Force and motion - OCR 21st CenturyFree body diagrams

Unbalanced forces on an object will change its motion. Ideas about force and momentum can be used to explain road safety measures.

Part of Physics (Single Science)Explaining motion

Free body diagrams

A models the forces acting on an object. The object or 'body' is usually shown as a box or a dot. The forces are shown as thin arrows pointing away from the centre of the box or dot.

Two free body diagrams show a cross with arrows on each end and a box in the middle. The second is a straight line with arrows on each end and a black dot in the middle.
Representing an object in a free body diagram as a box or a dot

Free body diagrams do not need to be drawn to scale but it can sometimes be useful if they are. It is important to label each arrow to show the magnitude of the force it represents. The type of force involved may also be shown.

Examples of free body diagrams

Drawing of situation

Free body diagram

A box rests on a table. Two arrows pointing in opposite directions act upwards and downwards from the point at which they meet on the table.
There is a black dot with equally sized arrows pointing upwards and downwards away from the dot.

Weight and normal contact force for a resting object. The normal contact force is equal to the weight.

Drawing of situation

Free body diagram

A box rests on an incline. There are three arrows; one acting vertically downwards from box’s base, one arrow acting perpendicular to the incline and one up the incline.
Three arrows of varying sizes coming out of a black dot. Each one is labelled with the relevant force.

Weight, normal contact force and friction for an object moving at constant speed down a hill. The effect of the horizontal force to the left is equal to the effect of the horizontal force to the right. The effect of the total upward force is equal to the weight.

Drawing of situation

Free body diagram

A boat rests on water. There are four arrows of different lengths coming out of the boat, pointing in different directions.
Arrows point upwards and downwards away from a square. Large arrow points to the right and smaller arrow points left.

Weight, upthrust, thrust and frictional forces for an accelerating speedboat. The upthrust is equal to the weight. The thrust is greater than the frictional forces.

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Resultant force

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