Resultant Forces (OCR GCSE Physics A (Gateway Science Suite)): Revision Notes

A single force that has the same effect as all of the original forces acting together.

These values are achieved by calculating the total force acting in each direction.

Definition:

• A single force represents the sum of all the forces acting on an object.
• If more than one force acts along a straight line, the resultant can be found by adding (acting in the same direction) or subtracting (acting in opposite directions) them.

Skydiver Example

• Forces acting: Air resistance and weight.

Stages of Skydiving

1. Diagram A:

• Initially, the skydiver has no air resistance, and the only force acting is weight.
• The resultant force is 833N down.

2. Diagram B:

• As he falls, he accelerates, increasing his speed.
• As air resistance increases, the resultant force from weight decreases.
• The resultant is 833 - 350 = 483N down.

3. Diagram C:

• So acceleration decreases, and he is not speeding up as quickly.
• The resultant is 133N down.

4. Diagram D:

• Eventually, they are equal and balanced, so there is no resultant force.
• Resultant = 0.
• No acceleration when the resultant force is 0; they travel at terminal velocity.

Free Body Diagrams: Show the forces (and their directions) acting on an object.

A force $F$ at angle $\theta$ to the ground can be resolved parallel and perpendicular to the ground.

Using Pythagoras' rule, the two components are shown: $a^2+b^2=c^2$