# Balanced forces

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

If the forces on an object are balanced (or if there are no forces acting on it), this is what happens:

• a stationary object stays still
• a moving object continues to move at the same speed and in the same direction

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

## Force diagrams

We can show the forces acting on an object using a force diagram. In a force diagram, an arrow represents each force. The arrow shows:

• the size of the force (the longer the arrow, the bigger the force)
• the direction in which the force acts

The arrow should be labelled with the name of the force and its size in newtons. Textbooks often show a force with a thick coloured arrow so that it looks nice, but it is more accurate if you just use a ruler and pen or pencil to draw an arrow with a single line.

Here are some examples of situations involving balanced forces.

## Hanging objects

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

The weight of the crate is balanced by the tension in the rope

## Floating in water

Objects float in water when their weight is balanced by the 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.

The weight of the boat is balanced by the upthrust from the water

## Standing on the ground

When an object rests on a surface such as the ground, the from the ground balances its weight. 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 the book is balanced by the reaction force from the table