(a) The diagram should show the following forces acting on the block:

• The weight of the block, \(4g\), acting vertically downwards.
• The normal reaction, \(R\), acting vertically upwards.
• The frictional force, \(F_r\), acting horizontally opposite to the direction of tension.
• The tension in the string, 30 N, acting at an angle of 24° above the horizontal.

(b) To find the coefficient of friction, resolve the forces horizontally and vertically:

Horizontally: \(30 \cos 24^{\circ} = F\)

\(F = 27.406 \ldots\)

Vertically: \(R + 30 \sin 24^{\circ} = 4g\)

\(R = 40 - 30 \sin 24^{\circ}\)

\(R = 27.797 \ldots\)

The coefficient of friction \(\mu\) is given by \(\mu = \frac{F}{R}\).

\(\mu = \frac{30 \cos 24^{\circ}}{40 - 30 \sin 24^{\circ}}\)

\(\mu = 0.986\) (rounded to three decimal places).