Newton's Laws



Inertia is a resistance to a change in motion. Objects with a greater mass have a greater inertia.

Mass is the amount of matter in an object and weight is the force of gravity on your mass.

Weight (N) = Mass (kg) x Gravitational field strength (N/kg)

On Earth, the gravitational field strength is 10 N/kg.

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Newton's 1st Law

"An object will remain at rest or continue with uniform motion in a straight line unless acted upon by an external force"

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Newton's 2nd Law

"The rate of change of momentum is proportional to the force applied, and takes place in the direction of the force"

Momentum = Mass x Velocity


Change in Momentum = Force x Time


Mass x Velocity = Force x Time

Velocity = Acceleration x Time


Force = Mass x Acceleration

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Newton's 3rd Law

"To every action there is an equal and opposite reaction"

In any interation between two bodies, A and B, the force exerted by body A on body B is equal and opposite. So if body A exerts a force on body B, B exerts an equal and opposite force on A

We call these action and reaction pairs.

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Balanced and Unbalanced Forces

Forces are measured in Newtons (N)

If two forces of the same size act in opposite directions on an object they are said to be balanced.

If the forces acting in opposite directions are different they are said to be unbalanced.

If there is more than on force in one line, they can be added together to give a total force. This total force is called the resultant force.

Unbalanced forces result in a change in a body's motion whereas balanced forces do not.

The acceleration of a body is directly proportional to the resultant force and inversely proportional to the body's mass, which can be seen in the equation:

Force = Mass x Acceleration

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Terminal Speed

When objects move faster in air (or water), air resistance or drag forces increase. When weight force is greater than drag the object accelerates. Moving through the air causes collisions with air molecules. The faster an object moves, the greater the number of collisions with air molecules which increases drag. As the drag forces increase, the resultant force is reduced. When the drag forces become the same as the weight, then the forces become balanced. There is now no resultant force and the object stops accelerating, but continues to move at constant speed. This constant speed is called terminal speed.

Opening a parachute provides a large surface area. This increases the drag force, slowing the object down. When the parachute opens the forces are unbalanced (drag is greater than weight) so the object deccelerates.

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