8th Class Science Force and Pressure Force

       Force

Force is required to move or stop an object from its current state. If a car is running on the road, driver applies a force on the break to stop the car. Again, force is also required on the accelerator to move the car form its rest. In both the conditions, the force is required. The unit for the measurement of applied force is Newton and it is represented by the first letter of its name, N.

Let a body P is in rest at point A, a force of 10 N is applied by a body J, and another force of 20 N is applied by a body K in the same direction as shown in the picture below.

Direction of possible displacement by the body P on resulting force

The net or resulting applied force on the body is the sum of both the applied forces = Applied force by J + Applied force by K = 10 N + 20 N = 30 N and the body P moves in the direction the applied forces.

If the body J applies the same amount of force in the same direction but the direction of applied force by the body K is changed, and the body K applies the same amount of force in opposite direction as shown in the figure below.

The net or resulting force on the body P = Applied force of 20 N by the body K -Applied force of 10N by the body J = 20 N-10 N = 10 N and the body P moves in the direction of applied force by the body K (greater applied force).

         Types of Force

Let us know about the different types of forces in detail.

         Gravitational Force

Everything is attracted by the Earth. The force with which the earth's pull the things towards itself is called gravitational force.

If an object is thrown upwards, it comes down on the earth due to the gravitational force.

           Magnetic Force

The force applied by a magnets on other magnetic materials is called the magnetic force. When an iron piece is placed near a magnet, it attracts the iron due to its magnetic force.

Two magnets can attract each other or repel each other. In both the conditions, the force which works between them is called magnetic force.

Look at the following picture of magnetic field around a magnet:

           Muscular Force

We can pull an object due to the force applied by the muscles of our body. The force applied by the muscles of human body is called the muscular force. Muscles fibers generate tension which causes the muscles to lengthen, shorten etc as per the requirement.

           Electrostatic Force

The force exerted by the electrically charged body is called the electrostatic force. A plastic comb is charged on rubbing with dry hair and charged comb attracts small pieces of paper due to the electrostatic force.

           Friction

A ball rolling on a rough surface stops after some time. If the same ball is rolled on a smooth surface, it takes long time to stop. This is because the force of friction between the ball and surface is larger on the rough surface as compare to that of the smooth surface. The force of friction acts in both the conditions but the amount of friction is less on smooth and large on rough surface. There are two forces that act on the ball, that is, the force by which the ball is rolled on the surface and the force by which the surface opposes the motion of the ball. The amount of force by which the ball is opposed by the surface is called force of friction. Therefore, the net acted force between two surfaces in contact and their tendency to oppose the motion of each other is called the force of friction.

Look at the following picture to understand the force of friction:

      Calculate the resulting force and direction which the door moves in the picture below.

(a) Resulting force of 40 N and door moves in the direction of A

(b) Resulting force of 50 N and door moves in the direction of B

(c) Resulting force of 100 N and door moves in the direction of A

(d) Resulting force of 20 N and door moves in the direction of B

(e) None of these

Answer: (a)

Explanation

The resulting force = (80 N + 60 N + 100 N) - 200 N = 40 N, and the door moves in the direction of A. Therefore, option (A) is correct and rest of the options is incorrect.

      What would be the resulting force, if two equal forces act on the body in opposite direction?

(a) Zero resulting force

(b) Unit resulting force

(c) 2 times of the applied forces

(d) All of these

(e) None of these

Answer: (a)

Explanation

If two equal forces are applied on a body then the resulting force on the body is always zero. Therefore, option (A) is correct and rest of the options is incorrect..