# Newton's Laws

## Newton's Laws Revision

**Newton’s Laws**

**Isaac Newton** was famous for many theories and models in Physics. In this section we look at his three **laws of motion**.

**Newton’s First Law**

**Newton’s first law** states:

“A body will **remain at rest** or move with a **constant velocity** unless acted on by a **resultant force**”.

In the** free-body diagram** above, we have balanced** forces **acting vertically. Therefore, according to **Newton’s first law**, the ball will remain at rest unless an external** force** acts upon it.

**Newton’s Second Law**

**Newton’s second law** states:

“The resultant force on an object is equal to its rate of change of **momentum**”.

This is often more commonly given by the following equation:

F = ma

- F is the
**force**in Newtons \left(\text{N}\right). - m is the
**mass**in kilograms \left(\text{kg}\right). - a is the
**acceleration**in metres per second \left(\text{ms}^{-2}\right).

In application, this means that for any object, if a **resultant force** is applied, the object will **accelerate**. It is important to note that a **change in direction** is also a form of acceleration. Therefore, if a force is applied an object may speed up, slow down or change direction.

We can also take from this that the acceleration experienced by the object is directly proportional to the resultant force exerted upon the object: F \propto a .

**Newton’s Third Law**

**Newton’s third law** states:

“If one body exerts a **resultant for****ce** on another body, then the second body will exert an equal and opposite force back on the first body”.

This is sometimes shortened to “every action has an equal and opposite reaction”.

If we take another look at this diagram we have the **gravitational force** acting downwards on the object. This may also be referred to as the ball’s weight.

According to **Newton’s third la****w**, the floor then produces an equal magnitude but opposite direction force back on the ball, keeping the forces** balanced**. This force is known as the **normal reaction force**.

## Newton's Laws Example Questions

**Question 1:** Describe one appropriate example of Newton’s first law.

**[1 mark]**

An object flying through space will continue to move at the **same speed**, in the** same direction** unless acted on by a **resultant force.**

Other examples could include a car moving at a constant speed, or a ball falling at a constant speed.

**Question 2:** During a car crash a 70 \: \text{kg} person is decelerated at 300 \: \text{ms}^{-2}. Calculate the force applied to the person.

**[2 marks]**

**Question 3:** Use Newton’s third law to explain how a rocket propels itself through the air.

**[3 marks]**

According to Newton’s third law, if an object exerts a force on another, the second object exerts a force back, equal and opposite.

So as the **rocket’s thrusters are expelling gases** out of them, the **gases collide with particles in the air, exerting a force.** The **air particles then exert an equal and opposite force on the thruster,** pushing the rocket through the air.