Examples of forces

Here we will consider the distinguishing features and examples of the normal force, weight, tension, friction, buoyancy and drag - all colourfully illustrated with Algodoo.

Much of this has already been described on the Forces page.

Key Concepts

Normal force (N)

The perpendicular force between two touching surfaces.

Weight (W)

The force of attraction between an object and the Earth. The size of this force is proportional to the mass of the object and gravitational field strength (constant as long as the Earth's surface appears flat!).

\(W=mg\)

Tension (T)

The force exerted when a string is used to pull an object (opposite to compression).

Essentials

Dynamic friction (F)

The force that opposes the relative motion of two touching surfaces in relative motion. The size of this force depends on the nature of the surfaces (which determines the coefficient of dynamic friction, μ_d) and the normal force between them.

\(F=\mu_dR\)

Static friction (F)

The force that opposes the relative motion of two touching surfaces without relative motion. The size of this force depends on the nature of the surfaces (which determines the coefficient of static friction, μ_s) and the normal force between them.

\(F=\mu_sR\)

The coefficient of static friction is larger than for dynamic friction, which you'll have noticed if you've ever pushed something hard to get it moving.

Drag (F)

Drag opposes the motion of bodies through a fluid. For slow moving bodies this is proportional to their velocity, v, but is proportional to v² for fast ones. Drag force is also dependent on the size and shape of the body and the properties of the fluid (find out more in the Engineering Option).

Summary

Pressure is not a force. Pressure is the ratio of force acting to the surface area. This is often calculated for the weight of an object acting on a horizontal surface.

\(P={F\over A}\)

Pressure has the SI derived unit Pascal (Pa), equivalent to N m^-2

No calculator THINK: How could a person exert a larger pressure than an elephant?