# Q & A: Tennis without Sliding

Q:
How is friction involved in tennis???
- Trista (age 12)
Cedar Rapids, Iowa US
A:
Friction is important in most sports. We often forget about friction, but it is always there. The friction that is most important is between our shoes and the ground.

You have to change your horizontal motion to get to the ball. According to Newton's second law (F=ma) this means that some force has to act on you in the horizontal direction. This force is the friction between your shoes and the ground.

So, it's friction that lets you move around in different ways. Without it you couldn't start or stop, unless you banged into the net or something. (That might sound familiar if you've ever tried to walk on ice.)

(published on 10/22/2007)

## Follow-Up #1: direction of friction

Q:
Why does friction have to act on you in a horizontal way
- Marichi Hernandez
Spain
A:
It's not always horizontal, just in this case. The reason is that the friction between surfaces is in the plane of the surfaces. Any force between the surfaces in the other direction is called a 'normal force', and has very different properties. For example, two surfaces can slide relative to each other along the direction of a continuing frictional force, but if two objects moved relative to each other along the direction of the normal force the objects would either interpenetrate or separate. The surface of the tennis court is a horizontal plane.

Mike W.

(published on 05/16/2013)