# Tennis Net Tension

1. Feb 13, 2008

### jgpufc

Hi

I'm wondering if you could help me with a problem im having. Im currently mid process of designing a device used to measure tension in a tennis net, and I am stuck at trying to calculate the deflection in the net cord on ball impact.

I have calculated that the ball will impact the net with a force of around 6492.14676 N by using the extremes for a mens serve, ball dimensions and the weight. The recommended tension in the net cord is 500lbs, i have tried to model the system as a beam and work out deflection, but have had no joy, can you please help me!!

Many Thanks

2. Feb 13, 2008

### Q_Goest

The ball has some kinetic energy which must be absorbed by the net/wire rope. For a first cut at this, neglect the mass of the net and wire rope. It can be added in later.

Neglecting mass, the wire rope has an initial tension and 'stretch' determined by load, modulus and geometry. Stress equals strain times modulus. Now consider that if you deflected this wire dx, it has to stretch further, creating a force in the direction of deflection. Resolve the force per unit distance and you have a spring rate for the wire in the direction of this deflection.

Once you have a spring rate for the wire which is based on a deflection in the direction of the impacting ball, you can simply equate the energy of the ball to the energy of the wire rope as it deflects. The wire rope stretches, storing energy just like any spring.

~

The above neglects mass, so for a second cut, you might try adding mass back in. Make some assumptions about the mass of the wire rope and net which is being moved by the ball. When the ball hits the net, assume conservation of momentum and determine the velocity of the wire rope/net/ball right after impact. Assume the three parts stick together. This initial velocity is imposed on the system immediately after impact. Let's assume the rope/net/ball system now move together with some slightly higher mass. This higher mass now has an initial velocity which will stretch the rope as in the first cut. Equate the energy of the rope/net/ball to the energy of your wire rope spring and solve for deflection.