7 - Power and Energy Applications Free Online FlashCards

What does power measure?

Power is the rate of energy transfer: $P_{\text{avg}}=\frac{\Delta E}{\Delta t}=\frac{W}{\Delta t}$.

What is the SI unit of power?

The watt is the SI unit of power: $1\ \text{W}=1\ \frac{\text{J}}{\text{s}}$.

How is energy found from a variable power graph?

For variable power, transferred energy equals the area under the power–time graph: $\Delta E=\int P\,dt$.

What is the formula for instantaneous power from a force?

Instantaneous power is $P=\vec F\cdot\vec v=Fv\cos\theta$, where $\theta$ is the angle between force and velocity.

What power does a perpendicular force deliver?

A force perpendicular to velocity delivers zero instantaneous power because $\cos 90^\circ=0$.

How is gravitational potential energy modeled near Earth?

Gravitational potential energy near Earth is $U_g=mgy$. Its zero level is arbitrary; only changes in $U_g$ affect predictions.

What is the elastic potential-energy formula?

The elastic potential energy of a spring is $U_s=\frac{1}{2}kx^2$, where $k$ is the spring constant and $x$ is compression or extension.

How is efficiency defined?

Efficiency is useful output divided by total input: $\eta=\frac{E_{\text{useful,out}}}{E_{\text{in}}}=\frac{P_{\text{useful,out}}}{P_{\text{in}}}$.

How do you find input power from useful output power?

The required input power is useful output power divided by efficiency: $P_{\text{in}}=\frac{P_{\text{useful,out}}}{\eta}$.

What energy-accounting equation applies to a mechanical system?

For a mechanical system, $\Delta K+\Delta U+\Delta E_{\text{th}}=W_{\text{ext}}$. For an isolated system, $W_{\text{ext}}=0$.

What happens to energy transferred by friction?

Friction transfers mechanical energy into thermal energy. It does not destroy energy or remove it from the total energy balance.

A 250 kg load rises 12 m in 8.0 s. What useful power is delivered?

The useful power is $P=\frac{mgh}{\Delta t}=\frac{(250)(9.8)(12)}{8.0}\approx3.7\ \text{kW}$.