# 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.

## 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.

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.

## Activity 2

### Gravity calculations

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

# There's more to learn

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