The concept of force

The concept of force

By the end of this topic you should be able to:

• state the difference between mass and weight;• draw vectors representing forces acting on a given body;• identify situations in which frictional forces develop and draw those frictional forces;• use Hooke’s law correctly.

Dynamics

Why do objects move like they do?

Weight

This force is the result of the gravitational attraction between the mass in question and the mass of the earth. The weight of a body is the gravitational force experienced by that body, which on earth is given by the formula

W = mg

TensionA string that is taut is said to be under tension. The force that arises in any body when it is stretched is called tension. A tension force in a string is created when two forces are applied in opposite directions at the ends of the string.

To say that there is tension in a string means that an arbitrary point on the string is acted upon by two forces (the tension T).

Normal reaction forces

If a body touches another body, there is a force of reaction or contact force between the two bodies. This force is perpendicular to the body exerting the force.

Drag forces

Drag forces are forces that oppose the motion of a body through a fluid (a gas or a liquid).

Upthrust

Any object placed in a fluid experiences an upward force called upthrust. If the upthrust force equals the weight of the body, the body will float in the fluid. If the upthrust is less than the weight, the body will sink.

FrictionFrictional forces oppose the motion of a body. Friction arises whenever one body slides over another. In this case we speak of kinetic friction. Friction also arises whenever there is just a tendency for motion, not necessarily motion itself. In this case, we speak of static friction.

Example question

Free-body diagrams

A free-body diagram is a diagram showing the magnitude and direction of all the forces acting on a chosen body. The body is shown on its own, free of its surroundings and of any other bodies it may be in contact with.

Hooke´s law

If we try to extend a spring, a force pulls the spring back to its original length; if we try to compress a spring, again a force tries to pull the spring back to its original length. The force in the spring has a simple relationship to the amount by which the spring is extended or compressed.

T = k

The extension or compression of the spring must not be too large, otherwise Hooke’s law isn’t applicable. That range is called as the elastic limit.

Questions