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The field concept questions
A field is a region in which a body feels a force without anything touching it, and the same idea covers gravity, electricity and magnetism. Gravitational and electric fields turn out to follow almost identical equations, with one difference that matters: gravity only ever attracts.
14 original questions · 36 marks · the field concept notes · Gravitational fields
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Define a force field.
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A region in which a body experiences a non-contact force (1); due to a property of the body such as its mass or charge (1).State the property of a body that gives rise to each of the following: a gravitational field, an electric field, a magnetic field.
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Gravitational field: mass; electric field: (static) charge (1); magnetic field: moving charge (or a permanent magnet) (1).A mass of 2.0 kg experiences a gravitational force of 19.6 N. Calculate the gravitational field strength at this point.
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g = F/m = 19.6/2.0 (1)
g = 9.8 N kg−1 (1)A diagram shows the gravitational field lines of a planet. State what is indicated by the direction of the arrow on a field line, and by the spacing of the lines.
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The arrow shows the direction of the (gravitational) force on a mass placed at that point (1); the spacing shows the strength of the field, with closer lines meaning a stronger field (1).Explain why two field lines can never cross.
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At a crossing point the field (force) would have two different directions at once (1); the force on a body at any point acts in only one direction, so lines cannot cross (1).Compare the gravitational force and the electric force between two point objects. State one similarity and two differences.
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Similarity: both obey an inverse-square law with separation (or both act at a distance) (1). Differences: gravitational force is always attractive whereas the electric force can be attractive or repulsive (1); the electric force between two charged particles is very much stronger than the gravitational force between them (1).A rock of mass 5.0 kg on the surface of a planet experiences a gravitational force of 40 N. Calculate the gravitational field strength at the surface of the planet.
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g = F/m = 40/5.0 (1)
g = 8.0 N kg−1 (1)Define gravitational field strength and state an appropriate unit for it.
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Force per unit mass on a small test mass placed at the point, g = F/m (1); unit: N kg−1 (or m s−2) (1).The table gives the gravitational field strength at the surface of three moons of a large planet.
moon P: 1.3 N kg−1
moon Q: 0.90 N kg−1
moon R: 2.2 N kg−1
A lander of mass 350 kg is damaged on touchdown if its weight exceeds 600 N. Deduce on which of the moons the lander can touch down safely.Explain why the Earth's gravitational field can be treated as uniform in a laboratory experiment, but must be treated as radial when analysing the motion of a distant satellite.
Describe the gravitational field line pattern around an isolated point mass, and the pattern in a uniform field close to the Earth's surface.
A space probe of mass 10 kg experiences a gravitational force of 82 N at a point above a planet. Calculate the gravitational field strength at that point and state the direction of the field.
Explain why gravitational forces are always attractive, whereas electric forces can be either attractive or repulsive.
A planet and its smaller moon are separated by a fixed distance. Explain why there is a point on the line joining their centres at which the resultant gravitational field strength is zero, and state, with a reason, whether this point lies closer to the planet or to the moon.
The same practice on paper: the printable workbook for this topic, questions and a worked answer book.
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