Case Study Based Questions: Friction | Science Class 8 PDF Download

Case Study

Read the given passage below and answer the question:

The force required to overcome friction at the instant an object starts moving from rest is a measure of static friction. On the other hand, the force required to keep the object moving with the same speed is a measure of sliding friction. The sliding friction is slightly smaller than the static friction.

When one body rolls over the surface of another body, the resistance to its motion is called rolling friction. Rolling reduces friction. It is always easier to roll than to slide a body over another.

Q1: The following options show arrangement of types of friction according to decreasing order of applied friction. Select the correct order.
(a) Rolling > Sliding > Static
(b) Static > Rolling > Sliding
(c) Sliding > Static > Rolling
(d) Static > Sliding > Rolling
Ans: (d)

Q2: In which of the following case one will have to apply maximum force?
(a) Riding a bicycle
(b) Trying to move a heavy rock from its resting place
(c) Swimming in a pool
(d) Drilling a hole in a wall
Ans: (b)

Q3: Oiling or greasing in machine parts:
(a) Increases the friction between the parts
(b) Decreases the friction between the parts
(c) Both (a) and (b) are correct.
(d) None of the above
Ans: (b)

Q4: Why static friction is more than sliding friction?
Ans: The friction is caused by the interlocking of irregularities in the two surfaces. It is obvious that the force of friction will increase if the two surfaces are pressed harder. The force required to overcome friction at the instant an object starts moving from rest is a measure of static friction. On the other hand, the force required to keep the object moving with the same speed is a measure of sliding friction. When the box starts sliding, the contact points on its surface do not get enough time to lock into the contact points on the floor. So, the sliding friction is slightly smaller than the static friction.

Q5: Classify the given examples into static, sliding, and rolling friction:
Static Friction: Papers on a table, car parked on a hill.
Sliding Friction: A box sliding on an inclined plane, child playing on slide in a park.
Rolling Friction: Ball moving on the ground, tyres of a bus.