The coefficient of static friction between a wooden block of mass 0.5 kg and a vertical rough wall is 0.2. What is the minimum horizontal force (in newtons) that must be applied to the block to keep it adhered to the wall? (take g = 10 m/s²)
- 2021
- JEE
- Laws of Motion
- Friction
- 11th
Step-wise Solution :
Problem Logic decode
A block cannot adhere by itself on the wall because the external unbalanced force namely gravitational force applied by the earth, tries to pull down the block. So, in order to adhere (hold) to the wall, an external force must be applied. That is, this external force tends to push the block horizontally and block in turn pushes the wall. By Newton’s 3rd law, “if block pushes wall, the wall will also pushes the block in opposite direction, right?” The force by the wall on the block is ‘Normal Reaction’ (comes into picture whenever there is a contact). Since, the wall is rough, frictional force will also come into play to hold the block on to the wall, which depends on the normal reaction. Logically, in order for the block to stay in rest, all the vertical forces must be balanced. That is, gravitational force must be balanced by the frictional force.
More the external force pushes the block —> More the Normal reaction (by Newton’s third law) —> More the frictional force —> More the gain in tendenc to hold the block.
Therefore, to adhere the block to the wall, frictional force must be greater than or equal to weight of the block (i.e., gravitational force of the block).
Block on Rough Wall (With Force Values)
Apply Horizontal Force: 0 N