Physics
Cambridge IGCSE
Cambridge IGCSE
TOPIC 1A: FORCES and MOTION
Mass is a measure of the quantity of matter in an object. In simple terms, it relates to the number of atoms present, and how big those atoms are. The international unit for mass is the kilogram (kg).
Figure 1. This astronaut has mass, but no weight.
Weight is a measure of the pull of gravity on an object. As it is a pull, it is also a force and is therefore measured in newtons (N). On Earth, if an object has mass, then gravity pulls on it and it has weight. However in space there is no gravity, and so nothing has any weight, only mass. Not many people have been to space, and this is why many consider weight and mass to be the same. However astronauts know from practical experience that they are very different.
On Earth, the connection between mass and weight is quite simple as shown by this diagram:
Figure 2. The link between mass and weight.
In this diagram, the orange juice carton has a mass of 1 kg and gravity is pulling it downwards with a force of about 10 N.
It is similar for the bag of fruit - 5 kg mass experiences a force of about 50 N. The pull of gravity, or the weight, is always about 10 times higher than the mass. It is actually 9.8 times higher. We say that - on Earth - the gravitational field strength (given the letter 'g') is 9.8 newtons per kilogram, or 9.8 N/kg.
Gravitational field strength is defined as the force of gravity (weight) per unit mass.
We can put this idea together in a formula:
| gravitational field strength (N/kg )= | weight (N) |
| mass (kg) |
| g = | W |
| m |
Notes:
Only a few people have ever been to the Moon. You may have seen some movie clips like the one below showing astronauts on the moon's surface. There is no air, but there is gravity - it is just much weaker than on Earth because the Moon has less mass.
YouTube video - Gene Cernan 'bunny hopping' on the Moon - Nasa 1972.
As the video shows, the gravitational field strength g is lower on the Moon - it is only 1.6 N/kg. (About 6 times lower than on the Earth).
The value of 'g' varies from planet to planet, as shown here:
| Solar System Object | Gravitational Field Strength - g |
|---|---|
| Mercury | 3.7 |
| Venus | 8.9 |
| Mars | 3.7 |
| Jupiter | 23 |
| The Moon | 1.6 |
Table 1. Variations in gravitational field strength (g).
Notice that Jupiter has a much higher field strength than the Moon. This is because the mass of Jupiter is much higher than the mass of the Moon.
It is much harder to figure out why two planets - Mars and Mercury - have similar values of 'g' even though they have different masses. Why is this?
You can use the values in table 1 to calculate the weight of objects on other planets, using the formula W = m x g.
For many of these questions, the mass of an object will remain the same (assuming it is the same object), but the weight can change as it depends on the strength of gravity acting on it.
Example:
Using data from table 1, how much would a 70 kg astronaut weigh on the Mars?
Answer:
We know g = W/m, and so W = mg
m=70 kg, g=3.7 m/s2
Therefore weight = 70 x 3.7 = 259 N.
Questions:
3. If we took the 5 kg bag of fruit on a space journey, how much would it weigh in the following locations? (Use information from table 1 above).
For all these questions, We know g = W/m, and so W = mg
4. On the Moon mission in 1972, the astronauts picked up a rock to bring back to Earth. The weight of the rock was 32 N on the Moon. Calculate:
| m = | W |
| g |
| m = | 32 |
| 1.6 |
m = 20 kg.
b) If m = 20 kg, and on the Earth g = 10 N/kg,
then W = m x g
W
= 20 x 10,
Therefore W = 200 N
Now test your understanding using these quick, 10 minute questions on this topic from Grade Gorilla:
