(a) Force has to be applied to change the direction of a moving object.

(b) When an elephant drags a wooden log over the land, the forces that are applied on the log are muscular force , gravitational force and frictional force.

(c) A ball was set rolling on a large table. If its motion is to be changed, a force will have to be applied on it.

(d) The force of friction always acts against the motion.

An object falling from a tall building: Gravitational force

An aeroplane flying in sky: Mechanical Force,  Gravitational force, Frictional Force, Gravitational Force

Squeezing sugarcane juice with a squeezer: Muscular force, Mechanical Force, Frictional Force

Winnowing food grains: Muscular Force, Gravitational force

Muscular force: The force applied by the action of muscles in our body is termed as a muscular force. The bones and muscles of our body help us to perform movements. Muscular force is used to perform all these movements.

Example:  when you pick up a book placed on the table using your hands, pulls the handcart, you apply muscular force. The coolie lifting the luggage uses muscular force.

Gravitational force: It is the force that is exerted by the Earth on every object, which is near or on its surface.

For example, the rocket that takes off from the ground, applies the mechanical force against the gravitational force. All the things on the earth, stay on the earth due to gravitational force.

Mechanical force: The force applied with the help of machines is called mechanical force. 

For example, when a car gets started, its engine creates a mechanical force on the tyres that help the car to accelerate. So here, the movement of car occurs due to the force generated by the machine on the tyres.

Some machines are used with the help of muscular force, while some machines work on electricity or fuel. These machines are automatic. Machines help us to carry out tasks with ease and speed.

Electrostatic force: Electrostatic force is the force that exists either between the two charged bodies, or between a charged and an uncharged body. Due to electrically charged materials like ebonite, plastic, rubber this force is produced.
For example, the charged scale attracts the pieces of paper by a non-contact force known as electrostatic force.

The force of friction: Frictional force is created by friction. For example, a ball rolling on a ground stops after sometime because of frictional force acting between the ground and the ball, the brakes used in stopping a running vehicle work on the frictional force.

The footpaths and slopes of the walk ways have rough surface to create more frictional force, so that people do not slip and fall as they walk.

Magnetic force: The force exerted by a magnet is known as magnetic force.

For example, separation of iron stuffs from junk is done with the help of magnetic force. Magnetic force is used in many gadgets such as doorbell, huge cranes that lift loads.

• The machines working on mechanical force should work efficiently.
• A machine is oiled from time to time to reduce friction between machine parts, reduces wear & tear of machine parts.
• By doing so, life span of a machine, speed and efficiency of the machine increases,

Therefore, machines are oiled from time to time.

• An object thrown upwards goes up due to muscular force.
• At one point during its ascent the muscular force becomes zero.
• The gravitational force then starts operating on this object. It pulls this object down.
• Thus, an object thrown upwards comes down after reaching a point.

• The coins on the carrom board should move swiftly during play.
• Powder is sprinkled on a carrom board to reduce friction between the striker/carrom coins and the carrom board.
• The powder smoothens the surface of the board and thus the striker and coins can move on it easily.

The ramp at a railway station has a rough surface so that the friction between our feet and the ground is enough for us to walk comfortably and without tripping.

If the surface is smooth and slippery, there will be no frictional force. People using ramp in a hurry will slip and fall down easily.

In order to avoid this and keep people safe, the ramp at a railway station has a rough surface.

Force can be applied to almost all things. Lifting, pulling, riding a bicycle, pushing a load, squeezing or bending something, driving vehicles, using different machines, and almost all the day-to-day activities are done by applying force.

Following things can be done by applying force:

• It can move a body from rest.
• It can bring a moving body to rest.
• It can change the direction of a moving body.
• It can change the speed of a moving body.

The gravitational force acting on an object is called the weight of that object.

Machines which run on muscular force are:

• Bicycle
• Rickshaw
• Handcart
• Stitching machine
• Plough.
• Bullock cart