Forces

A force is a push or a pull on an object. It is measured in Newtons, N. This push or pull is the result of an interaction between bodies, such as someone pushing the object, tension in a rope, friction with the ground, gravitational force, air resistance and many more.

Forces have a direction and a magnitude. This makes them vector quantities. A way of representing this is having the F in bold, F, underlined, F, or with an arrow over it.

All the forces on an object can be drawn on a free-body diagram. This is useful for working out the overall force on the object. Each force on the object is represented as an arrow in the direction of a force.

Here are some examples:

For a box resting on the ground, the weight of the box is commonly represented as the mass times the acceleration due to Earth’s gravity, g, so mg overall. Because the box is stationary, there must be a reaction force, or normal force, N, equal and opposite to mg.

scan30001

If there is then a force applied horizontally, the box will accelerate, since F = ma.

scan30002

In the next case, the applied force provides a force upwards, equal to Fsinθ and a force horizontally, equal to Fcosθ (these are obtained by resolving the force given). The horizontal force causes acceleration, while the vertical force means that N decreases.

scan30003

All forces which aren’t perpendicular to each other have to be resolved so that they are:scan30004

Back to Contents: Physics: Mechanics

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