Gravitational field strength

Question 1

What is the gravitational field?

Answer 1

the force field round a mass

any object with a mass attracts any other mass

the second mass also has a force field around itself and pulls on the first object with an equal force in the opposite direction

Question 2

What is the field line?

Answer 2

the path which a mass would follow when attracted by gravity

also called the line of force

directed at the centre usually

Question 3

What is the definition of the strength of a gravitational field?

Answer 3

g is the force per uni mass on a small test mass placed in the field

unit is N kg^-1

Question 4

Why does the test mass need to be small?

Answer 4

otherwise it might pull so much on the other object that it changes its position and alters the field

Question 5

What is the weigh of an object?

Answer 5

the force of gravity on it

F = mg

Question 6

What is g ?

Answer 6

g is the gravitation field strength at the objects position

if the object is not acted on by any other force it accelerates, the object is unsupported

a = force / mass = mg / m = g

the object therefore falls freely with acceleration g

Question 7

What is radial field?

Answer 7

a field pattern where the field lines are like the spokes of a wheel , always directed at the centre

e.g. a small mass is always directed towards the centre of a larger spherical mass

the magnitude of g in a radial field decreases with increasing distance from the massive body

Question 8

What is uniform firled?

Answer 8

a field patterns where the gravitational field strength is the same in magnitude and direction throughout the field

the field lines are therefore parallel to one another and equally spaced

Question 9

Is the Earth’s gravitational field uniform or radial?

Answer 9

• the force of g due to the Earth on a small mass decreases with distance from the earth
• so the field is therefore radial
• over small distances much less than the earths radius the change of gravitational field strength is inisgnificant
• only over distances that are small compared with the Earth;s radius can the Earth’s field be considered uniform

Question 10

What is thee quation for the variation of g with distance from the surface of a spherical body of mass?

Answer 10

g = g_sR² / r²

R is radius

r is distance from the position to the centre of the planet

derived from

• 𝑔 = 𝐺M / 𝑟²
• GM = constant
• therefore g_sR² = gr²