Question

Friction is not required in which of the following cases?
A. Man running on the ground
B. Friction between parts of a machine
C. Writing with a pen
D. Monkey climbing a tree

Answer 1

When there is a relative motion between two bodies, there are intermolecular forces present which oppose the relative motion, this is called friction.It is useful in some scenarios but also has its disadvantages in some scenarios.The smoother the surfaces, the lesser the frictional force between the bodies. Therefore, we can say that frictional force is higher when there are irregularities.It acts in the opposite direction to the force that causes the relative motion.Now, we will read every option and understand if friction is useful in that particular situation or not.

Complete step by step answer:
Frictional force arises when there is relative motion between the two bodies, it is directly proportional to irregularities between them.There are different types of friction – static friction, kinetic friction, rolling friction and sliding friction and also fluid friction.

Let’s check every option now and try to understand in which direction the relative motion is occurring.
Option A: Man running on the ground
When a man is walking on the ground, the feet push the ground backward. The relative motion between the feet and the ground is the force that pushes the ground backward. Hence, the frictional force that acts on the bodies is in the opposite direction to the pushing force. Hence it acts in the forward direction. Hence, we walk in the forward direction.
Since friction stops things from sliding apart, in the absence of it, there is no moving up or climbing anything. We will crawl or fall down to the lowest point under gravity.Therefore, friction is useful in this scenario.

Option B: Friction between parts of a machine
There are a lot of moving as well as static parts in a machine. When a part is moving, there is a relative motion which gives rise to friction. Friction always acts in the opposite direction hence it is bad for the motion. This reduces the performance of the machine. The parts lose their quality because of constant rubbing between the surfaces.We can reduce the friction by smoothening the surfaces of the moving parts.Therefore, friction is not necessary in this situation.

Option C: Writing with a pen
Frictional force is very useful while writing. The friction helps the ink particles to stay on paper and dry. If there is no friction present, then the ink particles just roll off and won’t stay on the paper like the same way we can’t write on glass with a normal pen.Hence, friction is useful in this situation.

Option D: Monkey climbing a tree
When a monkey climbs a tree, it tries to pull the tree downwards and hence there is a relative motion between the monkey’s limbs and the tree. Since there is a relative motion, there is frictional force that acts in the direction opposite to the downward pull. This friction helps the monkey climb upward.Hence, friction is useful in this scenario.

Therefore, the right answer is option B.

Note: Knowing the concept and having a clear grip of the concept of friction will help us answer the question. A very important misconception that students have is that:“The frictional force acts in the opposite direction of the relative motion but not in the opposite direction of motion.”We have to understand the options and try to visualize them and try to find the direction of relative motion.