Movement and Position » Practical: Investigating Motion

Investigating motion

This is a practical named in the specification: investigate the motion of everyday objects such as toy cars or tennis balls. Exam questions can ask you to describe the method, work with a set of results or suggest improvements. You can answer them all from this lesson, even if you have never done the practical in a laboratory. It is also easy to try at home with a plank, some books and the stopwatch on a phone.

Every motion experiment comes down to two measurements: a distance and a time. Remember, average speed is distance moved divided by time taken.

Key terms:

• Independent variable: the one thing you change on purpose.
• Dependent variable: the thing you measure to see the effect.
• Control variables: the things you keep the same so the test is fair.
• Mean: an average. Add the repeat readings together, then divide by how many there are.
• Reaction time: the short delay between seeing something happen and pressing the stopwatch button.
• Light gate: a sensor that shines a beam of light across the track and detects the moment an object breaks the beam.
• Data logger: an electronic device that records readings from sensors, such as the times from light gates.
• Ticker timer: a device that prints dots on a paper tape at a steady rate as a moving object pulls the tape through it.

Measuring distance and time

Measure short distances with a metre rule and longer ones with a tape measure. Mark a clear start line and finish line, and always measure from the same part of the object, such as the front of the car.

The simplest way to measure time is a stopwatch. Its weakness is you. Your reaction time is about 0.2 s, and you have to react twice: once to start the watch and once to stop it. If the journey takes 10 s this hardly matters. If it takes less than 1 s, the error is a large part of the reading.

These three methods remove reaction time.

Experiment 1: a toy car on a ramp

Aim: to find out how the height of a ramp changes the average speed of a toy car.

Apparatus: a long, smooth board resting on a pile of books to make a ramp, a toy car, a metre rule and a stopwatch (or two light gates and a data logger).

1. Set up the ramp. Measure its height at the raised end with the metre rule.
2. Mark a start line near the top of the ramp and a finish line near the bottom. Measure the distance between them.
3. Hold the car still with its front on the start line.
4. Let go of the car without pushing it and start the stopwatch at the same moment.
5. Stop the stopwatch when the front of the car reaches the finish line. Record the time.
6. Repeat twice more at the same height and work out the mean time.
7. Add a book to raise the ramp and repeat steps 1 to 6 for at least four different heights.
8. Calculate the average speed of the car for each height.

Here is a set of results. The distance between the lines was 1.20 m each time.

Ramp height (cm)	Time 1 (s)	Time 2 (s)	Time 3 (s)	Mean time (s)	Average speed (m/s)
10	2.04	1.97	1.99	2.00	0.60
15	1.52	1.47	1.51	1.50	0.80
20	1.23	1.27	1.25	1.25	0.96
25	1.10	1.14	1.12	1.12	1.1

Conclusion: as the height of the ramp increases, the mean time falls, so the average speed of the car increases.

Why repeat? No two timings are exactly the same. Taking a mean of three readings evens out the small random differences. Repeats also show up a reading that does not fit. If one time at 20 cm had been 1.61 s, it would stand out from the others. You would measure it again and leave the odd result out of the mean.

A distance-time graph for the car

A stopwatch gives only one time for the whole run. A ticker timer or a video shows where the car was at many moments on the way down. This table comes from a video of the car on the 20 cm ramp, taken at 20 frames each second. The position of the car was read from every fifth frame, which is every 0.25 s.

Time (s)	0	0.25	0.50	0.75	1.00	1.25
Distance (cm)	0	5	19	43	77	120

Plot time along the bottom and distance up the side, as you did in the lesson Distance-Time Graphs. The points make a smooth curve that gets steeper. This shows that the car speeds up all the way down the ramp. It is also why we call 0.96 m/s the average speed: the car was slower than this at the top and faster at the bottom.

Experiment 2: a tennis ball

A falling ball. Use a tape measure to mark a height of 2.0 m on a wall. Hold a tennis ball with its bottom edge level with the mark. Let go and time the fall to the floor. Three stopwatch times of 0.66 s, 0.61 s and 0.65 s give a mean of 0.64 s.

average speed = 2.0 ÷ 0.64 = 3.1 m/s

This answer is not very trustworthy. A reaction time of about 0.2 s is nearly a third of the 0.64 s being measured. To improve the experiment, film the fall next to the tape measure and count frames, use light gates, or drop the ball from a greater height so the time is longer.

A rolling ball. Mark two lines 5.0 m apart on a smooth, level floor. Roll the ball so it crosses both lines, and time it between them. A longer distance gives a longer time, so the reaction time error matters much less.

Safety

• Put a box or cushion at the bottom of the ramp to catch the car, so it does not shoot across the floor.
• Keep the ramp away from the edge of the table and make sure the pile of books is stable.
• Pick up cars and balls straight away so nobody steps on one and slips.
• Do not climb on a chair or table to drop the ball, and make sure nobody is underneath it.

Exam-style question

A student investigates how the height of a ramp affects the average speed of a toy car. She releases the car from a start line and uses a stopwatch to time it to a finish line 0.90 m further down the ramp. For one height, her three times are 1.48 s, 1.52 s and 1.50 s.

(a) State the independent variable in this investigation. [1 mark]

(b) State one variable the student should keep the same. [1 mark]

(c) Calculate the average speed of the car for this height. Use the mean of the three times. [3 marks]

(d) Explain why using light gates and a data logger instead of a stopwatch would improve this investigation. [2 marks]