Balanced and Unbalanced
Forces
What allows you to peddle
your bike, blow your nose,
chew gum or
swim in a pool?
So, What is a force?
A force is a push or pull
Force gives an object the energy
to move, stop moving,
or change direction.
You exert forces on other objects
We would never be able to move
without exerting forces on
other things
When you write with a pen
you exert a force
A flag blows because the
wind exerts a force
A magnet exerts a force
on iron
• Two forces that are very common are:
– Gravity- a force that pulls all objects
towards the center of the Earth.
– Friction – a force that resists motion
between two surfaces that are in contact.
Forces always occur in pairs
Forces can be balanced or unbalanced
Balanced Forces DO NOT cause
a change in motion.
Unbalanced forces do cause
a change in motion
Have you ever had an arm wrestling
competition?
There was a probably a time when
Both of you were pushing as hard as
You could but your arms stayed in
place
The forces exerted by each person
is equal, but opposite in direction
So they cancel each other out and
no motion takes place
What would happen during a game of
tug of war, if two teams exerted the
same amount of force on the rope?
The two forces would cancel each other
out and the result would be no motion
Unbalanced forces are not equal and this causes motion
When two unbalanced forces are
in opposite direction, the net force
is exerted in the direction of the
larger force (subtract)
15 Newtons (N)
40 Newtons (N)
Net Force
25 Newtons (N)
• When two unbalanced forces are
in same direction, the net force
is exerted in the direction of the force
(add).
Imagine your family’s car breaks
down on the road and you have to
push it into a parking lot.
If you push the car with a force of 25 N and
your sister pushes the car with a force of 15 N
It would look like this:
15 N and 25 N
Net force = 40 N
You both are pushing in the same direction so
you add the forces to get the net force
Two tugboats are moving a barge.
Tugboat A exerts a force of 3000 newtons (N)
on the barge. Tugboat B exerts a force of
5000 newtons (N) in the same direction.
1.What is the combined force on the barge?
2. Draw arrows showing the individual and
combined forces of the tugboats in #1.
3. Now suppose that Tugboat A exerts a force of
2000 Newtons (N) on the barge and Tugboat B
exerts a force of 4000 Newtons (N) in the
opposite direction. What is the combined force
on the barge?
4. Draw arrows showing the individual and
combined forces of the tugboats in #3.
5. Could there ever be a case when Tugboat A and
Tugboat B are both exerting a force on the
barge but the barge doesn't move?
Draw arrows showing the individual and combined
forces in such a situation.