What makes objects change
speed or direction?
What is a Force?
 Definition: A force is a push or pull that causes an
object to move, stop, or change direction
 In physics, a force is anything that makes an object
accelerate
What is a Force?
• The student is
pushing down on
the chair, but the
chair does not
move.
• The floor is
balancing the force
by pushing on the
chair.
What is a Force?
• In science, a force is a push or a pull.
• All forces have two properties:
–Direction and Size
• A newton (N) is the unit that describes
the size of a force.
Newtons? Sir Isaac Newton?
 Yes! The unit for measuring force is NEWTON(S).
 It is named for that famous scientist!
Forces in the Same Direction
• When forces are applied in the same
direction, they are added to determine
the size of the net force.
Forces in Different Directions
• When two forces act in opposite directions, you
subtract the smaller force from the larger force to
determine the net force.
• The net force will be in the same direction as the
larger force.
Balanced &
Unbalanced
Forces
Balanced Force
• When the forces on an object produce a net
force of 0 N, the forces are balanced.
• There is no change in the motion of the
object.
Balanced Forces
They are equal in size and opposite in
direction
Balanced Forces
(Balanced Forces = No Acceleration)
5N
Object
Notice that all the forces are equally pointed in
the opposite direction. Hence they balance
each other – or cancel each other.
5N
Equal Pushing
Forces may cancel each other and
produce No Acceleration!
Balanced Forces
10N
10N
10N
Balanced
Push
i.e. Pushing a
Car
No
Acceleration
10N
Balanced
Pull
i.e. Tug-o-war
No
Acceleration
Balanced Forces
(Balanced Forces = No Acceleration)
Ground pushes up
Gravity pulls down
Gravity pulls down on you…
The ground pushes back up…
THIS KEEPS YOU WHERE YOU ARE!
If these football players push
on each other equally as hard,
will either one move?
Unbalanced Force
• When the net force on an object is not 0 N,
the forces on the object are unbalanced.
• Unbalanced forces produce a change in
motion of an object.
Unbalanced Forces
Causes Acceleration
Adding Forces
5N
Object
5N
Notice that all the forces are pointed in the
same direction. Hence they add together.
Unbalanced Forces
Causes Acceleration
Subtracting Forces
5N
Object
5N
Notice that all the forces are unequal and
pointed in the opposite direction. Hence they
are unbalanced and in opposition to each
other – or one partially cancels the other.
5N
Unbalanced Forces
Causes Acceleration
Adding Forces
Subtracting Forces
Un-Balanced Forces
Additive
10N
10N
Un-Balanced
Same
Direction
Faster
Acceleration
Un-Balanced
Opposite
Direction
Slower
Acceleration
Subtractive
10N
10N
10N
Subtracting Forces
10 N
=5N 5N
• Two forces can subtract to produce a net
force in the direction of the larger force.
– Negative Acceleration occurs (Slow Down)
Balanced v. Unbalanced
Forces
• If all forces are balanced there is no
acceleration in any direction.
– (Either Zero Motion or Constant Velocity)
• If one force is greater than the opposing
force we get acceleration in a direction.
Are these Balanced or Unbalanced Forces?
Force
–
Examples:
Balanced Force
Examples:
Unbalanced Force
Examples:
Net Force –
Example:
Newton (N) –
Examples:
4 N, left – 10 N, right = 6N, right
Force
– A push or pull on an object
Examples:
pushing a shopping cart
or pulling a wagon
Balanced Force – Equal forces acting on an object in
opposite directions. (Object is still)
Examples:
Unbalanced Force
– Unequal forces acting on an
object which causes it to move
Examples:
Net Force
– a combination of the
magnitude (difference between 2 forces)
and direction (direction of largest force)
Example:
4 N, left – 10 N, right = 6N, right
Newton (N) – The metric unit of measuring
force
Examples:
Balance between
Gravity and Friction
Q: If the friction of the air and the pull of
gravity were in perfect balance, what
would the skydiver be doing?
A: The skydiver would be going a constant
velocity (moving with no acceleration) down
towards the ground until it hit the ground.
The skydiver is colliding with the
molecules of gas in the air!
Which slows down the fall of the skydiver!
Accelerating
Constant
Velocity
Accelerating
Air
Resistance
30 m/s
28
25
m/s
20
10
0 m/s
Speed:
Constant Velocity
0 m/s
How it works
Notice that as the skydiver
leaves the plane, the force
of gravity accelerates him
faster towards the earth;
however, the moment he
jumps out of the plane,
another force begins to
oppose gravity – Air
Resistance, (Friction).
This friction with the air
begins to slow down his rate
of acceleration. Meaning,
he’s still speeding up, just not
as fast.
Gravity
Once the force of air
friction equals the force of
gravity, the skydiver steadily
descends towards earth at a
constant velocity.
Force
Balanced Force
Unbalanced Force
Net Force
Newton
Spring Scale