WEEK 8:FORCES

TOPIC: FORCES

We often relate the word force to strength’. You know that the stronger you are the more force you can produce and the heavier are the objects that you can move. By exerting a larger force, you can kick a football further, cycle faster, drag a heavier load or jump heigher.The unit of force is Newton (N), which was named in honor of the English physicist Isaac Newton.

Types of forces

There are three main types of forces, namely pull, push or turning force.

Push forces move the object away from the source of the push force. For example, you push your book across the desk towards your friend.

Pull forces move the object towards the source of the pull force. For example, you pull a tug of water rope and the rope and the person holding the other end moves towards you.

Turning forces cause rotation of an object. For example, you exert a turning force when you open or close a tap.

THE EFFECTS OF FORCES.

We can also define a force by the effects it has. So, we can say that a force can

(1) Change the shape of the body it acts upon

(2)Make a body at rest, move.

(3) Change the speed of a moving body

(4) Change the direction of a moving body.

(5) Make a moving body stop.

USES OF FORCES

How many examples of using forces can you think of? There are many different ways in which forces can be used.

We use a pushing force to move a wheelbarrow, or to press the key on a computer keyboard.

We use a pulling or pushing for to shut a door

Dogs exert a pulling force on their leashes when walking.

In a tug of war, both teams exert a pulling force on the rope, but the forces are in opposite directions

We use push forces to lift masses upwards. Gravity pulls them down again

A cyclist uses push pull forces to move the handlebars and to make the petals go round.

A footballer uses a push force to kick the ball.

A car mechanic using a spanner applies a turning force.

CONTACT AND NON CONTACT FORCES

By now, we have established that all forces can change the shape, speed or direction of motion of an object. It is the push and pull forces acting on an object that keep things still or get them moving. Forces and energy are interrelated and when forces are at work, energy is transferred.

   Forces can further be categorized as contact and non- contact forces;

Contact forces.

Kicking a ball to make it move is a contact force. Some other types of contact forces are listed below.

Friction.is the force that prevents objects from moving or that slows them down when they are already moving.

Air resistance is the force you on your face when you are cycling fast. It is caused by friction between you and the air.

Upthrust is produced by water pushing upwards on an object. It keeps things floating. When a boat floats on water, it weight pulls it downwards but the opposite pushing force from the water prevent s it from sinking. You can feel it when you try to push your hand downwards through water.

Surface tension. This is a force that helps raindrops to keep their shape and make it possible for some insects to move on the surface.

NON-CONTACT FORCES

Non-contact forces can have an impact from a distance, so there does not have to be contact between objects. There are 4 types of NON-CONTACT forces.

Gravity is the force that pulls objects down to the center of the planet and that gives them weight. Gravity is present between any two objects, but it is only with very large objects such as stars and planets that we notice it. Gravity acts over very large distance.

Magnetic force.is produced by a magnet, which affects magnetic materials such as iron and steel. Usually it only works over a short distance. The area in which the magnetic force is applied, is called magnetic field.

Electric force.is the force from static electricity, which can attract objects to it .This force also tend to act over a short distance.

Nuclear force.it the force that operates within atoms to keep the nucleus together. It is the strongest of all the forces this force acts over  a very short distance.

GRAVITATIONAL FORCE

As we said earlier, forces act upon objects and we cannot necessarily see the force itself. Gravitational force is a further example of interaction.

When an object is released from a height above the ground, the object naturally falls to the ground. When a football is kicked upwards, it will reach a high point and then, fall to the ground. This is because the force of gravity is acting on the object and it's pulling it downwards. In a later unit you will explore how gravity pulls all objects downwards towards the center of the planet. Gravity is an example of a NON-CONTACT force and it works on all objects on the surface or in the atmosphere of the planet. It gives all objects weight.

The strength of the gravitational force acting on an object depends on three things;

The mass(m) of the only in kilograms

The acceleration due to gravity (g)

The height of the object (h) in metersq, so formula for gravity is

GF=mass (m)×acceleration(g)×height(h).

FRICTION

Sometimes when we are on slide it is difficult to move .At other times it seems to be much easier. When two objects are touching each other, there is friction between their surfaces. Friction prevents surfaces from sliding smoothly over each other. Friction occurs because the one surface resists the other surface.

When two smooth surfaces such as glass and ice rub together, there is very little friction. The force of friction is small for smooth surfaces. When two rough surfaces rub together, there is more friction. So, the rougher the surface, the more friction there is.

Friction is a force that opposes motion. Friction always acts in a direction opposite to the object's direction of movement.

ADVANTAGE OF FRICTION

(1) Friction between the tyres and the road makes it possible for cars to stay on the road, but it also limits the speed that the car can reach.

(2)When a vehicle brakes, the Friction between the tyres and the road allows it to stop. If a road is wet and slippery,there is less friction between the road and the tyres.The vehicle will then  need a longer  distance to stop.This is why car drivers need to be careful when they are driving on a wet roads.

(3) Tools can be sharpened  with a grindstone or a file.The grindstone has a rough surface and the Friction between this surface and the tools sharpens the tools

(4)Many machine use a driver belt to transfer energy between moving parts.The belt needs to have high friction to enable it  to transfer the energy and  not to let it  slip.

DISADVANTAGES OF FRICTION

(1) Friction can wear down surfaces.For example,the brake pad on a bicycle and a car need to be replaced after a period of time.Car tyres must also be replaced at times, because the tyres wear down due to friction between them and the road.

(2) Because friction need to be overcome to make machines work,some of the energy supplied to the machine is not usefully transferred to the machine.Instead it is used to overcome the Frictional force between the moving parts of the m.This also reduce the efficiency of the machine

REDUCING FRICTION

(1) Lubricating surface with oil or Grease help to reduce the Friction between them.

(2)Some machines have ball bearings and rollers to prevent the moving parts from rubbing against each other.

(3)we can reduce Friction by reducing the contact area.

EXERCISE

1. List three main types of forces

2. State four effects of forces