing in situations where there is an
unbalanced force. Spend some time clarifying and
providing examples that support the main ideas of
the reading.
Answers to Student Sheet 80.1, Anticipation Guide: “Newton’s Laws of Motion”
+
1. The heavier the car, the more force is needed to make it speed up.
This is true because a heavier car has more mass, and Newton’s second law states that more force is
needed to accelerate (speed up) the car.
—
2. A force is always needed to keep an object moving.
This is false because if there are no forces (including friction) acting on a moving object, that object
will remain in motion, as described by Newton’s first law.
+
3. A force is always needed to slow down an object.
This is true because Newton’s first law says an object in motion will remain in motion unless acted
upon by a force. This force is often provided by friction.
—
4. It takes more force to slow down a small car than a large truck because the truck is heavier.
This is not true for a given deceleration because a larger force is needed to slow down a large, massive, truck,
as Newton’s second law states. This law describes how mass and force are directly related to each other.
+
5. When a falling object hits the ground, the ground applies an upward force to it.
This can also be analyzed using Newton’s third law, which states that when one object applies a force on
a second object, the second object applies the same size force, in the opposite direction, on the first object.
—
6. Friction exists only when two solid objects rub against each other.
This is false because friction exists when any two substances in any state— solid, liquid, or gas—are touching.
E-71
Activity 80 • Newton’s Laws of Motion
SUGGESTED ANSWERS TO QUESTIONS
1.
3. An engine can exert a force of 1,000 newtons. How
fast can this engine accelerate:
Spaceships that travel millions of miles
into outer space use very little fuel. How
can they go so far on so little fuel?
a. a 1,000 kg car
F = m x a
1,000 N = 1,000 kg x a
Because there is no friction in outer space, an
object in motion remains in motion. Once a
space probe leaves behind the friction of Earth’s
atmosphere, it needs very little fuel to get it to its
final destination. (However, it uses fuel to slow
down if it is going to land on a planet or to
change direction.)
a = 1,000 N/1,000 kg
a = 1 m/s2
b. a 2,000 kg car
F = m x a
1,000 N = 2,000 kg x a
■ Teacher’s Note: This answer ignores gravity,
a = 1,000 N/2,000 kg
which is a force that affects space travel near
such massive objects as planets and moons.
a = 0.5 m/s2
4.
2.
(UC ASSESSMENT) Use Newton’s Laws to explain
why it is easier to turn a truck when it is empty than
when it is carrying a heavy load.
Level 3 Response:
Newton said that heavier things have more inertia. This means that the truck with the heavy
load has a greater tendency to keep moving at
the same speed and in the same direction. The
truck needs more force when it is full to make the
same change in motion as when it is empty. So
the inertia of the heavier truck keeps it from
turning as easily as the empty truck. More inertia explains why the full truck is harder to
maneuver, like the full cart in the class demonstration.
E-72
Use Newton’s third law to explain why a
blown up, but untied balloon flies around
the room when you let it go.
Newton’s third law states that forces come in
equal and opposite pairs. For every action force
there is an equal and opposite reaction force.
The balloon exerts a force on the air (the action
force), causing the air to rush out the opening,
while at the same time the air exerts an equal
and opposite reaction force on the balloon,
causing the balloon to fly around the room.
5. Motor oil, axle grease, and other lubricants are slippery. Why do you think people spend the money to
put these lubricants in their cars?
Lubricants reduce friction, and less friction
means that the engine and the wheels move
more smoothly (and stay cooler). This means
the engine needs to exert less force, which often
results in it lasting longer, using less fuel, and
requiring fewer repairs.