# Newton's First Law of Motion - Context by hjkuiw354

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2 .2
Context

Think about the last time you were in a car
parked at set of traffic lights. The lights turned
green and the car began to move, its speed
increasing. When a car increases its velocity
it is undergoing acceleration. And if you are                    on the other hand, you may feel as though a force
in a car that is accelerating you often feel as                  is causing you to be thrown forwards. Do you
though there is a force pushing you backwards into                     remember the last time you were in a car that was
your seat. When a car stops (decelerates) suddenly,                    turning a corner sharply? You may have noticed
the feeling of being thrown sideways as the car
turned. In this Focus you will investigate why
you experience these forces during accelerations,
decelerations and turns.

In order for a force to do these things, however,
two conditions must be met.
a The force must be unbalanced. In order for a force
to cause a change of velocity, direction or shape
there must be an unbalanced force in the direction
of the change. If the force is balanced by another
force, the forces cancel each other out because they
are acting in opposite directions.

Fig 2.2.1     The high accelerations and tight turns of stunt
aircraft can cause stunt pilots to experience
dangerously large forces.

Force
You may recall that a force can be thought of as a push               This box will experience a change in its motion      Fig 2.2.2
because the forces acting are not balanced.
or a pull. Forces can cause the following changes to                    The larger force to the left will cause the box
objects they act on. They can:                                                                to accelerate to the left.
• cause an object to accelerate—that is, to increase
its velocity                                                      b The force must be external. Can you move a car by
• cause an object to decelerate—that is, to decrease                   sitting inside it and pushing on the steering wheel?
its velocity                                                        Of course, the answer is no. If you wanted to push
• cause an object to change its direction                              a car, you would have to stand outside the car and
• cause a change of shape.                                             push it.

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Newton’s First Law of Motion                                                         >>>
Unlike the boy pushing on the box from the                                                                             Fig 2.2.4
The dinner-table party trick relying on inertia
outside in Figure 2.2.2, the man in this picture
will not move the boat because the force he
Fig 2.2.3              applies is not external to the boat.

To fully understand how forces can do these
things you need to learn about the work of a famous                                                          The strongman
physicist who in the 1680s studied the ways in                                           A famous carnival trick invo
lved a strongman with a larg
which bodies move. He determined many laws that                                       boulder on his chest. His ass                                    e
istant would hit the boulder
a sledge-hammer, breaking                                       with
describe motion, but perhaps                                                                the boulder, to the astonishmen
the crowd. This trick is mor                                      t of
e about inertia than strength
his most famous are referred                                    the strongman did have to be                              . True,
strong enough to support the
to as Newton’s Three Laws of                                  boulder on his chest in the
first place. But the boulder,
massive, had much inertia,                                   being
Motion.                                                         As long as the boulder was
so it did not want to start mov
ing.
hit hard and quickly enough
would not move downwards                                         it
before breaking, meaning that
Variables and
Newton’s apple
Newton’s First                                    strongman would feel little,
if any, extra force on his che
the
st.
The world of science
owes a great debt to Isaac
Law of Motion
Newton. His studies of        Newton’s First Law of motion
the motion of bodies have        is sometimes called the law of
given us many important         inertia. Inertia is a property of
laws that describe the ways
in which objects move on        matter by which things tend
Earth and in space. One of       to keep on doing what they
these laws explains gravity.      are already doing. Imagine a
It is said that Newton
came up with the idea of
heavy box at rest (not moving)
gravity one day as he was       on the floor. Because the box
sitting under an apple tree     has inertia, it will continue to
at his home. Observing        remain at rest until it is acted
a falling apple gave him
the idea that perhaps      on by an external unbalanced
gravity might not only      force. A famous party trick
bring an apple down to        that uses the principle of
Earth but also keep moons                                                                   A famous carnival trick—the                    Fig 2.2.5
orbiting planets, planets     inertia is shown in Figure
strongman and the boulder
orbiting stars and so on. A     2.2.4. If the tablecloth is
descendant of the original     pulled away quickly the                   Newton’s First Law also applies to moving
tree can be seen growing
outside the main gate of
dinner setting pieces, all of        bodies. If a body is travelling in a straight line
Trinity College, Cambridge,      which have inertia, remain           at a constant speed, it will tend to continue
below the room Newton         doing what they were                 moving in a straight line at a constant speed.                                Prac 1
p. 37
lived in when he studied      already doing—that is, they
there.                                                  But if an external unbalanced force starts
tend to remain at rest.              acting on it, its motion will change.

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2.2

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For instance, imagine you are in a car moving at                        Remember the last time you were parked at a set
a constant speed in a straight line. If the driver of the               of traffic lights. The lights turned green and the driver
car were to suddenly apply the brakes, what would                       accelerated forward. You might remember feeling that
you feel? You would feel as though you were being                       you were being pushed back into your seat. Actually
thrown forwards. Actually you are not being ‘thrown                     there was no force pushing you backwards. You were
forwards’. Your body simply has a tendency to keep                      at rest and so had a tendency to remain at rest. The
on doing what it is already doing—that is, moving in                    car, however, was accelerating forwards and because
a straight line at a constant speed. The car, however,                  you were in the car, the car seat would have exerted a
is slowing down around you and your seat belt in                        forward force on you. This forward force was the force
particular stops you from continuing to move forwards                   that caused you to accelerate forwards, but from your
onto the dashboard or through the front windshield.                     perspective you felt as though you were being pushed
back into the seat.
A passenger in a car demonstrates Newton’s
Fig 2.2.6           First Law when the car slows down rapidly.

When a car accelerates you feel as if you are          Fig 2.2.8
being pushed back into your seat.

Lastly, when your car turns left you feel as though
you are being pushed to the right-hand side of the
Crash test dummies                              car. Again, this is not the case. Your body simply has
t
el humans to investigate wha     a tendency to keep on moving in a straight line at a
Car manufacturers use mod                  -speed impacts.
in high
happens when a car is involved                               constant speed. The car, however, is turning away
improvements to car design
These tests have resulted in
dummies are used to measure
that ultimately save lives. The                          ally                         You feel as though you are being pushed
the force of impact. Hig h-speed photography dramatic
crash as the dummies      Fig 2.2.9          to the right as the car turns left.
t Law in a
demonstrates Newton’s Firs                  ething.
keep moving until they hit som                                passengers keep moving
in a straight line
car
turns
left
passengers appear to move to right

Fig 2.2.7           The crash test dummy on this motorbike clearly
shows the effect of inertia.

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Newton’s First Law of Motion                                                >>>
from under you. The seatbelt and seat supply a force
to the left to keep you turning with the car, but you               12 Why do you feel as if you have been thrown forwards
feel as though you are being pushed to the right.                      when you are in a car that is braking suddenly?
What you have learnt here can be summarised
into a single law. Newton’s First Law can be
stated as:                                                          13 What stops you from becoming injured when you are
in a car that decelerates rapidly?
A body will stay at rest or continue to move
14 Use Newton’s First Law and your knowledge of
in a straight line at a constant speed until an               the composition of space to suggest why a meteor
external unbalanced force acts on it.                         in space can maintain a constant speed through
space. Remember that a meteor has no means
of propulsion.
Homework book 2.2 Plotting car performance data
15 Do you think it is safe to hold a baby in your arms
Prac 2
Homework book 2.3 Calculating F = ma                p. 37      while you are a passenger in a car? Why or why not?
16 Why do you think seat belts and airbags are important
in vehicles?
17 Before astronauts went into space they had to

2.2                         [ Questions ]
experience a feeling of weightlessness. Scientists came
up with a way to simulate weightlessness. They flew an
aircraft upwards at a shallow angle of ascent, then slowly
FOCUS

levelled off. The astronauts were sitting without seat belts
Use your book                                                     on seats in a padded compartment in the plane.
a Explain what would happen and why they would
Force                                                                  feel weightless.
1 What is a force?
b This method would not work if the plane was flying
2 What can forces do to the objects they act on?                      up at a steep angle and suddenly levelled off.
3 What is meant by an ‘externally applied’ force?                     Explain why.
4 Describe an example of a force that fails to change an          c Would this method work if the plane was put into
object because it is not externally applied.                        a shallow dive and then suddenly levelled off?
Explain.
5 Why does a force have to be unbalanced in order to
produce a change?
Investigating questions
6 Describe an example of a force that does not change
an object because it is balanced.                            18 Research how the following work and make a comment
on how Newton’s First Law of Motion applies:
7 What is an acceleration?
a a centrifuge
8 What is a deceleration?
b the spin dryer from a washing machine.
Newton’s First Law of Motion                                    19 Use the Internet to find how a satellite is placed in
9 What is inertia?                                                orbit around the Earth. What does ‘in orbit’ mean?
10 State Newton’s First Law of Motion.                          20 What is a ‘whiplash’ injury suffered in a traffic accident,
11 Why do you feel as though you are pushed back into              why does it occur, and how are car seats designed to
your seat as your car accelerates?                              counteract this?

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2.2

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2.2                      [ Practical activities ]
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Lazy coins
Purpose                                               6 Now make a stack of about five 50 g masses. Place
Prac 1     To investigate inertia of coins and masses.             a file or a book about 50 cm behind the stack. With a
Focus 2.2
bit of waste plywood or masonite, try to hit the bottom
Requirements                                            mass out towards the book without toppling the other
Sheet of A4 or A3 paper, five 20 cent coins, five       masses over. (Don’t use a ruler because it will dent
50 gm masses, piece of plywood or masonite              the edge.)
about the size of a ruler.
Questions
Procedure
1 Place a piece of A4 or A3 paper on the edge of a desk           1 Write a sentence to explain why you could leave the
so that about 5 cm of one of the shorter sides hangs off          coin or coins almost unmoved when you pulled the
the edge of the desk.                                             paper quickly, and why you did not topple the masses
when you hit the bottom mass out.
2 Place a 20 cent coin on the opposite end of the paper.
2 Write a sentence to explain why you could not leave
3 Take hold of the end of the paper hanging from the end
the coin or coins unmoved when you pulled the paper
of the desk and pull it very quickly. If you do this fast
slowly.
enough the coin should remain unmoved.
3 What force acted between the paper and the coins as
4 Repeat steps 1–3 but this time place a second
the paper slid beneath the coins?
20 cent piece on top of the first. Keep going to see how
many coins you can do the experiment with.                      4 What property is possessed by the coin or coins that
allows them to remain almost stationary when paper is
5 Try repeating the activity (back to a single coin) but this
pulled quickly from under them?
time pull on the paper slowly rather than quickly.

Crash test food
Purpose                                               2 Show your design to your teacher and when you are
Prac 2     To build a capsule that will allow a grape (or egg      given permission, test out your capsule. Your teacher
Focus 2.2
if you can afford it) to be dropped from at least       will tell you how the testing will be conducted. (You may
2 metres without splitting.                             only have one try if each group conducts the test of their
capsule in front of the class.)
Requirements
Your choice of materials, grapes or eggs (or whatever your         Questions
teacher suggests), metre rule or tape measure, balance.
1 Make a comment about the design feature of the most
Procedure                                                            successful capsule in the class.
1 You are to design (using commonly available materials)          2 Explain the relevance of this task to Newton’s First Law
a capsule to hold a fresh, uncooked hen’s egg, or                 of Motion.
a grape (or other food) so that the capsule can be
dropped from a height of 2 metres without damaging
the egg or grape. The following specifications must be         You can increase the height dropped of the more successful
adhered to.                                                    capsules in the class.
a The container must be small enough to fit into a             SAFETY NOTE: Ensure that you are supervised by your teacher
space 30 cm × 30 cm × 30 cm.                                if you are using a ladder or any other apparatus to increase the
b The container must not exceed a weight of 200 grams.         height from which your capsule is dropped.
c The capsule must allow for the egg or grape to be
inserted into the capsule just prior to the test.
d The egg or grape must be easily removed from the
capsule after the test.

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