Resultant Force Information
A stationary object remains stationary if the sum of the forces acting upon it resultant force - is zero. A moving object with a zero resultant force keeps
moving at the same speed and in the same direction.
If the resultant force acting on an object is not zero, a stationary object begins
to accelerate in the same direction as the force. A moving object speeds up,
slows down or changes direction.
Acceleration depends on the force applied to an object and the object’s mass.
Zero resultant force
You should be able to use the idea of the resultant force on an object to determine its
movement.
The resultant force
An object may have several different forces acting on it, which can have different
strengths and directions. But they can be added together to give the resultant force.
This is a single force that has the same effect on the object as all the individual forces
acting together.
A resultant force acting on an object may cause it to begin moving or to change its
movement. In a force diagram, the longer the arrow the bigger the force.
Zero resultant force
When all the forces are balanced, the resultant force is zero. In this case:
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A stationary object remains stationary
A moving object keeps on moving at the same speed and in the same direction
For example, in the force diagram for the weightlifter,the arrows are the same length.
As they are the same size and in opposite directions, the resultant force on the bar
is zero. Its weight acting downwards is balanced by the upward force provided by the
weightlifter. This means that:
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If the bar was already still, it will not start to move
If the bar was moving, it will continue to move at the same speed and in the same
direction
Non-zero resultant force
When all the forces are not balanced, the resultant force is not zero. In this case, an
object accelerates in the direction of the resultant force. This means that:
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A stationary object begins to move in the direction of the resultant force
A moving object speeds up, slows down or changes direction depending on the
direction of the resultant force
In this diagram of the weightlifter, the resultant force on the bar is not zero. The
upwards force is bigger than the downwards force. The resultant force acts in
the upwards direction.
There are three possibilities:
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The bar was still but begins to move upwards
The bar was already moving upwards and so moves upwards faster than before
The bar was already moving downwards and so moves downwards more slowly than
before
Note that you cannot tell which possibility is correct unless you know if and how the
bar was already moving.
In the next diagram of the weightlifter, the resultant force on the bar is again not zero.
The upwards force is smaller than the downwards force. The resultant force acts in
the downwards direction.
Again there are three possibilities:
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
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The bar was still but begins to move downwards
The bar was already moving downwards and so moves downwards faster than before
The bar was already moving upwards and so moves upwards more slowly than before