Gravity - AQAFree body diagrams - Higher

Gravity is one of the most important forces in the universe. An object with mass in a gravitational field experiences a force known as weight.

Part ofCombined ScienceForces

Free body diagrams - Higher

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:

Weight and reaction force for a resting object

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.

Drawing of situation

Free body diagram

Weight, reaction force and friction for an object moving at constant speed down a hill.

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.
Drawing of situation
Three arrows of varying sizes coming out of a black dot. Each one is labelled with the relevant force.
Free body diagram

Weight, upthrust, thrust and air resistance for an accelerating speedboat.

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