Download Chapter 4 Motion

Lesson 2
What Are the Laws of Motion?
Fast Fact
Crash Test Dummies Car manufacturers use crash tests to ensure that new vehicles
are safe to drive. Traveling at 48 km/hr (30 mi/hr), a car that hits a concrete wall
will cave in about 0.6 m (2.0 ft). Even though the car stops, the crash test dummies
keep moving forward. In the Investigate, you will model a crash test.
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Momentum Crash Test
Materials
 game board
 meterstick
 dime
 several books
 small tov car
 quarter
Procedure
1. Make a ramp by setting one end of the game board on a stack of books about 15 cm
high. Place another book as a barrier about 10 to 15 cm from the bottom of the
ramp. Be sure the barrier is lower than the front end of the car.
2. Put the car at the top of the ramp. Put the dime on the front end of the car. Let the
car roll down the ramp and hit the barrier. Observe what happens to the dime.
Measure and record the dime's distance from the barrier. Repeat several times.
3. Repeat Step 2, but use a quarter. Repeat several times.
4. Predict how the results will differ if you make the ramp higher. Add another book to
the stack under the ramp. Repeat Steps 2 and 3 several times. Was your prediction
correct?
Draw Conclusions
1. Put your data in a table. Compare data for the dime with data for the quarter. How
does a coin's mass relate to the distance it travels?
2. What happened when you made the ramp higher? Infer why this happened.
3. Inquiry Skill What was the control variable in Steps 2 and 3? What was the test
variable?
Investigate Further
Experiment with several methods of keeping the coin on the car when the car
strikes the barrier. Conduct several trials for each method.
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Reading in Science
VOCABULARY
inertia p. 155
SCIENCE CONCEPTS
 how inertia affects motion
 how force, mass, and acceleration are related
READING FOCUS SKILL CAUSE AND EFFECT Find out how forces affect
motion.
Newton's First Law of Motion
A soccer ball doesn't roll across a field on its own. It takes a force, such as a kick from
you, to get it moving. But once it's moving, the ball doesn't go on forever. Sooner
or later, it stops.
You might think that the natural state of an object is to be still. You might also think
that once it's moving, it stops only because the force that pushed it stops, too. In
other words, the ball stops because you aren't kicking it anymore.
However, objects stop because a force acts on them. On Earth, gravity and friction stop
moving objects. Gravity pulls objects toward Earth. If you throw a ball, it
eventually curves and hits the ground. Suppose gravity didn't exist. The ball would
keep moving through the air until drag stopped it.
Friction acts in the direction opposite to motion. The less friction between the ground
and a rolling ball, the farther the ball will move. When pushed with the same force,
the ball will roll farther down a paved street than down a grassy field.
---see pictures
Isaac Newton (1642-1727) is a giant in the history of science. He is most famous for
his three laws of motion, his law of gravitation, and his experiments with light
and prisms.
The ball doesn't move until a force hits it. Once the ball has been hit, will the ball
keep moving forever? Why not?
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When an ice hockey player hits a puck, the puck slides a long way because ice is very
slippery. If the friction between the puck and the ice disappeared, the puck would
slide forever. It would move in the same direction and at a constant speed.
Isaac Newton described the effects of forces in three laws of motion. Newton's first law
of motion describes inertia. Inertia is the tendency of objects to resist a change in
motion. Objects at rest don't move unless a force moves them. Objects in motion
don't slow down, stop, or turn unless a force makes them do so. A simple way to
state the first law of motion is No acceleration can happen without a force.
Have you ever seen a performer pull a tablecloth from under the dishes and silverware
on a table? The dishes and silverware remain in place, amazing the audience. This
trick works because of Newton's first law of motion. The inertia keeps the dishes
and silverware in place. Only a small force is applied by snapping away the
tablecloth. This force isn't large enough or long enough to move the table setting
very far.
CAUSE AND EFFECT Why are safety belts such an important safety feature in
cars?
---see pictures
Inertia keeps you moving forward if the car stops suddenly. Safety belts apply the
force needed to stop you.
The bulletproof vest is made of strong material that can absorb the force of a
bullet, slowing it down.
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Newton's Second Law of Motion
Newton's first law tells you that when you kick a ball, the ball will move. Newton's
second law tells you that if you kick the ball harder, it will move faster. It also tells
you that a heavy ball is harder to move than a lighter ball.
Newton's second law says that when a force acts on an object, the object accelerates.
The law also tells you how much the object accelerates. Recall that acceleration is
the rate at which velocity changes. In other words, the second law says An object's
acceleration depends on the object's mass and the force applied to it.
Let's look at how force and mass affect acceleration. The larger the force is, the greater
the acceleration is. Suppose you push a scooter gently. The scooter speeds up
slowly. If you use more strength to push, the scooter's speed changes quickly. In
both cases, the scooter moves in the direction in which it is pushed.
Let's take another example. A car will stop suddenly if the driver slams on the brakes.
But if the driver applies the brakes gently, the car gradually slows down.
---see pictures
The long row of empty shopping carts requires more force to accelerate than one
empty cart requires.
The ribbon requires only a little bit of force to accelerate because it has a small
mass.
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The less an object's mass is, the less force is needed to move it. It's much easier to push
an empty shopping cart than a full one. Light cars are used in drag racing because a
car with less mass accelerates faster than a car with more mass.
For a force to move an object, the force must overcome other forces. Suppose you want
to pick up a box of books. The force you use to lift the box is the strength of your
muscles. The force acting against your muscles is gravity.
Recall that weight is the measure of the force of gravity on an object. If the weight of
the books is greater than your muscle strength, you can't move the box. However,
you could take out some of the books to decrease the mass of the box. Or you could
get someone to help you, increasing the force applied to the box. To lift an object,
the lifting force must be equal to or greater than the weight of the object.
You can write the second law of motion as an equation.
acceleration = force/mass
The standard unit of force is the newton.
CAUSE AND EFFECT What is the effect on a car's acceleration if six people ride
in the car instead of one?
Science Up Close
Impact Between a Baseball and a Bat
When a bat and a baseball strike each other, acceleration results.
1. The baseball moves toward the bat as the bat moves toward the baseball.
2. The baseball applies a force to the bat as the bat applies a force to the baseball.
The force from the bat gives different accelerations to individual particles of
the baseball. This causes the baseball to change shape.
3. Both the baseball and the bat accelerate. The push of the bat on the baseball
changes the baseball's speed and direction. The push of the baseball on the bat
slows the bat.
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Newton's Third Law of Motion
Suppose you're running without watching where you're going. You run into a wall.
Ouch!
Are you hurt because you hit the wall? No—you can't feel the force on the wall. You
can feel forces only on your body. You're hurt because the wall hit you.
The wall didn't reach out and hit you, of course. But it did push you with the same
amount of force with which you pushed it. If it hadn't, you would have moved the
wall.
The third law of motion states: Whenever one object applies a force to a second object,
the second object applies an equal and opposite force to the first object.
A simple way to state this law is that forces always occur in pairs. For every action
force, there is an equal and opposite reaction force. When your feet touch the
ground, the ground touches your feet. When a hammer hits a nail, the nail hits the
hammer. When Earth pulls on the moon, the moon pulls back on Earth—all with
equal, but opposite, force.
Here's a way to make Newton's third law of motion easier to understand. Just remember
that when one object pushes on another, the other object pushes back on the first.
---see picture
As the boy pushes off the boat, the boat pushes back on him in the opposite
direction.
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---see picture
As the runner’s foot pushes against the starting block, the block pushes against the
runner’s foot.
A horse pulls a cart. Can you name the opposing forces? Just think of the rule. The
horse pulls on the cart. So Newton's third law tells you that the cart pulls on the
horse.
Newton's third law shows that forces always happen in pairs. An action force can't
occur without an equal and opposite reaction force. The forces push with the same
strength, but in opposite directions, at exactly the same time.
CAUSE AND EFFECT You push a broom. What pushes on you, and in what
direction?
Insta-Lab
Make a Balloon Rocket
Blow up a balloon, but don't tie it. Let it go, and observe its motion. Compare the
direction in which the balloon moves to the direction in which the air travels. How
does this demonstrate Newton's third law?
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Motion in Space
Astronauts inside the space shuttle float as if no gravity were acting on them. However,
Earth's gravity at the shuttle isn't zero or even near zero. The shuttle's orbit is close
enough to Earth that gravity is almost as strong there as it is on Earth.
Why do the astronauts seem to be weightless? This happens because inertia and gravity
are balanced. After the shuttle reaches orbit, its engines shut down. Inertia carries
the shuttle forward. Space near Earth has only a little drag to slow down objects.
Gravity causes the shuttle to fall. The shuttle falls continuously toward Earth. But
at the same time, it moves forward fast enough to keep from hitting Earth. The
curve of the shuttle's path as it moves forward and toward Earth is the same as the
curve of Earth, so the shuttle doesn't get any closer to Earth's surface.
This same principle explains why planets and moons stay in orbit. Earth's moon, for
example, moves forward because of inertia. At the same time, Earth's gravity pulls
the moon toward Earth. The result is that the moon moves in a curved path around
Earth. If gravity didn't pull the moon, it would continue moving forever in a
straight line at a constant speed. If the moon didn't have inertia, it would crash into
Earth.
CAUSE AND EFFECT What causes Earth to orbit the sun?
---see pictures
A microgravity, or nearly weightless, environment occurs when the shuttle falls
around Earth at a constant speed. The astronauts don't feel the effects of
Earth's gravity.
Drag acts on this space capsule as it reenters Earth's atmosphere.
The satellite orbits Earth. It's pulled toward Earth by Earth's gravity.
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Reading Review
1. CAUSE AND EFFECT Make a graphic organizer like the one shown. To fill out
the chart, use Newton's laws to explain A how inertia affects motion, B how force
affects acceleration, and C how one object affects a second object that applies a
force to it.
---see chart
2. SUMMARIZE Write a summary of this lesson. Begin with the sentence There are
three laws of motion.
3. DRAW CONCLUSIONS The first law of motion states that you don't have to apply
a force to maintain motion. Why, then, do you have to keep pedaling your bike to
keep moving?
4. VOCABULARY Make up a riddle or a quiz question about inertia. Provide the
answer.
Test Prep
5. Critical Thinking Give an example from your everyday life of the third law of
motion.
6. A girl throws a ball. If the ball's acceleration is 12 m/s/s and its mass is 0.5 kg, how
much force did the girl apply to the ball?
A. 0.04 N
B. 0.6 N
C. 6 N
D. 12.5 N
Writing
Narrative Writing
Find out how microgravity affects the daily activities of astronauts. Write a skit that
describes a typical day in an astronaut's life.
Math
Solve Problems
Forces that act in the same direction add like regular numbers. A girl and a boy pull a
wagon to the right. The girl applies 10 newtons. The boy applies 8 newtons. What's
the total force on the wagon?
Social Studies
History of Science
Use library resources to research the life of Isaac Newton. Write a story for younger
students that tells how he came up with the three laws of motion.
For more links and activities, go to www.hspscience.com
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