GCSE Physics Revision notes

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IN general... A force is a push or a pull

A force can:

change the shape of an object

or

can change its state of rest or motion

or

change the direction of an object

The unit of force is the Newton (N)

Balanced forces When forces acting in opposite directions on an object are equal, we say they are balanced.

If forces are balanced an object will...

(Newtons first law)

Move at a constant speed

Be stationary

Unbalanced forces When the forces acting on an object are not equal we say they are unbalanced.

If the forces are unbalanced:

A stationary object will move and speed up

A moving object speeds up or slows down

Resultant Force A number of forces acting on a body may be replaced by a single force which has the same effect on the body as the original forces all acting together. The force is called the resultant force.

If two forces are heading in the same direction they must be added together to get the resultant force.

If two forces are heading in opposite directions then they must be subtracted from each other to get the resultant force.

Resultant Force and Movement

1. If the resultant force acting on a body is 0, the object will be either:

At a constant speed or

Stationary

2. If the resultant force acting on a stationary object is not 0 the body will:

Change speed; it will get faster

Newtons Laws When a body remains at rest (stationary) or it is moving at a constant speed then the forces on it are balanced and there is no resultant force.

Linking unbalanced forces, masses and acceleration

When there is an unbalanced force the object accelerates (or changes speed)

The size of the force needed to accelerate a mass can be worked out using Newton's Second Law

Resultant Force = Mass x Acceleration

Resultant force is measured in Newtons - N

Mass is measured in KIlograms - kg

Acceleration is measured in metres per second squared - m/s ²

From the equation we can find that:

Mass = Force/Acceleration

Acceleration = Force/Mass

This law tells us for that a given body, the bigger the unbalanced force, the greater is the acceleration.

This helps to explain why very large objects take a long distance to stop.

One Newton is the force which causes an object of 1kg to accelerate at 1 m/s ²

Terminal Velocity and falling objects When an object is dropped, the force of gravity is greater than air resistance and so the object accelerates.

The force of air resistance gradually increases. As force of gravity does not change, the object accelerates slower.

Eventually the force of air resistance and the force of gravity become equal and the object travels at a constant speed. This is known as terminal velocity.

Terminal velocity is....

Where the force of air resistance and the force of gravity are equal and the object moves at a constant speed.

Mass and weight Weight and mass are terms which are often used in our daily lives. However we have to be careful to distinguish the differences between these two terms.

Mass - The amount of matter that an object is made from

Mass is a scalar quantity

Inertia-

A bigger object accelerates more slowly than a smaller object for the same force applied. Bigger objects have an in-built reluctance to start moving.

GCSE physics revision CH.1 Forces Contents:

In general...

Balanced forces

Unbalanced forces

Resultant force

Newton's Laws

Terminal velocity and falling objects

Mass and weight