Download The Nature of Force

The Nature of Force
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Force causes an object to stop moving, start moving,
or change direction.
In science, the word force has a simple and specific
meaning. Force is a push or pull.
When one object pushes or pulls another object, we
say that the first object is exerting a force on the
second object. Examples of force is you exert force on
a nail when you hammer it, on a wagon when you pull
it, and on a pebble when you skim it across a pond.
Like velocity and acceleration, forces are described not
only by how strong they are, but also by the direction
in which they act. If you push a door open, you exert
force in a different direction than if you pull on the
door.
Unbalanced Forces
 Turn to page 45 and we will read about unbalanced
forces. Answer the following questions.
 When you and a friend push a box across the room,
you (add or subtract) the two forces?
 If you are pushing one way and your friend is
pushing the other way, you (add or subtract) the
forces?
 According to the book, the (width or length) of each
arrow tells you the strength of the force?
 The (wider or narrower) the arrow, the more strength
the force has.
Try it yourself
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Unbalanced forces in opposite directions
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Balanced forces in opposite directions
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 The overall force on an object after all the forces are
added together is called the net force. When there is
a net force acting on an object, the forces are said to
be unbalanced.
 Unbalanced forces can cause an object to start
moving, stop moving, or change directions.
 Unbalanced forces acting on an object will change
the object’s motion. In other words, an unbalanced
force will cause an object to accelerate.
 For example, if two unequal forces acting in the
opposite directions are applied to a box, the box will
accelerate in the direction of the greater force.
Balanced Forces
Equal forces acting on one object in opposite
directions are called balanced forces.
One force is exactly balanced by the other
force.
Balanced forces acting on an object will not
change an object’s motion.
When you add equal forces exerted in the
opposite directions, the net force is zero.
Therefore, the box will not move.
Take a Look
Open your books to page 47 to see
examples of unbalanced forces in the
same direction, unbalanced forces in
opposite directions, and balanced forces
in the same direction.
Newton’s First Law of Motion
The ancient Greeks observed that
objects have natural resting places.
Objects move toward those places. For
example, a rock falls to the ground. A
ball rolls to the bottom of the hill.
Once an object is in its resting place, it
cannot move by itself. For an object ot
move, a force has to act on it.
Inertia
In the early 1600’s an Italian astronomer,
Galileo Galilei, questioned the idea that a
force is needed to keep an object moving.
He suggested that once an object is in
motion, no push or pull is needed to keep it
in motion.
Whether an object is moving or at rest, every
object resists any change to its motion.
This resistance is called inertia.
Inertia is the tendency of an object to resist
change in its motion.
Sir Isaac Newton
 Galileo’s ideas paved the way for the English mathematician, Sir
Isaac Newton.
 Newton discovered the three basic laws of motion in the late
1600’s.
 Newton’s first law of motion states that an object at rest will
remain at rest, and an object that is moving at constant velocity
will continue moving at constant velocity unless acted upon by
an unbalanced force.
 Newton’s first law of motion is also called the law of inertia.
 Inertia explains why when a car suddenly stops, inertia causes
you to continue moving forward. Passengers in a moving car
have inertia. Therefore, a force is required to change their
motion. That force is exerted by the safety belt. If the safety
belt is not worn, that force may be exerted by the windshield
instead. What do we have now in most cars that will exert the
force to change the passenger’s motion?
Mass
 Think about this… Which is more difficult to move, a
jar of pennies or a jar of foam peanuts? What is the
difference between the jar of pennies and the jar of
foam peanuts? They both have equal volume which
means they take up the same amount of space. The
difference is the amount of mass each one has.
 Mass is the amount of matter in an object.
How do we Measure Mass?
 Mass is the amount of matter in an object.
 The standard unit of measurement of mass is kilograms. For
example, a bicycle might have a mass of 10 kilograms and a
rider might have the mass of 45 kilograms. Which one has
more mass? The rider of course.
 A kilogram is equal to 1000 grams. The mass of a nickel equal
about 5 grams.
 The amount of inertia an object has depends on its mass. The
greater the mass of an object, the greater its inertia. Mass,
then, can also be defined as a measure of the inertial of an
object.