Originally Posted by sureshs
Very interesting, and something I wanted to do for a long time.
Do you know what is the main cause of friction? Your weight. The frictional force is usually a coefficient (less than 1) times the weight.
So this is a physics question I always had. If a person standing on a scale tenses his leg muscles to try to "increase" the force on the ground, will the scale show a higher reading? No.
I just tried it now and it is confirmed.
So, pressing against the ground cannot increase friction.
It can only provide a springboard to rise.
A niggle and one bigger issue in the above.
First the niggle:
Your weight doesn't cause friction. Friction is caused by the characteristics of the two materials in contact, namely a tennis court and a shoe.
One way to model the frictional force created by two materials in contact is to assume that the force generated is less than or equal to the force between the two surfaces multiplied by a coefficient (as suresh said called the coefficient of friction). For most material combinations it is less than 1.
Now the bigger issue:
While just tensing the muscles in your leg won't change the force between your shoe and the ground, bending your leg and lowering your body, like you would setting up for a forehand, and then straightening your leg most certainly will increase the force. Try it on your scale.
As far as hitting forehands goes I 'm not sure how much of a difference any of this makes. I think the more salient point is that bending your back/outside leg in your set-up allows your leg to help drive your hips around. The legs become an important part of the kinetic chain and add to your power.