Resultant Force

Key Stage 3

The Resultant Force is the overall force on an object.

A resultant force can be calculated by taking forces that act along the same line and add them together.

If the forces are in the same direction as each other, then they have a value.

If one force is acting in the opposite direction to another one of the forces is given a negative value.

	If the 'up' direction is chosen to be positive then any vector pointing 'up' has a positive value and any vector pointing 'down' has a negative value. In this diagram the green vector has a value of +2 while the red vector has a value of -4. If the number lines represent the number of Newtons force then the Resultant Force is the two values added together. F_Resultant = (+2N) + (-4N) F_Resultant = -2N upwards


If the 'right' direction is chosen to be positive then any vector pointing 'right' has a positive value and any vector pointing 'left' has a negative value. In this diagram the blue vector has a value of +3 while the yellow vector has a value of -2. If the number lines represent the number of Newtons force then the Resultant Force is the two values added together. F_Resultant = (+3N) + (-2N) F_Resultant = 1N to the right


Choose which directions are positive. Up is positive. Right is positive. Calculate the Resultant Force. There is 4N up and the 7N down. F_Resultant = (+4N) + (-7N) F_Resultant = -3N up	Choose which directions are positive. Up is positive. Right is positive. Calculate the Resultant Force. There is 4N left and the 3N right. F_Resultant = (+3N) + (-4N) F_Resultant = -1N right	There is 6N up and the 6N down. 6N - 6N = 0N. The resultant force up and down is 0N. There is 6N left and the 4N right. 6N - 4N = 2N. The resultant force is 2N to the left.

The resultant force is the sum of all forces acting on an object.

When forces act in the same direction their magnitudes are added together.

When forces act along the same line but in opposite directions; one is subtracted from the other.

When forces are at right angles to one another they can be added using Pythagoras theorem to find the magnitude of the resultant force and then trigonometry can be used to find the direction of the resultant force.