Force and Motion

Programme Outline

This programme defines speed and acceleration. A glider on a linear air track is used to measure speed and acceleration in the laboratory, and the results are shown graphically. Students are asked to speculate on the effects of increasing and decreasing speeds on speed graphs and increasing pulling powers on acceleration graphs. Finally, the factors that affect falling objects are considered and you see how a falling object accelerates under the force of gravity, until it reaches a maximum acceleration.

The programme is divided into three parts:

13.1 Speed
13.2 Acceleration
13.3 Gravity and Falling

13.1 Speed

Speed is defined and measured using a glider on an almost friction-free linear air track. The track has gates every 0.5m. Light beams, connected to timers, are shone across these gates. When the passing glider breaks the light beam across a gate, the timer stops. The average speed (distance/time) of the glider, between each gate, can then be calculated and plotted. When speeds are constant, the result will be a straight-line graph.

v (speed) = s (distance travelled) / t (time taken)

A bicycle computer records its average speed at regular distances as the bike increases speed. These speeds are plotted and produce a steeper graph.

What will the graph of a slower moving vehicle look like?
Answer: It will be less steep.

13.2 Acceleration

Very few things travel at constant speed. When speed increases we call it acceleration. This is demonstrated using time-lapse photography of a golf club swing. The glider, on the linear air track, is attached to a tape weighted at the end, which makes it accelerate. The time it takes to travel each 0.5m is recorded. When distance travelled is plotted against time, the graph is a curve. When the pulling mass is increased, the graph curve is steeper. A further mass is added and the following readings were taken, which students can use to plot the acceleration graphs.

	Distance travelled (m)	0.5	1.0	1.5
Test 1	Time taken (s)	1.8	2.6	3.3
Test 2	Time taken (s)	1.4	2.0	2.4
Test 3	Time taken (s)	1.1	1.6	1.9

13.3 Gravity and Falling

Bungee jumping, rain falling and high diving are shown as examples of the effects of gravity as a pulling force. We also see the interior of a space capsule to show the effect of free fall. Do all divers fall at the same rate? Does their mass make a difference?

Two balls, of the same size but differing mass, are dropped from the same height, to see if mass affects the rate of fall. It does not. Balls of similar mass, but very different sizes are dropped from the same height. Will they fall at the same or different rates? The larger ball is slower because of the frictional resistance of the air molecules against it.

Watching a film of a melon dropped from a tall building, you can see that its speed increases as it falls. It accelerates at a constant rate of 10 m/s/s.