BASIC SCIENCE FOR CLASS 8 AMARNATH MISHRA,PHD GEETA MEHTA ,PHD DEEPTI LEHRI,PHD RAJESH MOHAN, PHD

                    Physic

               Chapter -1

               Force and pressure

              Only answer

                Exercises

A. Answer the following in not more than 20 words. 

1. Apply a force.

2. Friction.

3. Gravity.

4. Friction, air resistance, and gravity.

5. Friction causes wear and tear, generates heat, and can cause noise.

6. Pressure is force per unit area. The SI unit of pressure is the pascal (Pa).

7. A sharp edge concentrates the force of the knife, making it easier to cut through objects.

8. The air pressure inside the glass is greater than the air pressure outside the glass, which creates a force that pushes the coaster against the rim of the glass.

9. The SI unit of force is the newton (N)

B. Answer the following in not more than 40 words. 

1. Situations in which it is helpful to increase friction:

    * When you walk, you need friction between your shoes and the ground to keep from slipping.

    * When you drive a car, you need friction between the tires and the road to keep the car from skidding.

2. Streamlined body: A streamlined body is a body that has a smooth, rounded shape. This shape helps to reduce drag, which is the force that opposes the motion of an object through a fluid.

3. How does oiling and using ball bearings help reduce friction:

    * Oiling helps to reduce friction by creating a thin layer of oil between the two surfaces that are rubbing together. This layer of oil reduces the amount of contact between the two surfaces, which reduces the amount of friction.

    * Ball bearings help to reduce friction by using balls to support the weight of the object. The balls roll against each other, which reduces the amount of friction between the object and the surface it is resting on.

4. When you push something heavy, the resistance you feel decreases as the body starts moving. Explain what happens:

    * When you push something heavy, the force of friction between the object and the surface it is resting on opposes the motion of the object. As the object starts to move, the force of friction decreases because the object is no longer in contact with the surface as much. This decrease in friction allows the object to move more easily.

5. (a) What is the characteristic that magnetic, electrostatic and gravitational forces have in common?

    * The characteristic that magnetic, electrostatic and gravitational forces have in common is that they are all forces that act between two objects.

    * (b) What happens when a body leaves the earth's gravitational field? Give one example.

    * When a body leaves the earth's gravitational field, it will continue to move in a straight line until it is acted on by another force. For example, if a satellite is launched into orbit around the earth, it will continue to orbit the earth until it is either slowed down by atmospheric drag or until it is hit by another object.

6. (a) If everything in the universe attracts everything else, why do we not attract each other?

    * We do attract each other, but the force of attraction is very small because we have very little mass.

    * (b) Why is the moon's force of gravity less than that of the earth?

    * The moon's force of gravity is less than that of the earth because the moon has less mass than the earth

C. Answer the following in not more than 100 words. 

1. A spring balance measures force. To demonstrate the principle on which it works, you can hang a weight from the spring and measure the length of the spring. The longer the spring is stretched, the more force is being applied to it.

2. The pressure in a liquid is related to depth by the equation $P = \rho g h$, where $P$ is the pressure, $\rho$ is the density of the liquid, $g$ is the acceleration due to gravity, and $h$ is the depth. To demonstrate this, you can fill a glass with water and then lower a weight into the water. The deeper the weight is, the more pressure it will exert on the bottom of the glass.

3. Friction helps us to walk, run, and cycle by providing traction. It also helps us to grip objects and to control our movements. However, friction can also create problems, such as when it causes objects to wear down or when it makes it difficult to move objects. When you cycle, friction between the tires and the ground provides traction, which allows you to control the direction of your bike. However, friction also causes the tires to wear down over time. 

D. Complete the following. 

1. Rolling friction is less than sliding friction.

2. A body has weight because of gravity force.

3. The force which opposes the motion of one body over another is frictional force.

4. The smoother the surface, the less is the frictional force.

5. Kinetic friction is less than static friction.

6. The smaller the area over which a force is applied, the greater is the pressure.

7. The pressure at a particular depth in a liquid is equal in every direction. 

E. Choose the correct option in each of the following. 

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