Balanced forces

Force diagrams

We can show the forces acting on an object using a force diagram. In a force diagram, each force is shown as a force arrow. An arrow shows:

• the size of the force (the longer the arrow, the bigger the force)

• the direction in which the force acts.

The arrow is usually labelled with the name of the force and its size in newtons. Text books often show a force with a thick coloured arrow, but it is best if you just use a pencil and ruler to draw an arrow with a single line.

Balanced forces

When two forces acting on an object are equal in size but act in opposite directions, we say that they are balanced forces.

So notice that an object can be moving even if there are no forces acting on it.

Examples

Here are some examples of balanced forces.

Hanging objects

The forces on this hanging crate are equal in size but act in opposite directions. The weight pulls down and the tension in the rope pulls up.

The forces on this hanging crate are balanced.

Floating in water

Objects float in water when their weight is balanced by the upthrust from the water. The object will sink until the weight of the water it pushes out of the way is the same as the weight of the object.

A boat floats because its weight is balanced by the upthrust from the water

Standing on the ground

When an object rests on a surface such as the ground, its weight is balanced by the reaction force from the ground. The ground pushes up against the object. The reaction force is what you feel in your feet as you stand still. Without this balancing force you would sink into the ground.

The weight of a book lying on a table is balanced by the reaction force from the table top