Page 284 - Physics Coursebook 2015 (A level)

Cambridge International A Level Physics
Gravitational forces and fields
According to Newton, all masses create a gravitational field in the space around them. This field gives rise to a force on any object having mass which is placed in this field. The Moon orbits the Earth because it experiences a gravitational force due to the Earth’s gravitational field.
Moon
Earth
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Figure 18.1 Skydivers balance the forces of gravity and air resistance.
We live our lives with the constant experience of gravity. We know that things fall when we drop them. The free-fall parachutists in Figure 18.1 are enjoying the experience of falling through the air under the influence of gravity.
The Earth’s gravitational force extends well beyond its surface. The Moon stays in its orbit, at a distance
of about 400 000 km away, because of the Earth’s gravitational pull (Figure 18.2). The Earth orbits the Sun at a distance of 150 000 000 km because of the gravitational force between them.
Figure 18.2 The Moon orbits the Earth. There is an attractive gravitational force acting on the Moon due to its mass and the mass of the Earth.
In physics, the idea of a field is a very general one.
If an object is placed in a gravitational field, a force will act on the object because of its mass. In Chapter 8, we saw that a charged object experiences a force when it is placed in an electric field. You are probably familiar with the idea that a magnet produces a magnetic field around itself, and that this will produce a force (attractive or repulsive) on another magnet placed nearby.
Representing a gravitational field
In Chapter 8 we saw how to represent an electric field using field lines. In a similar way, we can represent the Earth’s gravitational field by drawing field lines, as shown in Figure 18.3. The field lines show two things:
■■ The arrows on the field lines show us the direction of the gravitational force on a mass placed in the field.
■■ The spacing of the field lines indicates the strength of the gravitational field – the further apart they are, the weaker the field.
The drawing of the Earth’s gravitational field shows that all objects are attracted towards the centre of the Earth. This is true even if they are below the surface of the Earth. The gravitational force gets weaker as you get further away from the Earth’s surface – this is shown by the greater separation between the field lines. The Earth is almost a
field lines
Figure 18.3 The Earth’s gravitational field is represented by field lines.
uniform spherical mass – it bulges a bit at the equator. The gravitational field of the Earth is as if its entire mass was concentrated at its centre. As far as any object beyond the Earth’s surface is concerned, the Earth behaves as a
point mass.