Student Name:__________________________
Grade & Division:__________________
Lesson Objective:
1. Understand how to combine forces acting on a system and To calculate
Net force (in One dimension)
2. Identify whether forces on a system are balanced or unbalanced.
3. Draw Free-body diagram for Forces (in One dimension)
Student must know from previous lesson: Meaning of Free-body diagram, Unit of force, Force
is a vector
NET FORCE
What is NET FORCE?
Net Force is the vector sum of forces acting on an
object.
(Vector sum means to add up the effects of both the
magnitude and direction of the forces)
Can you tell what is the NET FORCE on the fridge in
this picture?
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In the above picture, the two opposite forces cancel each other’s effect. We call such group of
forces which cancel each other’s effect as BALANCED FORCES. (Likewise, if a set of forces don’t
cancel each other’s effect, we call them -UNBALANCED FORCES).
IMPORTANT POINT: When Forces on an object are balanced, it does not necessarily mean that
it will stop moving. It rather means that it will have Zero acceleration (ie; Zero change in velocity).
So, if the forces are balanced, depending on the situation, it could either be moving at a constant
velocity or be stationary.
EXAMPLE: Let us consider the example of a skydiver in the
following picture. The downward gravitational force on the
diver is 500 N. As he falls, the upward air resistance (friction
between his body and the wind pushing him upwards) is
250N.
1. Can you draw a free-body diagram of the diver below?
Al Shabhana School, Al Sila
Student Name:__________________________
Grade & Division:__________________
2. What is the net force on the diver? Are the two forces balanced or unbalanced?
3. CHALLENGE QUESTION: You know that an object falling down due to Earth’s gravity accelerates,
so its speed keeps on increasing. This means that the air resistance (wind) pushing him upward
must also keep on increasing as he falls more distance.
(a) What will happen to the velocity of the diver, at a certain point, when upward air resistance
becomes equal to downward gravitational force?
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(b) Will he float in the air or keep falling down at this point?
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4. Test whether you can correctly draw a Free-body diagram for the following situation.
Two brothers are moving a chair. The one
wearing blue shirt is pulling with a force of
500 N and the other wearing red is pushing
with a force of 700 N. The contact between
the chair and the floor resists these forces by
exerting a force of 500 N in the opposite
direction.
(a) Draw the free-body diagram of the forces on
the chair.
(b) What is the net force acting on the chair?
(c) Are the forces balanced or unbalanced?
Al Shabhana School, Al Sila