12.2 CHAPTER 12. FORCE,MOMENTUM AND IMPULSE
Examples of Forces in Physics ESBER
Most of Physics revolves around the study of forces. Although thereare many different forces, all
are handled in the same way. All forces in Physics can be put into one of four groups. These are
gravitational forces, electromagnetic forces, strong nuclear forces and weak nuclear forces. Youwill
mostly come across gravitational or electromagnetic forces at school.
Gravitational Forces
Gravity is the attractiveforce between two objects due to the mass of the objects. When you throw a
ball in the air, its mass and the Earth’s mass attract each other, which leads to a force between them.
The ball falls back towards the Earth, and the Earth accelerates towards the ball. The movementof the
Earth towards the ball is, however, so small thatyou couldn’t possibly measure it.
Electromagnetic Forces
Almost all of the forcesthat we experience in everyday life are electromagnetic in origin. Theyhave
this unusual name because long ago people thought that electric forces and magnetic forces were
different things. After much work and experimentation, it has been realised that they are actually
different manifestationsof the same underlyingtheory.
Electric or ElectrostaticForces
If we have objects carrying electrical charge, which are not moving, then we are dealing withelec-
trostatic forces (Coulomb’s Law). This force isactually much strongerthan gravity. This mayseem
strange, since gravity isobviously very powerful, and holding a balloonto the wall seems to bethe
most impressive thing electrostatic forces havedone, but if we think about it: for gravity to be de-
tectable, we need to have a very large mass nearby. But a balloon rubbed against someone’s hair can
stick to a wall with a force so strong that it overcomes the force of gravitybetween the entire Earthand
the balloon—with just the charges in the balloonand the wall!
Magnetic Forces
The magnetic force is adifferent manifestation of the electromagnetic force. It stems from the interac-
tion between moving charges as opposed to the fixed charges involved in Coulomb’s Law. Examples
of the magnetic force inaction include magnets,compasses, car enginesand computer data storage.
Magnets are also used inthe wrecking industry topick up cars and move them around sites.
Friction
According to Newton’sFirst Law (we will discuss this later in the chapter) an object moving without
a force acting on it will keep on moving.Then why does a box sliding on a table cometo a stop?
The answer is friction.Friction arises where two surfaces are in contact and moving relative toeach
other as a result of the interaction between the molecules of the two contact surfaces—for instance the
interactions between themolecules on the bottomof the box with molecules on the top of the table.
This interaction is electromagnetic in origin, hence friction is just another view of the electromagnetic
force. Later in this chapter we will discuss frictional forces a little more.
Drag Forces
This is the force an object experiences while travelling through a medium like an aeroplane flying
through air. When something travels through theair it needs to displace air as it travels and because of
this, the air exerts a forceon the object. This becomes an important force when you move fast and alot
of thought is taken to tryand reduce the amountof drag force a sports caror an aeroplane experiences.