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A mass of 1 kg is kept on a inclined plane with 30° inclination with respect to horizontal plane and it is at rest initially. Then the whole assembly is moved up with constant velocity of 4 m/s. The work done by the frictional force in time 2 s is ________ J. (Take g = 10 m/s²)
$$f = mg \sin\theta$$
$$f = 1 \times 10 \times \sin(30^\circ) = 10 \times \frac{1}{2} = 5\text{ N}$$
$$s = v \times t = 4\text{ m/s} \times 2\text{ s} = 8\text{ m}$$
$$\alpha = 90^\circ - 30^\circ = 60^\circ$$
$$W_f = f \cdot s \cdot \cos\alpha$$
$$W_f = 5\text{ N} \times 8\text{ m} \times \cos(60^\circ)$$
$$W_f = 40 \times \frac{1}{2} = 20\text{ J}$$
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