The interaction between objects leads to actions which can be seen by the observer, these actions are caused by forces between the objects in question. A force is a push or pull.
Some of the interactions involve contact between the objects, where the objects are physically touching. These are called contact forces. An example of a contact force is friction. In the gif below the stickman is able to walk on the surface due to the friction (contact) between his feet and the surface. Put a layer of grease on the surface and friction is reduced and the stickman slips over.
Other examples of contact forces include air resistance, tension, and normal contact force.
Other interactions involve no contact – the objects are physically separated. These are called non-contact forces. An example of a non-contact force is magnetic force. Below you can see the effect of the magnetic force bewteen the magnets but there is no contact between the objects.
Other examples of non-contact forces are gravitational force and electrostatic force.
When measuring forces, the physical quantities measured fall into two categories: scalars and vectors. Scalar and vector quantities are treated differently in calculations.
Scalar quantities only have a magnitude or size. For example, energy (e.g. 100 J), mass (e.g. 15 kg), temperature (e.g. 10oC) or distance (e.g. 1m).
Vector quantities have both magnitude and an associated direction. For example, displacement (e.g. 2 m west), velocity (e.g. 23m/s upwards), acceleration (e.g. 9.8 metres m/s²downwards).
Speed can be thought of as the rate at which an object covers distance. A fast-moving object has a high speed and covers a relatively large distance in a given amount of time, while a slow-moving object covers a relatively small amount of distance in the same amount of time.
Speed is also a scalar quantity, as it does not refer to direction.
Speed can be calculated by knowing the values for distance and time.
