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Science

Force, mass and acceleration

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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.

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.

When all the forces are balanced, the resultant force is zero. In this case:

  • a stationary object remains stationary
  • a moving object keeps on moving at the same speed in the same direction

For example, in the diagram of the weightlifter, the resultant force on the bar is zero, so the bar does not move. Its weight acting downwards is balanced by the upward force provided by the weightlifter.

The longer the arrow, the bigger the force. In this diagram, the arrows are the same length, so we know they are the same size.

When all the forces are not balanced, the resultant force is not zero. In this case:

  • 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, so the bar moves upwards.

Resultant force is not zero

In this next diagram of the weightlifter, the resultant force on the bar is also not zero. This time, the upwards force is smaller than the downwards force. The resultant force acts in the downwards direction, so the bar moves downwards.

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