1. Explain giving two examples each of:
(a) Contact forces, and (b) Non-contact forces.
Answers
Answer:
(a) Contact forces :
If two objects have to be touching for the force to act, the force would be described as a contact force. The two objects will be pushing or pulling on each other e.g.
(1) Friction is a contact force between two surfaces rubbing against each other, the force moving an object forwards is partially countered acted by a force of friction acting in the opposite direction e.g.
the tyre of a car in contact with the road,
pressing the brake pad onto the disc of a car's braking system,
rubbing your hands together.
(2) Air resistance is also a contact force as an object moves through the atmosphere, friction between the object and air, even though the downward motion is caused by gravity.
(3) Tension and compression in bridges
(b) non contact force:
If the objects are subjected to a force, but do not need to be in contact with each other, the force would be described as a non-contact force e.g. three classic non-contact force fields acting between objects that are not touching ...
Gravity - the gravitational attraction force between any two objects.
Any object in a gravitational field experiences the force of gravity.
note:
(i) A falling object in air, experiences the non-contact force of gravity and the contact force of friction between the object and air - air resistance.
(ii) Objects in contact with each other will still experience the 'non-contact' force of gravitational attraction - any object standing motionless on a surface.
Magnetism - the magnetic attraction between a magnetised material and another magnet material.
Any object that is magnetic or can be magnetised, will experience a force from the magnetic field of a magnet.
This can be the magnetic force of attraction of iron towards a magnet (permanent and induced N-S pole attraction), two magnets attracted by their opposite poles (N-S <= N-S), or two like poles of magnets repelling each other (N<=>N or S<=>S poles) - possible magnetic field effects.
You see the magnetic field non-contact force in action as a magnet attracts and picks up an iron nail. Initially there is no contact, but once attached there is contact, but the attractive force is still present even if they are in contact.
You can have an object is suspended by magnetic repulsion, where there is no contact at all.
Electrostatic force - the attraction (+ -) or repulsion (- - or + +) interaction of two objects carrying an electrical charge - electrical field effects
A simple example is a rubbed charged plastic rod picking up tiny bits of paper.