Forces
Force
A force is a push or pull exerted on an object.
The effect of a force
What forces do, the effect thereof. Two or more forces can be exerted in opposite directions so that they neutralize or balance one another's effect.
A force can have the following effects:
A force can cause a stationary object to start moving. A moving object to move faster (accelerate). A moving object to move slower (decelerate). A force can change the shape of an object (deformation). A force can change the direction of an object (change of direction). A force can also cause an object to rotate/spin (rotation).
The unit of force
The newton (N) is the unit in which force is measured. The unit is derived from the name of the English scientist, Sir Isaac Newton.
The measuring of force
A newton meter (newton spring balance) isused to measure force. It is a spring balance containing a metal spring. The newton meter is equipped with a scale marked or calibrated in newton (the unit of force).
Forces work in pairs
Forces always work in pairs and simultaneously on various objects and never occur as single forces.
First example
When you push against a wall with your hands, your hands exert a force on the wall, for example to the right. The wall also exerts a force to the left on your hands. The forces (an action and a reaction force) act in pairs (on various objects), are equal in size, act in opposite directions, act simultaneously on your hands and on the wall.
Second example
When you sit on a chair you exert a downwards force on the chair. The chair also exerts an upwards force on you. The forces (an action and a reaction force) act in pairs, are equal in size, act in opposite directions, act simultaneously on you and on the chair.
Balanced and unbalanced forces
When two or more forces act on an object, the forces combine to form a net force (also called resultant force). These two forces are not force pairs (action and reaction) because they act on the same object. When the two forces are equal, but act in opposite directions, they balance each other. We say that the nett force (resultant force) is 0 N. When the forces act in opposite directions, but are not equal, we say that the net force (resultant force) is greater than 0N. If they exert forces of an equal magnitude, the rope will stand still, the forces balance one another and the net fore=0 N. If one group can exert a stronger force than the other, the rope will move in the direction of the stronger force. This is because the forces are unbalanced. It is greater because the forces are unbalanced and there is a nett force (resultant force) working in the direction of the greater force. When forces are not balanced, there is a nett force. When forces are unbalanced, there is no nett force (resultant force).
Two types of forces
Contract force and Non-contact force (or field forces)
Contact force
A contact force is exerted when two objects are in contact with one another, i.e. when they touch one another.
Types of contact force
Frictional force, tensile force, compression force, thrust force or tension and air resistance.
Non-contact forces ( or field forces)
A non-contact force or field force is exerted over a distance, i.e. it is a force as a result of an action at a distance between two objects. The two objects that non-contact forces (field forces) exert on one another do not touch one another.
Types of non-contact forces (or field forces)
Gravitational force, magnetic force and electrostatic force.
Field forces: Gravitational force
When one throws a ball into the air, it always comes down to the surface of the Earth again. Force of gravity is always a force of attraction and is a non-contact force (field force). All objects that have mass, exert a gravitational force of attraction upon one another.
Gravitational force (1)
Gravitational force is a force of attraction that is exerted by ojects (bodies) on one another as a result of their mass. When two books lie next to one another on a table, the books, although they exert a gravitational force of attraction on one another, do not move towards one another because the frictional force between the books and the table is far greater than the gravitational force of attraction between the two books
Gravitational force (2)
Gravitational force keeps everything on earth in place. There is gravitational force between the Earth and the Moon - it keeps the Moon in its orbit around the earth. The pull between the Earth and the Moon causes the tides of the sea. The gravitational force of the Earth keeps satellites in their orbits above earth. Gravitational force is always directed at the centre point of the mass of the object.
The magnitude of gravitational force is determined by two factors:
The mass of the object and the distance between the objects. Gravitational force is measure just as any other in newton (N).
Mass
The mass of an object is the amount of matter in the object and is measured in kilograms (kg). A truck has greater mass (more matter) than a bicycle. The mass of an object stays the same everywhere, even though it may be measured at different places on earth or at any place in the universe.
Unit of mass
The main unit of mass is the kilogram (kg). The smaller mass units are gram (g) and miligram (mg). A greater mass such as the cargo of a ship is for instance indicated in tons: 1 kilograms (kg)=1 000 000 miligrams (mg), 1 kilogram (kg)=1 000g (grams) 1 000 kilograms (kg)=1 ton
Appuratus used to measure mass
Scale
Weight
The weight of an object is the gravitational force that is being exerted by the Earth (or the Moon, or another planet) on the object. Weight (Fg) is measured in newton (N). The weight of an object will change if the gravitational force (gravity) on the object changes. On the Moon the gravitational force is one sixth of the gravitational force on earth.
Formula for weight
Weight=mass x gravitational acceleration
Fg=mg
Gravitational acceleration
The gravitational acceleration for earth is g=9,8 m/s2.
Field forces: Magnetic forces
Magnetic force or magnetism is the force that magnets exert on magnetic materials that contain iron, steel, cobalt and nickel. Objects that are attracted by a magnet, are called magnetic materials. Objects that are not attracted by a mangnet, are called nonmagnetic materials.
Magnetic forces
Magnets can attract magnetic materials through materials such as paper, carton, plastic and glass. Materials that contain iron, steel, nickel and cobalt, will block or shield magnetic forces. Watches and electric meters are shielded by being mounted in iron housing or iron ring. Magnets can magnetise another object- you can see this by touching a nail that is not magnetic with a magnet in order to magnetise it.
Magnetic forces
Similar poles repel one another. Disimilar poles attract one another. Magnets indicate direction- a magnet that is suspended freely and is horizontal, comes to rest in a North South direction. Mangetic force is attractive or repulsive and acts over a distance.
Magnetic fields
The area around the magnet in which magnetic forces are exerted we call the magnetic field of a magnet. The magnetic field surrounds the entire magnet ad is the strongest at the poles of the magnet. The magnetic field around a magnet is invisible, but can e made visible by means of iron fillings that arrange themselves around the magnet, forming lines.
Magnetic field lines
These lines are called magnetic field lines- the are therfore magnetic field lines in a magnetic field. Magnetic field lines never cross one another. A magnetic field line is the path along which a N-pole would move in a magnetic field; for that reason we always choose the direction of magnetic field from north to south.