CHAPTER 2
Scientists’ Ideas
NEWTON’S LAWS
When scientists study the interactions between objects they sometimes find it
convenient to analyze and explain them in terms of the forces the objects exert on each
other, rather than in terms of the energy transfers between them. The link between the
forces acting on an object and its motion fascinated scientists for a long time.
In the 4th century BC the Greek philosopher Aristotle concluded, from watching the
world around him, that in order for an object to move at a constant speed, it was
necessary to maintain a constant push on it. As a consequence he said that if no such
push acted, it was the natural motion of an object to slow and stop. Amazingly,
Aristotle’s ideas stood almost unchallenged for some 2000 years!
It was not until the early 17th century AD that Aristotle’s ideas were seriously
challenged, by the Italian scientist Galileo. Among Galileo’s many contributions to
science was his assertion that moving objects tend to slow and stop because of some
outside resistance to their motion, not because it is their natural tendency to do so.
In the late 17th century Sir Isaac Newton, building on the work of Galileo and others,
published his ideas, which have come to be known as Newton’s Laws of Motion.
Written in the language we have been using in this course, Newton’s first two laws
state:
1ST LAW:
If no forces (or a combination of forces that is balanced) act on an object then, if it
is at rest, it will remain at rest, and if it is in motion, it will continue to move in a
straight line path at a constant speed.
2nd LAW:
If a single force (or a combination of forces that is unbalanced) acts on an object,
its speed and/or its direction will change. How quickly the speed (and/or
direction) changes is directly related to the strength of the net force and inversely
related to the object’s mass.
These statements of Newton’s Laws contain many ideas. These are listed below. Read
through these ideas with your group and, below each idea, make a note of the evidence
or examples you have seen in your investigations that support each idea.
©2007 PET
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Idea N1 – Interactions described in terms of forces:
The interaction between two objects may be described in terms of the force (a push or
pull) that one object exerts on the other. To describe a force fully we need to know its
strength and its direction. The strength of a force is measured in units of newtons (N).
Forces are exerted between objects only while the objects interact, and are not
transferred from one object to another. Energy can be transferred to (or from objects),
but not forces.
A force diagram uses arrows to represent the forces acting on an object at a particular
moment. The length of the arrows represents the relative strengths of the forces. When
an object is in motion, its force diagram should also include a speed arrow to show in
which direction it is moving. Two examples are shown below:
Force diagram for a cart moving to
the right and being pushed in the
direction of motion by a fan unit.
Force diagram for a cart moving to
the right but being pushed to the
left by a frictional force that opposes
its motion.
Direction of motion
Force exer ted on cart by
fan u nit
Direction of motion
Frictional f orce exe rted
on cart by track
Evidence/examples:
Idea N2 - Force on an object at rest:
If a single force acts on an object at rest, the object will begin to move in the direction of
the force.
Evidence/examples:
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Scientists’ Ideas: Newton’s Laws
Idea N3 - Force in direction of motion:
If a single force acts on an object in the same direction as its motion the object’s speed
will increase. (When the speed of an object changes scientists say it is accelerating.)
Evidence/examples:
Idea N4 - Force in direction opposite to motion:
If a single force acts on an object in the direction opposite to its motion the object’s
speed will decrease. (Scientists call this accelerating too, since a decrease in speed is
also a change.)
Evidence/examples:
If the force continues to act, the object may eventually stop and even reverse direction.
Evidence/examples:
Friction
An example of a force that always opposes motion is friction. It is caused by the contact
interaction between tiny bumps and irregularities in the surfaces of objects as they rub
together.
Evidence/examples:
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Idea N5 - Strength of force and change in speed:
When a single force acts on an object, the greater the strength of the force the higher the
rate of change of the object’s speed.
Evidence/examples:
Idea N6 - Mass and change in speed:
When a single force acts on an object, the more mass the object has, the lower the rate of
change of the object’s speed. (Scientists call this property of objects their inertia.)
Evidence/examples:
The effects of both force-strength and mass on the rate of change of speed of an object
(Ideas N5 and N6) can be combined into a single relationship:
Rate of change in speed~
Strength of force
Mass of object
Scientists define the rate of change in speed of an object (that is, how much it’s speed
changes in one second) to be its acceleration.

Idea N7 - Effect of no forces:
If there are no forces acting on an object at rest then the object will remain at rest
(constant speed of zero).
If no forces act on an object in motion then the object will remain in motion at a constant
speed (greater than zero).
If no forces act on an object in motion then the path of the object will be a straight line.
Evidence/examples:
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Scientists’ Ideas: Newton’s Laws
Idea N8 - Force from the side:
If a force acts on a moving object in a direction that is neither along the line of the
object’s motion, nor directly opposed to it, then the object’s path will change direction.
Evidence/examples:
Idea N9 - Curved path:
If a continuous force acts on a moving object in a direction that always points toward a
common center, the object’s path may curve around the center.
Evidence/examples:
Idea N10 - Combinations of forces:
When more than one force acts on an object, the effect they have on the object’s motion
is due to the combination of these forces.
If the strengths and directions of the individual forces are such that their combination
results in an exact balancing of forces in opposite directions, the forces are said to be
balanced. When balanced forces act on an object, its speed and direction remain
unaffected, in exactly the same way as if no forces acted on it. (This is an extension of
Idea N7 above.)
If the strengths and directions of the individual forces are such that their combination
does not result in an exact balancing of forces in opposite directions, the forces are said
to be unbalanced. When unbalanced forces act on an object, the object’s speed and/or
direction changes, just as it does when an equivalent single force acts on it. (This is an
extension of Ideas N2 to N6, N8 and N9 above.)
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When the forces acting on an object are added and/or subtracted to give an equivalent
single force, scientists call the result the net force.
Unbalanced forces on an object in motion
Balanced forces on an object at rest
Dire ction of mo tion
200 N
150 N
Equivalent to single force
200 N
200 N
Equivalent to no forces
Direction of mo tion
50 N
Net force = 0 N
Net force = 50 N to the left
Evidence/examples:
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