Topic 10: Fields

Question 1

How does a diagram showing gravitational field lines show that the gravitational field strength of the planet decreases as you move further from the surface of the planet?

Answer 1

The field lines grow further apart as you move away from the surface

Question 2

How would you calculate the speed of a meteor very far from Planet X with a negligibly small speed if the planet has no atmosphere?

Answer 2

• Calculate the loss in gravitational potential energy, GMm/r
• Equate it to kinetic energy as the energy is being transferred from gravitational to kinetic, 1/2mv^2
• rearrange to calculate v, ignoring the mass as it can be cancelled from both sides of the equation

Question 3

Explain what is meant by the gravitational potential at the surface of a planet.

Answer 3

The gravitational work done per unit mass in moving an object from infinity to that point

Question 4

What does Keppler’s 3rd Law express?

Answer 4

The farther a planet is from the Sun, the slower its orbital speed, and vice versa