# Showing friction opposes gravityHelpppp

1. Nov 23, 2005

### suspenc3

Hi, im doin a lab report finding the accelration of gravity. The setup uses an air track, and a rider, photogates etc. One question asks: If there is friction in the system, will it increase or decrease your experimental value of g.

I know that the answer is that it will decrease my experimental value of g because friction opposes gravity.
How can I show some work/Prove This?

2. Nov 23, 2005

### Staff: Mentor

That is not quite true.

Friction is due to a normal force on a surface. The magnitude of the friction force is given by a coefficient of friction * the normal force. Friction opposes the motion of an object, so without an external force the object would decelerate.

One could incline the air track at two different angles (at least), and perhaps assume that the coefficient of friction is the same. Measure the rate of acceleration at two angles and solve g. The angle would the other second unknown.

And don't forget air resistance, which becomes more significant as v increases.

Alternatively to measure the acceleration of gravity, one would simply drop an object from various heights and determine the time taken to reach the ground. Then plotting the acceleration vs t2, one would get a value for g.