Gravity between every pair of masses

Science · Forces & Motion · ages 16-17

Name ______________________ Date ____________

Gravity between two masses is always attractive.
Circle one: True False
Along which line does gravity act?
- The line joining the centres
- The line to the nearest surface
- A tangent to the orbit
What are the units of gravitational field strength?
- joules per kilogram
- newtons per kilogram
- newtons per metre
Surface g is 9.8 N/kg. At two Earth radii from the centre, g falls to a quarter. Give g there in N/kg.
Answer: ______________
Two asteroids double their separation. What happens to the force?
- It quarters
- It halves
- It doubles
An apple falls while the Moon orbits. What do the two motions share?
- Both need air to push against
- Both need the same speed
- Both are caused by Earth's gravity
Take surface g as 9.8 N/kg. A moon orbits at 4 Earth radii from the centre. Give g there in N/kg.
Answer: ______________
A planet has Earth's mass but twice Earth's radius. How does its surface g compare?
- Half of Earth's
- The same as Earth's
- A quarter of Earth's

Answer key

For grown-ups. Fold this page away before handing over the rest.

True · There is no repulsive form of gravity between masses.
The line joining the centres · Each mass pulls the other along their joining line.
newtons per kilogram · Field strength is force per unit mass.
2.45 · Quarter the surface value: 9.8 divided by 4 is 2.45.
It quarters · Inverse square means double distance gives one quarter force.
Both are caused by Earth's gravity · One force explains both the falling apple and the orbiting Moon.
0.6125 · Four times the radius gives one sixteenth: 9.8 over 16 is 0.6125.
A quarter of Earth's · Same M with doubled R gives one quarter from G M over R squared.