Friction compensated runway
Classroom Activity for 11-14
What the Activity is for
Forces on a moving object.
The purpose of this activity is to revise with pupils the idea that even when an object is moving there can be a number of forces acting on it which add to zero.
What to Prepare
- motion sensor light gates and software
- books or clamps to adjust height of runway
What Happens During this Activity
This is a demonstration activity in which the slope of a runway is adjusted to make a trolley move down it at a constant speed.
With the runway horizontal, push the trolley and note that it slows down. The final speed is less than the initial speed. Why does it slow down? The slowing down indicates that frictional forces are acting on the trolley in a horizontal plane. Raise the runway and repeat the experiment so that the trolley speeds up. Now the force of gravity pulling down the slope is greater than the frictional force.
Finally, reduce the slope so that the trolley runs down the runway at a constant rate. At this point the runway has been friction compensated.
This means that the gravitational force acting to pull the trolley forwards is opposed by the frictional force acting against the moving object. If these forces add to zero then the trolley moves at a constant speed.
This demonstration can be carried out so that pupils judge with their own eyes whether the trolley is speeding up, slowing down, or moving at a constant speed. Alternatively use a pair of light gates or other motion sensors linked to a suitable data logger to record the times taken for the trolley to pass through the gates.