Mass and weight
Imagine pulling a 50 kg crate across a smooth floor. It's
difficult because the mass of the crate is large, 50 kg.
Now think about what
happens when you take the same box to the Moon and try to drag it across a floor of just the
same type as the one on the Earth – it will be just as difficult because the MASS of the box
has not changed. It is still 50 kg.
(Ignore any friction between the box and the
floor)
Now imagine
LIFTING the crate on the Earth. This time the important thing is the force of the Earth's
gravity on the crate. As you know gravity acts downwards. So, lifting up the crate
means that we have to move it against the force of the Earth's gravity.
The force of the Earth’s gravity on every kilogram is about 10 N.
Put another way the
gravitational attraction of the Earth is about 10 N/kg. (At sea level this attraction is nearer 9.8
N/kg).
We call the force of attraction of the Earth’s gravity on an object the WEIGHT of the object
The weight of an object
is a force and is so is measured in Newtons, while the mass of an object is measured in
kilograms.
To find the weight of an object you simply multiply its mass (in kilograms)
by the force of the Earth's gravity on 1 kg (10N).
Weight (W, Newtons) = Mass (m, kg) x Gravitational field strength (g, N/kg)
On the Earth the weight of our crate will be 50x10 = 500
N
Now imagine taking the crate to the
Moon and lifting it up there. Its mass is still 50 kg but the Moon's gravitational pull is only
about 1/6 of the Earth's – in other words about 1.6 N/kg. This means that the weight of our
crate on the Moon will be 50x1.6 = 80 N and so it will be much easier to lift up.
Since
Force = mass x acceleration we could also find the weight of an object by multiplying its
mass by the acceleration of free fall. You would get the same numbers as in the examples
above because the acceleration in free fall is 10 m/s2 on the Earth and 1.6 m/s
^{2} on the
Moon.
Some examples of other masses are shown in the
table.
Mass on the Earth 
Weight on the Earth 

Mass on the Moon 
Weight on the Moon 
100 g 
1 N 

100 g 
0.16 N 
60 kg 
600 N 

60 kg 
9.6 N 
100 kg 
1000 N 

100 kg 
160 N 
1000 kg 
10 000 N 

1000 kg 
1600 N 
On other planets
the strength of the gravitational field and the acceleration in free fall is different from that on
the Earth and so our crate would weigh different amounts if taken to these planets. The table
below gives you some weights of our 50 kg crate on other
planets.
Planet 
Weight of the 50 kg crate 
Mercury 
190 N 
Venus 
440 N 
Earth 
500 N 
Mars 
190 N 
Jupiter 
1245 N 
Saturn 
520 N 
Uranus 
520 N 
Neptune 
690 N 
(Pluto) 
14.5 N 
(Remember that its
MASS is the same everywhere including in deep space or in orbit round any planet where it
would be weightless!)
It's interesting to look at the weights of our crate on Earth
and on Saturn or Uranus. They are almost the same. That means if you were to go to Saturn
or Uranus you would weigh just about the same as you do here. However on Pluto you would
be lighter than on the Moon. What do you think that means about astronauts' athletic records
on Pluto?
On the surface of our Sun the gravity pull is so strong that our crate would
weigh an enormous 13 700 N!
The mass of an object stays the same no matter where it is in the Universe. The weight of an object changes depending on the gravitational pull at the place where it is.