Unit – 4 Physics : Work, Energy and Power (MCQs)

1. A uniform force $(2\hat{i}+\hat{j})$ N acts on a particle of mass 1 kg. The particle displaces from $(3\hat{j}+\hat{k})$ m to $(5\hat{i}+3\hat{j})$ m. The work done by the force is

a) 9 J
b) 6 J
c) 10 J
d) 12 J
Answer: c) 10 J

2. A ball of mass 1 kg and another of mass 2 kg are dropped from a tall building. After a fall of 40 m, their kinetic energies will be in the ratio

a) $\sqrt{2}:1$
b) $1:\sqrt{2}$
c) $2:1$
d) $1:2$
Answer: d) 1 : 2

3. A body of mass 1 kg is thrown vertically upwards with velocity $20~ms^{-1}$. It comes to rest at a height of 18 m. How much energy is lost due to air friction? (Take $g=10~ms^{-2}$)

a) 20 J
b) 30 J
c) 40 J
d) 10 J
Answer: b) 30 J

4. An engine pumps water continuously through a hose. Water leaves the hose with velocity $v$ and $m$ is the mass per unit length of water. The rate at which kinetic energy is imparted is

a) $\frac12 mv^2$
b) $mv^3$
c) $\frac32 mv^2$
d) $\frac52 mv^2$
Answer: a) $\frac12 mv^2$

5. A body of mass $4m$ at rest explodes into three pieces. Two pieces each of mass $m$ move perpendicular to each other with equal speed $v$. The total kinetic energy generated is

a) $mv^2$
b) $\frac{3}{2}mv^2$
c) $2mv^2$
d) $4mv^2$
Answer: b) $\frac{3}{2}mv^2$

8. The work done by a conservative force for a closed path is

Closed path conservative force diagram
a) always negative
b) zero
c) always positive
d) not defined
Answer: b) zero

13. A particle is placed at the origin and a force $F = kx$ acts on it. If $U(0)=0$, the graph of $U(x)$ versus $x$ is

Potential energy U(x) vs x graphs
a) a
b) b
c) c
d) d
Answer: c)