Newton's Laws

This section explains Newton’s First, Second and Third Laws for GCSE Physics.

Newton's First Law

Newton's First Law is also known as the Law of Inertia. It states that:

"An object will remain at rest, or continue to move with a constant velocity, unless acted upon by a resultant external force."

In simpler terms, an object will not change its state of motion (i.e., it will not start moving, stop moving, or change direction) unless a force is applied to it. This is because objects have inertia—the tendency to resist changes to their motion.

Key Points:

• If no external force acts on an object at rest, it will stay at rest.
• If no external force acts on a moving object, it will continue to move at a constant speed in a straight line.
• The greater the mass of an object, the greater its inertia.

Newton's Second Law

Newton's Second Law describes the relationship between the force applied to an object, its mass, and the resulting acceleration. It states that:

"The force acting on an object is directly proportional to the acceleration of the object and is in the same direction as the acceleration. The force is also inversely proportional to the object's mass."

In mathematical terms, this law is expressed as:

$$F = m \times a$$

Where:

• F is the force applied to the object in Newtons (N).
• m is the mass of the object in kilograms (kg).
• a is the acceleration of the object in metres per second squared (m/s²).

Explanation:

• The greater the force applied to an object, the greater the acceleration it will experience, assuming the mass remains constant.
• The greater the mass of an object, the smaller the acceleration for a given force (because the object resists acceleration more due to its larger mass).

Example of Newton's Second Law Formula

A car of mass 800 kg is subjected to a force of 4000 N. To calculate the acceleration, we can use Newton's second law:

$$F = m \times a$$

Rearranging the formula to solve for a:

$$a = \frac{F}{m}$$

Substitute the known values:

$$a = \frac{4000 \, \text{N}}{800 \, \text{kg}}$$

$$a = 5 \, \text{m/s}^2$$

So, the acceleration of the car is 5 m/s².

Newton's Third Law

Newton's Third Law states that:

"For every action, there is an equal and opposite reaction."

This means that if object A applies a force on object B, object B applies an equal but opposite force on object A. These forces are always equal in magnitude and opposite in direction, but they act on different objects.

Key Points:

• Forces always come in pairs: If you push on a wall, the wall pushes back on you with an equal and opposite force.
• The action and reaction forces do not cancel each other out because they act on different objects.

Example:

If you jump off a boat, you push the boat backwards with a certain force. At the same time, the boat pushes you forward with an equal and opposite force, which allows you to move forward.

Key Points:

• Newton's First Law (The Law of Inertia) states that an object will remain at rest or continue at constant velocity unless acted on by an external force.
• Newton's Second Law relates force, mass, and acceleration, with the formula $F = m \times a$.
• Newton's Third Law explains that for every action, there is an equal and opposite reaction, with forces always acting in pairs on different objects.

These three laws are fundamental in understanding how forces affect the motion of objects and are essential concepts in GCSE Physics.