Combined Science
Physics
Cambridge IGCSE
Physics
Cambridge IGCSE
TOPIC 1A: FORCES and MOTION
This entire topic is about forces and how force affects the movement of all things. There are many types of forces, and many ways we can measure movement.
Although this topic seems common sense to many, there are several areas of science in this topic not fully understood: Magnets can push and pull, but gravity can only pull. Why is that? What if we could control gravity like we control electricity, and we could make it push instead of pull? Recently the discovery of gravity waves was announced, which produce a varying pull on all objects in as the wave passes Earth.
Did you know that light rays can produce a force? Scientists have proposed a method of using light from the Sun to drive spaceships in the future as shown here:
Fig 1 . A Solar Sail for future space travel
(NASA - public domain)
The study of how forces affect motion is still at the forefront of science.
To begin this topic, we need to know how to take measurements effectively, especially when investigating motion: Some of this you should have covered already in previous learning, but basic measuring techniques are still included in the IGCSE syllabus.
Many of the experiments required for this IGCSE course require you to accurately measure mass, time, length and volume. There are standard ways of doing this and some subject specific vocabulary that you should be aware of:
For small masses, always use a laboratory 'electronic balance'. This can also be called a digital scale or 'top pan balance' but avoid 'weighing scales'. Some measure to the nearest gram, some to 0.001 g accuracy, so take care when you record your results to always use the right number of sig.figs.
In this example, the mass of the block is 103.00g. DO NOT round this up to 103 g in your results table! You want to show that you used a very accurate balance, so show all of the decimal points.
You should be able to use stopwatches for experiments, as well as other forms of timer. One very useful timing device used in motion experiments is called a light gate - these connect to a computer or device that records when the infrared beam has been broken as something moves through it. The video here explains how they can be used.
YouTube: How to use light gates (Underdog Physics)
Light gates are a good device to name in the evaluation section of a motion experiment - they always give more accurate times and do not rely on human reaction times, as do stop watches.
Another useful technique is to record an experiment on a video (using a phone or laptop) and then watch it back in slow motion - the exact time an event occurs can be seen from the video timer and recorded.
To measure length we clearly need a ruler. Take care to measure from a position at 90° to the ruler, to avoid what is called 'parallax error' as shown in this diagram. If you are viewing the ruler scale from any other position, you can measure the incorrect reading as shown here.
Regular solid volumes such as a wooden block can be measured using a ruler, and the volume calculated. For lab experiments, volume is usually given in cm3, although the official international unit is the m3. Liquids are usually measured in litres and millilitres. Did you know that 1 ml is the same volume as 1 cm3?
Section 1.4 on density explains some standard methods for measuring the volume of irregular objects.
One of the most common mistakes on IGCSE science coursework or in the practical exam is describing how to measure the volume of a liquid using a beaker. These have guide measurement lines but they are NOT accurate! Always use a measuring cylinder!
During all of your IGCSE science practical work, you should aim to collect the best results you can, giving the most accurate results possible. There are several standard techniques to help you do this. The most obvious one which is often overlooked, is to take repeat readings and then take an average. If you have random errors in your experiment, perhaps due to human error pressing the stop watch button at the end of an event, then taking repeats and an average value can reduce the effect of these random errors.
Note that scientists always do more than 2 repeats, so that if an anomalous result is collected, it is easily spotted when compared to the others.
The second standard technique common in physics is to measure multiples rather than individual measurements. For example, rather than measuring the thickness of a single page of a book, measure the thickness of 200 pages and then divide by 200 to find the answer for one page. This can also be used for timing events. Here is a simple pendulum. Instead of measuring the time period T for one swing, time for 10 swings and then divide by 10 to find a more accurate answer.
This technique reduces errors - for example if you can only time to an accuracy of within ±0.2 seconds, when you divide by 10, the accuracy becomes ±0.02 s! Much improved.
Questions:
1. What is the time period T of the pendulum shown above? (Note: One swing is 'there and back again')
| Time Period, T (seconds) |
| 27.88 |
| 27.75 |
| 27.90 |
| 25.23 |
| 27.70 |
There is an anomalous result for the 4th timing. Maybe the student only timed 9 swings by accident? Ignoring this result, the other 4 give an average for 10 swings of 27.8075 seconds.
Therefore T = 27.8075 /10 = 2.78075 seconds.
We now need to round up as the student was clearly not that accurate! For 10 swings it looks like the results varied by about ±0.1 seconds for 10 swings, so ±0.01 seconds for 1 swing, so it makes sense to round to the nearest 1/100th of a second:
T = 2.78 seconds
(Note: You will not be expected to justify why you rounded up - 3 significant figures is the best advice for nearly all experiments).
