IB Physics Work, energy and power Practice Questions

$50\,\text{J}$. Work $W=Fd\cos\theta=10\times 5.0\times\cos 0^{\circ}=50\,\text{J}$.

$9.0\,\text{J}$. $E_{k}=\frac{1}{2}mv^{2}=\frac{1}{2}(2.0)(3.0)^{2}=9.0\,\text{J}$.

$98\,\text{J}$. $\Delta E_{p}=mg\Delta h=5.0\times 9.8\times 2.0=98\,\text{J}$.

$20\,\text{W}$. Power $P=\frac{W}{t}=\frac{600}{30}=20\,\text{W}$.

$75\%$. Efficiency $=\frac{\text{useful output}}{\text{total input}}=\frac{150}{200}=0.75=75\%$.

$1.0\,\text{J}$. $E_{p}=\frac{1}{2}kx^{2}=\frac{1}{2}(200)(0.10)^{2}=1.0\,\text{J}$.

60000 / 6.0x10^4 / 60 000. Energy $=Pt=1000\times 60=6.0\times 10^{4}\,\text{J}$.

Question 1

A force of $10\,	ext{N}$ moves a box $5.0\,	ext{m}$ in the direction of the force. How much work is done?

Accepted Answer

$50\,	ext{J}$. Work $W=Fd\cos	heta=10	imes 5.0	imes\cos 0^{\circ}=50\,	ext{J}$.

Question 2

A $2.0\,	ext{kg}$ object moves at $3.0\,	ext{m s}^{-1}$. What is its kinetic energy?

Accepted Answer

$9.0\,	ext{J}$. $E_{k}=\frac{1}{2}mv^{2}=\frac{1}{2}(2.0)(3.0)^{2}=9.0\,	ext{J}$.

Question 3

A $5.0\,	ext{kg}$ mass is lifted $2.0\,	ext{m}$ ($g=9.8\,	ext{m s}^{-2}$). What is the gain in gravitational potential energy?

Accepted Answer

$98\,	ext{J}$. $\Delta E_{p}=mg\Delta h=5.0	imes 9.8	imes 2.0=98\,	ext{J}$.

Question 4

A machine does $600\,	ext{J}$ of work in $30\,	ext{s}$. What is its power output?

Accepted Answer

$20\,	ext{W}$. Power $P=\frac{W}{t}=\frac{600}{30}=20\,	ext{W}$.

Question 5

A motor receives $200\,	ext{J}$ of electrical energy and outputs $150\,	ext{J}$ of useful mechanical energy. What is its efficiency?

Accepted Answer

$75\%$. Efficiency $=\frac{	ext{useful output}}{	ext{total input}}=\frac{150}{200}=0.75=75\%$.

Question 6

A spring of stiffness $200\,	ext{N m}^{-1}$ is stretched by $0.10\,	ext{m}$. What elastic potential energy is stored?

Accepted Answer

$1.0\,	ext{J}$. $E_{p}=\frac{1}{2}kx^{2}=\frac{1}{2}(200)(0.10)^{2}=1.0\,	ext{J}$.

Question 7

The SI unit of work and energy is the _____.

Accepted Answer

joule / joules / J. One joule is the work done when a force of $1\,	ext{N}$ moves its point of application $1\,	ext{m}$.

Question 8

A $1000\,	ext{W}$ heater runs for $60\,	ext{s}$. The energy it transfers is _____ $	ext{J}$.

Accepted Answer

60000 / 6.0x10^4 / 60 000. Energy $=Pt=1000	imes 60=6.0	imes 10^{4}\,	ext{J}$.

Question 9

A ball is released from rest and slides down a smooth track through a vertical drop h. Its speed at the bottom is

Accepted Answer

sqrt(2 g h). Equating m g h to (1/2) m v squared gives v = sqrt(2 g h).

Question 10

For a frictionless slide, the final speed of an object released from rest is independent of

Accepted Answer

the mass of the object. By conservation of mechanical energy, speed depends only on vertical drop, not mass.

IB Physics: Work, energy and power — Practice Questions & Answers