Elastic Force, Hooke's Law and Adding Forces
Why a stretched spring pulls back, and how to add pushes and pulls together
Big Idea: When you stretch, squeeze or bend a body, it fights back. That "fighting back" push or pull is the elastic force. For a spring, the more you stretch it, the harder it pulls — and that simple rule is called Hooke's law.
By the end you will be able to:
- explain what deformation is and where the elastic force comes from,
- use Hooke's law F = kx to find the elastic force,
- describe how a dynamometer measures force with a spring,
- add two or more forces acting along one straight line,
- find the resultant force and say which way a body will move.
1. Deformation: changing a body's shape
Push on a lump of clay and it changes shape. Pull on a rubber band and it gets longer. Any change in the shape or size of a body is called a deformation. It happens whenever one body acts on another with a force.
There are two big kinds of deformation:
- Elastic deformation. The body springs back to its old shape once you let go. A stretched rubber band, a squeezed sponge and a bent ruler all return to normal.
- Plastic deformation. The body keeps the new shape. Squeezed clay, a bent paper clip and a dented can stay changed.
In this topic we care about elastic deformation, because that is where a very useful force appears.