9. Work & Energy

If a constant force $F$ acts on a sled moving at an average speed $v\_\mathrm{avg}$, what is the average power $P\_\mathrm{avg}$ delivered by the force in terms of $F$ and $v\_\mathrm{avg}$?

A gardener pushes a 12 kg lawnmower whose handle is tilted up 37° above horizontal. The lawnmower's coefficient of rolling friction is 0.15. How much power does the gardener have to supply to push the lawnmower at a constant speed of 1.2 m/s? Assume his push is parallel to the handle.

Which of the following is the correct SI unit for $power$?

Which of the following is a likely result of using a more powerful motor to run an elevator?

If you do $90$ joules of work in $30$ seconds, how much power have you generated?

A circuit contains a $5\Omega$ resistor with a current of $2A$ flowing through it. How much power is the resistor dissipating?

If a light draws $220W$ on a $120V$ circuit, what is the current flowing through the light?

A $1500-W$ electric heater is connected to a $120-V$ power line. How much heat energy, in joules, does it produce in $2.0$ hours of operation?

Which of the following correctly defines one watt in terms of base $\mathrm{SI}$ units?

A $10$ horsepower DC motor draws $31$ amps at $300$ volts. What is the efficiency of the motor? ($1$ horsepower = $746$ watts)

If the heating element of an electric dryer is rated at $4.1\mathrm{kW}$ when connected to a $240V$ line, what is the current drawn by the dryer?

An 85-kg football player traveling 5.0 m/s is stopped in 1.0 s by a tackler. What average power is required to stop him?

When you ride a bicycle at constant speed, nearly all the energy you expend goes into the work you do against the drag force of the air. Model a cyclist as having cross-section area 0.45 m² and, because the human body is not aerodynamically shaped, a drag coefficient of 0.90. Use 1.2 kg/m³ as the density of air at room temperature. The food calorie is equivalent to 4190 J. How many calories does the cyclist burn if he rides over level ground at 7.3 m/s for 1 h?

Some electric power companies use placement of water to store energy. Water is pumped from a low reservoir to a high reservoir. To store the energy produced in 1.0 hour by a 180-MW electric power plant, how many cubic meters of water will have to be pumped from the lower to the upper reservoir? Assume the upper reservoir is 380 m above the lower one. Water has a mass of 1.00 x 10³ kg for every 1.0 m³ ( 1MW = 10⁶ W).

How much work must you do to push a 10 kg block of steel across a steel table at a steady speed of 1.0m/s for 3.0 s? What is your power output while doing so?