Examples of Work Done

Q.

The work done by a centripetal force

1. increases by decreasing the radius of the circle

2. decreases by increasing the radius of the circle

3. increases by increasing the mass of the body

4. is always zero

Q.

A body of mass m is moving in a circle of radius r with a constant speed v. The force on the body is $\frac{m{v}^2}{r}$ and is directed towards the centre. What is the work done by this force in moving the body over half the circumference of the circle

1. $\frac{m{v}^2}{r}\times \pi r$

2. $Zero$

3. $\frac{m{v}^2}{r^2}$

4. $\frac{\pi r^2}{m{v}^2}$

Q. A person trying to lose weight by burning fat lifts a mass of $10\text{kg}$ upto a height of $1\text{m}$ for $1000$ times. Assume that the potential energy lost each time he lowers the mass is dissipated. How much fat will he use up, considering work is done only when the weight is lifted up? Fat supplies $3.8\times {10}^7\text{J/kg}$ which is converted to mechanical energy at an efficiency of $20$% . Take $g=9.8{\text{ms}}^{-2}$. Consider weight to be lifted slowly.

1. $2.45\times {10}^{-3}\text{kg}$
2. $6.45\times {10}^{-3}\text{kg}$
3. $9.89\times {10}^{-3}\text{kg}$
4. $12.89\times {10}^{-3}\text{kg}$

Q. The work done to raise a mass $m$ from the surface of the earth to a height $h$, which is equal to the radius of the earth is

1. $mgR$
2. $2mgR$
3. $\frac{1}{2}mgR$
4. $\frac{3}{2}mgR$

Q.

A body of mass 100 g is rotating in a circular path of radius r with constant velocity. The work done in one complete revolution is

1. $100rJ$

2. $\left(\frac{r}{100}\right)J$

3. $\left(\frac{100}{r}\right)J$

4. Zero

Q. A person trying to lose weight by burning fat lifts a mass of $10\text{kg}$ upto a height of $1\text{m}$ for $1000$ times. Assume that the potential energy lost each time he lowers the mass is dissipated. How much fat will he use up, considering work is done only when the weight is lifted up? Fat supplies $3.8\times {10}^7\text{J/kg}$ which is converted to mechanical energy at an efficiency of $20$% . Take $g=9.8{\text{ms}}^{-2}$. Consider weight to be lifted slowly.

1. $2.45\times {10}^{-3}\text{kg}$
2. $6.45\times {10}^{-3}\text{kg}$
3. $9.89\times {10}^{-3}\text{kg}$
4. $12.89\times {10}^{-3}\text{kg}$