1.4 Forces at a distance
Summary: If you let go of your pen and it moved up towards the ceiling, you would be very surprised. We are so familiar with the force of gravity that sometimes we do not even think of it as a force. Tags: science oxford-science student-book-7 physics Created: 2026-05-19 Last Updated: 2026-05-19
After this topic, you will be able to:
- describe the effects of fields acting on an object
- describe the difference between Weight and mass
- calculate weight.
Think back
- What do unsupported objects do?
- What is the force that acts on falling objects?
- Is Gravity a contact or non-contact force?
Key idea Non-contact Forces act from a distance in a field. Magnetic forces, electrostatic forces, and gravity are all non-contact forces.
Key words magnetic force, electrostatic force, field, weight, mass, kilogram (kg), Gravitational Field Strength
If you let go of your pen and it moved up towards the ceiling, you would be very surprised. We are so familiar with the force of gravity that sometimes we do not even think of it as a force.
Which forces act ‘at a distance’?
A Gravitational Force acts on your pen when you hold it up and let it go. Gravity is a non-contact force, and it is always attractive. There are other types of non-contact force.
Magnets exert a magnetic force on magnetic materials or other magnets without touching them. If you rub a balloon, you can pick up bits of paper with it or make your hair stand on end. You can see this happening in Figure 1. This is an electric or electrostatic force. Magnetic and electrostatic forces are non-contact forces that can be attractive or repulsive.
Figure 1A magnet picks up metal filings, and a balloon rubbed on a jumper can attract a baby’s hair.
A Compare the force of gravity acting on you when you lie in bed and when you sit on a chair.
What is a field?
In physics, a field is a special region in which something experiences a force. In a magnetic field, magnetic things experience a force. In a gravitational field, things with mass experience a force.
Gravitational, magnetic, and electrostatic fields have something in common. As you get further away from the mass, magnet, or charge, the field gets weaker.
Contact forces only act when objects are touching each other. Non-contact forces act at any distance, if the objects are touching or if they are not touching.
B Explain how you know that there is a gravitational field around you.
What do I weigh?
You can use a newton meter to find the weight of an apple. Earth pulls the apple down, towards the ground. Measuring the weight of the apple means measuring the force of Earth on it.
What is the difference between weight and mass?
Weight is a force, so it is measured in newtons (N). Mass is the amount of ‘stuff’ something is made up of. It is a measure of how hard it is to get something to move. Mass is measured in kilograms (kg). Figure 2 shows one way of measuring weight.
Figure 2An apple has a weight of about 1 N.
You can calculate weight using an equation.
On Earth, the gravitational field strength is about 10 N/kg. This means that the force acting on one kilogram is 10 N. If your mass is 50 kg, for example, then your weight on Earth is:
Gravitational field strength is different on other planets and stars. Your weight would be different on different planets because g would be different.
The Apollo astronauts could jump much higher on the Moon. This is because g on the Moon is about one-sixth of g on Earth.
Maths skills Smaller masses are measured in grams (g). There are 1000 g in 1 kilogram (kg). Convert these masses into grams: a 2 kg b 3.5 kg c 0.4 kg Convert these masses into kilograms: d 4700 g e 250 g
What would happen to my weight in space?
Imagine firing off from Earth in a spacecraft. As you move away from Earth, its gravitational field gets weaker. If you stood on scales (a balance) in the spacecraft, the reading would be less than it would be on Earth. Eventually the scale would read zero, but the amount of ‘you’ would not change — your mass stays the same. It is the force of Earth on you, your weight, that gets smaller.
C Compare your mass on Earth with your mass on the Moon.
Stretch zone
- Copy and complete the sentences. The force of gravity acts on things that have charge/mass. You can feel a magnetic/gravitational force between two magnets. Your weight is a force/stuff and is measured in kilograms/newtons. Your mass/weight is the amount of stuff you are made up of and is measured in kilograms/newtons.
- Give one reason why your weight on Jupiter is 2.5 times your weight on Earth.
Stretch zone
- Use the information on this page to: calculate the mass of an applecalculate the weight of the same apple on the Moon.