Force and Acceleration

A force may cause a mass to accelerate. This acceleration can be a change in speed, a change in direction or a change in both speed and direction.


Resultant force

The resultant force is the sum of all of the forces acting on an object.

The change in an object’s motion is caused by the resultant force.

If the forces acting on an object are unbalanced (not equal) it means that a resultant force is acting on the object.

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Newton's 2nd Law.

A resultant force causes an acceleration.

The acceleration can be calculated with this equation.

Resultant force (F) = mass (m) x acceleration (a).

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Newton’s 1st Law

Newton's 1st Law says that the velocity of an object will only change if a resultant force is acting on the object. This applies to a still or moving object.

Stationary: If an object is stationary (not moving) and there is no resultant force acting on it, it will stay stationary.

Moving: If an object is moving and there is no resultant force acting on it, the object will continue moving in the same direction at the same speed.

Newton's 3rd Law - Table Example

Newton’s Third Law says that: whenever 2 objects interact, the forces that they exert on (apply to) each other are equal and opposite.

If one object exerts (applies) a force on another object, then the other object must be exerting (applying) a force back.

If a hand pushes on a table, the table will push back on the hand with an equal force, but in the opposite direction.

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Acceleration - Examples

Rugby scrum


The scrum accelerates forwards as the players exert more force.

Newton 1st Law - Example

When a vehicle is travelling at a steady speed, the resistive forces (such as friction and air resistance) are balancing the driving force.

When there are balanced forces, there is no resultant force.

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