Power

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AP Physics 1 › Power

Questions 1 - 10
1

A car engine provides a constant driving force $F$ while the car moves at constant speed $v$ for time $t$. If the time doubles while $F$ and $v$ stay constant, average power is

quadrupled

unchanged

doubled

halved

Explanation

This question tests understanding of average power versus instantaneous power. Power is the rate of energy transfer, calculated as P = Fv when force and velocity are constant and aligned. Since both F and v remain constant, the instantaneous power P = Fv is constant throughout the motion. Average power equals total work divided by total time, but when instantaneous power is constant, average power equals instantaneous power regardless of duration. Choice A incorrectly assumes power depends on time duration. When force and velocity are both constant, power remains constant regardless of how long the process continues.

2

A winch raises a $m$-kg bucket vertically at constant speed. Trial 1 lifts it height $h$ in time $t$; Trial 2 lifts the same bucket height $h$ in time $2t$. Compare average power.

Trial 1 has twice the average power of Trial 2

They have equal average power because $mgh$ is the same

Trial 2 has greater power because it acts longer

Trial 2 has twice the average power of Trial 1

Explanation

This question tests understanding of power as the rate of doing work. Power equals work divided by time: P = W/t. In both trials, the winch lifts the same mass m through the same height h, so the work done against gravity is W = mgh in both cases. Trial 1 completes this work in time t, giving power P₁ = mgh/t. Trial 2 takes twice as long (2t), giving power P₂ = mgh/(2t) = (1/2)(mgh/t) = P₁/2. Therefore, Trial 1 has twice the average power of Trial 2. Choice C incorrectly assumes equal work means equal power, ignoring time. To find power, always divide work by the time taken to do that work.

3

In two trials, a force does work $2W$ in time $t$ (Trial 1) and work $W$ in time $t/2$ (Trial 2). Which trial has greater average power?

Cannot be determined without the force

Trial 1, because it has more work

Trial 2, because it takes less time

They have equal average power

Explanation

This question tests understanding of average power as work divided by time. Power is defined as P = W/t, where W is work done and t is time taken. In Trial 1, power is P₁ = 2W/t. In Trial 2, power is P₂ = W/(t/2) = 2W/t. Both trials have the same average power because doubling the work while doubling the time (Trial 1) gives the same result as halving the work while halving the time (Trial 2). Choice A incorrectly focuses only on work without considering time. To compare powers, always calculate the ratio of work to time for each situation.

4

A box is pulled at constant speed $v$ by a horizontal force $F$ for time $t$. What is the average power delivered by $F$?

$F$

$Fv$

$F/v$

$Ft$

Explanation

This question tests the concept of average power in the context of constant velocity motion. Power is defined as the rate at which work is done, given by P = W/t, where W is work and t is time. In this scenario, the work done by the force F over distance d = v t is W = F d = F v t, so average power P = (F v t)/t = F v. This shows that power depends on force and velocity, not directly on time for the average over that interval. A common distractor like choice A, Ft, might confuse power with impulse, which is force times time, but power involves energy transfer rate, not momentum change. To approach similar problems, always derive power from work divided by time or use P = F v for constant speed cases.

5

Two students pull identical carts at constant speed $v$ on level ground. Student 1 pulls with force $F$; Student 2 pulls with $2F$. Which statement about their powers is correct?

Both have the same power because the speed is the same.

Student 2 has the same power because power depends only on work, not force.

Student 2 has greater power because $P=Fv$ and $2F$ at the same $v$ doubles $P$.

Student 1 has greater power because using less force is more efficient.

Explanation

This question assesses the concept of power in AP Physics 1, which is the rate at which work is done or energy is transferred. Power is defined as P = W/Δt, but for constant force and velocity, it simplifies to P = Fv, where F is the force and v is the velocity. In this scenario, both students pull carts at the same constant speed v, but Student 2 applies twice the force, 2F, to overcome presumably greater friction or other resistance. Therefore, Student 2's power is 2Fv, which is double that of Student 1's Fv. A common distractor is choice A, which incorrectly assumes power depends only on speed, ignoring the role of force in the P = Fv formula. To approach similar problems, always identify whether power is calculated using energy transfer over time or force times velocity, depending on the given constants.

6

A student pushes a cart with constant horizontal force $F$ at constant speed $v$ for distance $d$. Which change increases average power?

Increase $v$ while keeping $F$ the same

Decrease the time by pushing for a shorter distance

Decrease $v$ while keeping $F$ the same

Increase $d$ while keeping $F$ and $v$ the same

Explanation

This question evaluates the application of power in constant force and velocity pushing. Power is the rate at which work is done, with average P = (F d)/t and since t = d/v, P simplifies to F v. Increasing v while keeping F constant directly increases P, as power scales with velocity. Keeping F and v the same but increasing d increases both work and time proportionally, leaving P unchanged. Choice A is a distractor, as decreasing v would reduce P, mistakenly thinking slower speed means more power due to longer time. Remember, for constant force problems, express power in terms of F and v to see which variables affect it.

7

A cart is pushed on a level track at constant speed $v$ by a horizontal force $F$ for time $t$. What is the average power delivered by the push?

$Ft$

$\dfrac{F}{t}$

$Fv$

$F$

Explanation

This question tests understanding of power for constant velocity motion. Power is the rate of energy transfer, calculated as P = W/t where W is work done. For constant velocity horizontal motion, the applied force F equals the friction force, and work done is W = F·d where d is distance. Since the cart moves at constant speed v for time t, the distance is d = vt, making work W = F(vt). Therefore, average power is P = W/t = F(vt)/t = Fv. Choice B (Ft) incorrectly represents impulse, not power. The key insight is that for constant velocity, instantaneous power P = Fv equals average power.

8

A student pushes a crate with horizontal force $F$ at constant speed across a floor. If the student pushes for twice as long at the same speed and force, how does average power change?

It stays the same because $P=Fv$ and both are unchanged

It quadruples because work doubles and time doubles

It doubles because the time doubles

It halves because power is inversely proportional to time

Explanation

This question tests understanding of power at constant velocity. Power equals force times velocity: P = Fv. Since both the force F and speed v remain constant throughout, the power P = Fv stays the same regardless of how long the student pushes. Doubling the time doubles the work done but doesn't change the rate at which work is done. Choice B incorrectly assumes power depends on total time rather than instantaneous conditions. For constant force and velocity, power remains constant regardless of duration.

9

A sled is pulled at constant speed by force $F$ at angle $\theta$ above horizontal. The sled’s speed is $v$. What is the power delivered by the pull?

$Fv\cos\theta$

$Fv\sin\theta$

$F\cos\theta$

$Fv$

Explanation

This question evaluates power with angled forces at constant speed. Power is the rate of work done, given by P = F · v = F v cosθ, where θ is the angle between force and velocity. Here, the horizontal velocity v and force at θ above horizontal yield P = F v cosθ, as only the horizontal component contributes to work along the direction of motion. This applies because at constant speed, power matches the rate of energy dissipation, like against friction. Choice A, F v, is a distractor that neglects the angle, assuming full force contributes. A strategy is to use the dot product for power when force and velocity are not parallel.

10

A battery transfers energy to a device at a constant rate. In $5\ \text{s}$ it transfers $E$. In $10\ \text{s}$ it transfers $2E$. What happens to average power?

It stays the same because $\frac{E}{5} = \frac{2E}{10}$

It halves because the time is longer

It cannot be determined without the force

It doubles because more energy is transferred

Explanation

This question tests understanding of average power with constant energy transfer rate. Power is the rate of energy transfer, so average P = ΔE / Δt. In 5 s, P = E/5, and in 10 s, P = 2E/10 = E/5, remaining the same as expected for constant rate. This shows average power is consistent when energy scales linearly with time. Choice A is a distractor, mistakenly thinking more total energy means higher power without considering extended time. For such problems, check if rates are constant by seeing if energy over time is proportional.

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