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Table of Contents
Chapter: Forces and Changes
in Motion
Section 1: Motion
Section 2: Forces and Motion
Section 3: The Laws of Motion
Forces and Motion
2
Force
• A force is a push or a pull that one object
exerts on another object.
• Objects like floors, chairs, and Earth also
exert forces on other objects, including
people.
Forces and Motion
2
Force Has Direction and Size
• Just like velocity and acceleration, a force
has both size and direction.
• The direction of a force is the direction of the
push or pull.
• Pushing or pulling harder increases the size
of the force you exert. The size of a force is
measured in newtons (N).
Forces and Motion
2
How Forces Combine
• When more than one force acts on an object,
the forces combine.
• The combination of all the forces acting on
an object is the net force.
Forces and Motion
2
Combining Forces in the Same
Direction
• When two forces act in the same direction on
an object, like a box, the net force is equal to
the sum of the two forces.
Forces and Motion
2
Combining Forces in Opposite
Directions
• If two forces of equal strength act on the box
in opposite directions, the forces will cancel,
resulting in a net force of zero.
Forces and Motion
2
Combining Forces in Opposite
Directions
• When two unequal forces act in opposite
directions on the box, the net force is the
difference of the two forces.
Forces and Motion
2
Balanced and Unbalanced Forces
• When the net force on an object is zero, the
motion of the object doesn’t change.
• The forces acting on an object are balanced
forces if the net force is zero.
• The forces acting on an object are unbalanced
forces if the net force is not zero.
Forces and Motion
2
Contact and Non-contact Forces
• A force is exerted when
one object pushes or
pulls on another.
• A force that is exerted
only when two
objects are touching is
a contact force.
• Non-contact forces are forces that can be
exerted by one object on another even when
the objects aren’t touching.
Forces and Motion
2
Gravity
• Gravity is a non-contact force that every
object exerts on every other object due to
their masses.
• The gravitational force between two objects is
an attractive force that tends to pull the two
objects closer together.
Forces and Motion
2
Gravity
• The size of the gravitational force between
two objects depends on the masses of the
objects and the distance between them.
• The gravitational force between two objects
increases if the mass of one or both of the
objects increases.
• If two objects move closer together, the
gravitational force between them increases.
Forces and Motion
2
Mass and Weight
• Because weight is a force, it is measured in
newtons.
• Weight is not the same as mass. Mass is the
amount of matter an object contains, and is
measured in kilograms.
Forces and Motion
2
Friction
• Friction is a contact force that resists the
sliding motion of two surfaces that are
touching.
• Friction causes a sliding object to slow down
and stop.
• Friction also can prevent surfaces from sliding
past each other.
Forces and Motion
2
Static Friction
• Static friction
is the force
between two
surfaces in
contact that
keeps them
from sliding
when a force
is applied.
Forces and Motion
2
Sliding Friction
• Sliding friction is the force that opposes the
motion of two
sliding surfaces
in contact.
• Sliding friction
exists between
all sliding
surfaces that
are touching.
Forces and Motion
2
What causes friction?
• When two surfaces are in contact, the
surfaces stick to each other where the dips
and bumps on one surface touch the dips and
bumps on the other surface.
• Friction is caused by the sticking of the two
surfaces at these bumps and dips.
Forces and Motion
2
The Buoyant Force
• The buoyant force is a force exerted by a
fluid on an object that is in the fluid.
• The buoyant force is always upward.
• If you are
floating in
water, the
buoyant force
is large enough
to balance your
weight.
Forces and Motion
2
Air Resistance
• Air resistance is a contact force that opposes
the motion of objects moving in air.
• Just like friction, air resistance acts in the
direction opposite to an object’s motion.
• Air resistance is less for a narrow, pointed
object than for a wide, flat object.
Section Check
2
Question 1
A force is a _______ or a _______.
Answer
Force is a push or a pull.
FL: SC.C.2.3.2
Section Check
2
Question 2
A force exerted between two objects that
are not touching is known as _______.
Answer
The answer is a long-range force. Gravity is a
long-range force; although we are not touching
the Sun, our planet is being acted on by the Sun
to keep it in orbit.
FL: SC.C.2.3.1
Section Check
2
Question 3
You try to push a desk across the floor one
direction but a friend is trying to push it the
opposite direction. As a result, the desk
doesn’t move. This is an example of what
kind of forces?
A. balanced forces
B. continuing forces
C. extended forces
D. unbalanced forces
FL: SC.C.2.3.6
Section Check
2
Answer
The answer is A. When forces are balanced, the
net force is zero.
FL: SC.C.2.3.6