Common Forces – Friction, Tension, Normal - UNSOLVED PRACTICE SET

Q7. Define normal reaction force. Why is it called a "reaction" force? Give an example where normal reaction is not equal to the weight of the body.

Q8. Explain why the tension is the same throughout a massless, inextensible string passing over a frictionless pulley.

Q9. A 10 kg block is placed on a horizontal surface. A horizontal force of 30 N is applied, but the block does not move. Calculate the force of static friction acting on the block.

Q10. In your school, a student pulls a box of books across the floor with a string at an angle of 30° above the horizontal. Explain how this affects the normal reaction compared to pulling horizontally.

Q11. A 5 kg block is suspended by a string from the ceiling. Calculate the tension in the string. If the string is pulled sideways so that it makes 30° with the vertical, what is the new tension?

Q12. Why is it easier to pull a lawn mower than to push it? Explain using the concept of normal reaction.

Q13. Explain the three common forces in mechanics — normal reaction, tension, and friction. For each force, discuss:

(i) Its origin and nature

(ii) How it is calculated

(iii) Its direction

(iv) Two examples from daily life

Include free body diagrams where appropriate.

Q14. A 10 kg block is placed on a rough horizontal surface (μₛ = 0.5, μₖ = 0.3). A horizontal force F is applied.

(a) Calculate the maximum static friction force.

(b) Calculate the force required to start moving the block.

(c) If F = 60 N, calculate the acceleration of the block.

(d) If the force is reduced to 40 N after the block starts moving, what happens?

(e) Draw a graph showing friction force vs applied force.

Q15. A system consists of two masses m₁ = 3 kg and m₂ = 2 kg connected by a light string passing over a smooth pulley. m₁ is on a rough horizontal table (μ = 0.2) and m₂ hangs vertically.

(a) Draw free body diagrams for both masses.

(b) Write the equations of motion for both masses.

(c) Calculate the tension in the string and the acceleration of the system.

(d) Calculate the minimum value of μ for which the system does not move.

(e) What would happen if the table were frictionless?

Q16. A 50 kg student stands in a lift.

(a) Calculate the normal reaction when the lift moves up at constant velocity.

(b) Calculate the normal reaction when the lift accelerates upward at 2 m/s².

(c) Calculate the normal reaction when the lift accelerates downward at 2 m/s².

(d) Calculate the normal reaction when the lift falls freely.

(e) Explain why the student feels "heavier" or "lighter" in each case.

Q17. A 20 kg block is placed on an inclined plane of angle 30°. The coefficient of static friction is 0.4 and kinetic friction is 0.3.

(a) Calculate the component of weight parallel to the plane.

(b) Calculate the maximum static friction.

(c) Will the block slide down? If yes, calculate its acceleration.

(d) Calculate the force required to push the block up the plane at constant velocity.

(e) If a horizontal force is applied to keep the block stationary, calculate its magnitude.

Q18. In a physics lab, students set up an Atwood machine with masses 400 g and 300 g connected by a light string over a frictionless pulley.

(a) Calculate the acceleration of the system.

(b) Calculate the tension in the string.

(c) Calculate the distance travelled by the heavier mass in 2 seconds.

(d) Calculate the velocity of each mass after 2 seconds.

(e) If the string can withstand a maximum tension of 5 N, will it break?