Newton's second law and gravity

Home learning focus

Learn about Newton's second law of motion.

This lesson includes:

  • one video
  • one activity

Learn

In this video, two students investigate Newton's 2nd law of motion.

Investigating F = ma

Newton's second law of motion

Newton's second law of motion can be described by this equation:

Force = mass × acceleration

This is when:

  • force is measured in newtons (N)
  • mass is measured in kilograms (kg)
  • acceleration is measured in metres per second squared (m/s²)

This means that acceleration of an object:

  • increases if the resultant force on it increases
  • decreases if the mass of the object increases.

Example

Calculate the force needed to accelerate a 22 kg cheetah at 15 m/s².

Force = mass × acceleration

Force = 22 × 15

Force = 330 N

Gravitational field strength

Gravitational field strength is the acceleration of an object due to gravity. Gravity is the force that keeps us on the ground and always acts downwards, towards the centre of the Earth.

Watch this video where Jon Chase explains gravitational field strength in more detail.

Explaining gravitational field strength

All objects with mass have a gravitational field around them. A gravitational field is where a mass experiences a force.

The formula that links mass and gravitational field strength is:

gravity force = mass × gravitational field strength (g)

  • gravity force is measured in newtons (N)
  • mass is measured in kilograms (kg)
  • gravitational field strength (g) is measured in newtons per kilogram (N/kg)

The Earth’s gravitational field strength (g) is 10 N/kg. This means that for each kg of mass, an object will experience 10 N of force.

Weight

Weight and gravity force are the same. Therefore we again use F=mg to calculate the weight of an object.

Example:

Calculate the gravity force (weight) of a skydiver with a mass 70 kg falling towards the Earth?

(Remember that gravitational field strength = 10 N/kg on Earth).

weight = mass × gravitational field strength

weight = 70 × 10

weight = 700 N

Practise

There are lots of ways to try out your science skills.

Activity 1

Gravity worksheet

See how much you remember about gravity and gravitational field strength with this downloadable worksheet from Beyond.

Gravity questions

Activity 2

Gravity calculations

See how much you remember about gravitational field strength with this downloadable worksheet from Teachit Science.

Gravity calculations

There's more to learn

Have a look at these other resources around the BBC and the web.

KS3 Physics
11-14 Physics
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