Q # 1. A person holds a bag of groceries while standing still, talking to a friend. A car is stationary with the engine running. From the stand point of work, how are these two situations similar?
Ans. In both the above two cases, since there is no displacement, therefore the work done will be zero. Hence in this respect, the two situations are similar.
Q # 2. Calculate the work done in kilo joules in lifting a mass of $10 \mathrm{~kg}$ (at steady velocity) through a vertical height of $10 \mathrm{~m}$.
Ans. The work done $\mathrm{W}$ on the object will be stored in the form of P.E. Therefore:
$$\begin{aligned}W & =m g h \\& =(10)(9.8)(10) \\& =980 \mathrm{~J} \\& =0.98 \mathrm{~kJ}\end{aligned}$$
Q # 3. A force $F$ acts through a distance $L$. The force/is them increased to $3 F$, and then acts through a further distance of 2L. Draw the work diagram to scale.
Ans. The force-displacement graph is shown in the figure.
As the work done is equal to the area under the force-displacement curve. Hence
$$\begin{aligned}W & =(F \times L)+(3 F \times 2 L) \\& =F L+6 F L \\& =7 F L\end{aligned}$$
Q # 4. In which case is more work done? When a $50 \mathrm{~kg}$ bag of books is lifted through $50 \mathrm{~cm}$, or when a $50 \mathrm{~kg}$ crate is purhed through $2 \mathrm{~m}$ across the floor with a force of $50 \mathrm{~N}$.
Q # 5. An object has $1 \mathrm{~J}$ of potential energy. Explain what does it mean?}
Ans. It means that work has been dome on the body by the force of $1 \mathrm{~N}$ which has lifted the body through a distance of $1 \mathrm{~m}$. This work has been stored in the body in the form of P.E. which is $1 \mathrm{~J}$.
Q # 6. A ball of mass $m$ is held at a height $h_{1}$ above a table. The tabletop is at a height $h_{2}$ above the floor. One student says that the ball has potential energy mgh but another says that it is $\operatorname{mg}\left(h_{1}+h_{2}\right)$. Who is correct?
Ans. The $1^{\text {st }}$ student has taken the table as the point of reference for calculating the P.E. While the $2^{\text {nd }}$ student has taken the floor as the point of reference So both are correct according to their own points of references.
Q # 7. When a rocket re-enters the atmosphere, its nose cone become very hot. Where does this heat energy come from?
Ans. The atmosphere of earth contains a large number of dust particles and water vapors. So when a rocket enters into the atmosphere and passes through these particles, due to the force of friction, the kinetic energy of the rocket is lost in the form of heat. That's why its nose cone becomes very hot.
Q # 8. What sort of energy is in the following:
a) Compressedspring
b) Water in a high dam
c) A moving car
Ans.
a) Elastic Potential Energy
b) Gravitational Potential Energy
c) Kinetic Energy
Q # 9. A girl drops a cup from a certain height, which breaks into pieces. What energy changes are involved?
Ans. When the cup was in the hands of girl, it had gravitational P.E. When the cup is dropped, its P.E. is converted into the K.E. On striking the ground, this energy is converted into sound energy, heat energy and work done in breaking the cup into pieces.
Q # 10. A boy uses a catapult to throw a stone which accidentally smashes a green house window. List the possible energy changes.
Ans. Initially, the catapult had elastic P.E. when the stone is thrown, its P.E. is converted intr K.E. On striking the window, this energy is converted into sound energy, heat energy and work dome in breaking the window into pieces.
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