JAMB Physics 1981 Question 19

Question 19

Friction

A body of mass 6 kg rests on an inclined plane. The normal reaction is R and the limiting frictional force is F as shown in the diagram (Fig. 1). If F is 30 N and g = 10 m s⁻², then the angle of inclination θ is

A.15°
B.30°
C.45°
D.60°
E.75°.

Show the answer and explanation

Answer: B (30°)

Explanation

At rest, friction balances the weight's component down the slope: F = mg sin θ, so 30 = 60 sin θ, sin θ = 0.5 and θ = 30°.