Class 8 Science Chapter 12: Friction - NCERT Solutions

(a) Motion

Friction is the opening force which change in state of the motion.

(b) Nature

Friction depends on the nature of the surface. Rough surface have high friction and smooth surface have less friction.

(c) Heat

Friction produces heat, it is the opposing force acting on the surface.

(d) Reduce

Powder is used as lubricant to smoothen the surface.

(e) Less

Sliding friction is less than the static friction, sliding friction is less than the static friction, sliding friction is the friction when one slides over another body.

(c) Static, sliding, rolling

Static > sliding > rolling

Because in rolling the interlocking time between two surfaces are very less as compared to the sliding and static friction. In static friction, the interlocking time between two surfaces are greater. So, static friction is higher than sliding and rolling friction. The area of contact is very less in rolling friction as compare to static and sliding friction.

(a) Wet marble floor, dry marble floor, towel newspaper

Force of friction depends upon the nature of the surface. Friction is more when more irregularities are present on the surface. A smooth surface have less friction. So, as in the increasing order of the force of friction, a wet marble floor have more irregularities so it shows the greater force of friction than the other whereas newspaper have less irregularities on its surface as compared to other. So, it exerts less force of friction.

Frictional force is the opposing force it acts in opposite direction of the object.

When the book slides on the desk, then the friction acts on the opposite direction of the book. In sliding friction, the interlocking between two surfaces is less than the static friction.

It is difficult to walk on the floor when the soapy water spills on the marble because it reduces the friction of the floor. Soapy water makes the surface of the marble is smooth and slippery. So, it is difficult to walk on the floor.

Sportsmen uses shoes with spikes because it increases the force of friction. Spikes makes the grip better of the shoes with the ground. More irregularities plays important role in increase in force of friction. Spikes helps the sportsmen to run and protect them from slipping.

Irregularities increase the surface area for the friction. Force of friction depend on the interlocking of irregularities between two surfaces. Iqbal has to apply less force to push a lighter box because here the interlocking between the floor and the lighter box is very small. But Seema have to apply a greater force to push the heavy box because here the interlocking between the floor and the heavy box is very strong. Seema have to apply the larger force in comparison to Iqbal.

Sliding friction is less than the static friction because force of friction depends on the interlocking of two surfaces. In sliding, interlocking between two surfaces is weak, so to overcome this we have to apply less force. But in static friction, the interlocking between the irregularities of two surfaces is very strong. So we can say that sliding friction is less than the static friction.

Advantages of friction are as follows:

1. We are able to walk.
2. We can write.
3. Friction is used to stop the vehicle.
4. It produces heat.
5. Helps in different types of motion.

Disadvantages of friction are as follows:

1. Forest fires occurs due to friction between branches of trees.
2. It is an opposing force, so energy is needed to overcome it.
3. Tyres and soles of shoes tears out.
4. Damage machines.
5. It produces a lot of heat.

When a body moves in fluid, it experiences a force of friction called a drag force. It fluids, to overcome the frictional force, objects given special shapes. Frictional force can be minimized by changing in shape of the object. The special shape of the body exerts less friction and move easily in the fluid. That’s why objects moving in fluids must have special shapes.