# Compensating for friction

##### Class practical

It is possible to arrange for trolleys to behave as if they are moving without friction.

#### Apparatus and materials

For each student or group of students

Runway, with means to produce a uniform slope

Dynamics trolley

Elastic cords for accelerating trolley

Rod for attaching elastic cord to trolley

Ticker-tape

Ticker-timer with power supply unit

#### Health & Safety and Technical notes

A long runway requires two persons to carry it and set it up on the bench.

Check that a string is placed across the bottom of the runway to stop the trolley.

#### Procedure

a Set up the runway with a slope that you can adjust. Place the trolley on it.

b Give the trolley a short and gentle push. If it slows down and stops that it is because of friction. If it speeds up it is because the effect of gravity is stronger than the effect of friction.

c Adjust the slope so that when you start the trolley it keeps going at a steady speed. Then the effects of friction and gravity are in balance.

d Test this with ticker-tape attached, and using the ticker-timer. This might increase the frictional effect slightly. Start the trolley and allow it to run down the slope. If it travels for most of its journey so that the dots are equally spaced, then its speed is constant.

e Attach an elastic cord to the trolley. By pulling on the elastic cord and always stretching it by the same amount you can exert a constant force on the trolley.

f Attach a new length of ticker tape. Produce a ticker-tape record of a trolley journey with a constant force.

g Use the ticker-tape record to work out the trolley's acceleration.

#### Teaching notes

Students often 'overdo' the slope and overcompensate for friction. That is why they should use a ticker-tape record to test whether speed is constant.

This experiment was safety-checked in March 2005