NCERT 8th Class (CBSE) Science: Force and Pressure

Question: What is electrostatic force?  What kind of a force is it?

Answer: Electrostatic force is exerted by a charged body on another charged or uncharged body. For this it is not necessary for the two bodies to be in contact with each other. This force is applicable from a distance so it a non contact force.

Question: Why does a moving ball gradually come to rest?

Answer: A moving ball comes to rest on its own due to force of friction. When ball moves on the ground, their surface come in contact to each other. The ball and the ground exert equal and opposite force on other, Which gradually brings the moving ball to a stop.

Question: What is pressure? What is the relation of pressure with area on which it is applied?

Answer: Force exerted on per unit area is called pressure. Pressure is related with area on which it is applied. When the area is increased the pressure exerted is less. But when the area on which pressure is exerted is decrease the pressure increase. So we conclude that pressure increase with decrease in area.

Question: Why the trucks and buses have broader wheels than the wheel of scooters or cycles?

Answer: The truck and buses are heavy and need to exert less pressure. As we know that pressure decrease with increase in area of contact, so the tyres are broadened so, that they exert less pressure on earth and move easily, if they exert more pressure, they will sink in earth. Scooters and cycles have less wide wheels because they need more pressure for gripping.

Question: How will you prove that pressure of liquids decrease with increase in height of its column?

Answer: For this to be proven take a plastic container. Take A pin and make holes in the container in horizontal line from bottom to middle of the container. Now put water in the container and observe, you will see the water will come out with maximum pressure from bottom holes while the hole at the top will have the minimum pressure of water.

Question: What changes are caused due to application of force on object?

Answer: Force when applied on an object cause many changes on the objects. Following changes occur due to application of force:

1. Force change the state of motion: Forces can changes the state of motion of the object. It can increase or decrease the speed of the moving object. it can change the direction of the moving object.
2. Force can change the shape of the object: Force can change the shape of the object on which it is applied. When force is applied on some object, it always have some effect on the shape of the object. The change in the object is some time visible and some time, it is not visible, but changes in shape of the object is always accompanied by application of force.

Question: What are the contact forces? Explain different forces which are contact forces.

Answer: Contact forces are the force which come in play when two objects come in contact with each other. Following forces are the contact forces:

1. Muscular Forces: The forces which is exerted due to action of muscles is called muscular forces. Action like bending running moving etc are the example of the muscular forces. It is a contact force because it comes in play when it comes in contact of the object.
2. Friction: Equal and opposite force being exerted on an object when its surface comes in contact with other surface. It is also a contact force because it is exerted when two surface come in contact with each other. Halting of a moving ball on its own is an example of forces of friction.