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.

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 weight pulls down and the tension in the rope pulls up.

A crate is held by a rope. The two forces are the pull of the rope and the weight of the crate.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 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.

The two forces on a boat are the upthrust of the water and the weight of the boat.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 reaction force 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 two forces on a book on a table are the reaction force of the table and the weight of the book.The weight of the book is balanced by the reaction force from the table