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.
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:
an object that is not moving stays still
an object that is moving continues to move at the same speed and in the same direction
So notice that an object can be moving even if there are no forces acting on it.
Here are some examples of balanced forces.
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.
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.
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.
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