Name three sources of force fields at A Level.

Mass (gravitational), static charge (electric) and moving charges (magnetic).

How is the direction of a field vector determined?

By inspection, for example towards the mass for gravity or away from a positive charge.

State Newton's law of gravitation.

The force between two point masses is F = Gm1m2/r^2, attractive and along the line joining them.

What is the value of G?

6.67 \times 10^{-11}\ N\,m^2\,kg^{-2}.

What happens to the gravitational force if the separation is trebled?

It falls to one ninth of its original value.

Which distance is used for spheres?

The distance between their centres, since a uniform sphere acts as a point mass at its centre.

How do the forces on two attracting masses compare?

Equal in magnitude and opposite in direction: a Newton's third law pair.

Give one similarity between gravitational and electrostatic forces.

Both obey an inverse square law for point sources (also field lines, potential, equipotentials).

Give one difference between gravitational and electrostatic forces.

Gravity is always attractive; electrostatic forces may attract or repel.

Where is the resultant force zero for a mass between two other masses?

Where the two gravitational forces are equal in magnitude and opposite in direction.

Force fields and Newton's law of gravitationAQA A-Level Physics: Flashcards

Card 1 of 120 of 12 known

Question

What is a force field?

Tap or press Space to reveal

Tap card or press Space to flip

See all 12 cards

What is a force field?: A region in which a body experiences a non-contact force.
Name three sources of force fields at A Level.: Mass (gravitational), static charge (electric) and moving charges (magnetic).
How is the direction of a field vector determined?: By inspection, for example towards the mass for gravity or away from a positive charge.
State Newton's law of gravitation.: The force between two point masses is $F = Gm_1m_2/r^2$ , attractive and along the line joining them.
What is the value of G?: $6.67 \times 10^{-11}\ \mathrm{N\,m^2\,kg^{-2}}$ .
What happens to the gravitational force if the separation is trebled?: It falls to one ninth of its original value.
What happens to the force if both masses are doubled and the separation is unchanged?: It becomes four times larger.
Which distance is used for spheres?: The distance between their centres, since a uniform sphere acts as a point mass at its centre.
How do the forces on two attracting masses compare?: Equal in magnitude and opposite in direction: a Newton's third law pair.
Give one similarity between gravitational and electrostatic forces.: Both obey an inverse square law for point sources (also field lines, potential, equipotentials).
Give one difference between gravitational and electrostatic forces.: Gravity is always attractive; electrostatic forces may attract or repel.
Where is the resultant force zero for a mass between two other masses?: Where the two gravitational forces are equal in magnitude and opposite in direction.

Exam questions on Force fields and Newton's law of gravitation

A torsion-balance experiment measures the gravitational attraction between a large lead sphere of mass 15 kg and a small lead sphere of mass 0.015 kg. The centres of the spheres are 0.060 m apart and the spheres are uncharged.
Calculate the magnitude of the gravitational force on the small sphere. Use $G = 6.67 \times 10^{-11}\ \mathrm{N\,m^2\,kg^{-2}}$ .2 marks
A student compares the gravitational field around a planet with the electric field around an isolated, positively charged metal sphere. Both bodies are treated as point sources.
Explain what is meant by a force field, and why a small mass near the planet and a small charge near the sphere are both described as being in a force field.2 marks
The Earth has mass 5.97 × 10²⁴ kg and the Moon has mass 7.35 × 10²² kg. Their centres are 3.84 × 10⁸ m apart. Treat both bodies as point masses at their centres.
Calculate the magnitude of the gravitational force of the Earth on the Moon. Use $G = 6.67 \times 10^{-11}\ \mathrm{N\,m^2\,kg^{-2}}$ .3 marks

See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).