Power AP Physics 1 FRQ

Power AP Physics 1 FRQ – Exam Style Questions etc.

Unit: 3. Work , Energy and Power

Weightage : 10-15%

AP Physics 1 Exam Style Questions – All Topics

Question

A pile of bricks of mass M is being raised to the tenth floor of a building of height H = 4y above the ground by a crane that is on top of the building. During the first part of the lift, the crane lifts the bricks a vertical distance \(h_1=3y\) in a time \(t_1=4T\). During the second part of the lift, the crane lifts the bricks a vertical distance \(h_2=y\) in \(t_2=T\). Which of the following correctly relates the power \(P_1\) generated by the crane during the first part of the lift to the power \(P_2\) generated by the crane during the second part of the lift?

A \(P_2=4P_1\)

B \(P_2=\frac{4}{3}P_1\)

C \(P_2=P_1\)

D \(P_2=\frac{3}{4}P_1\)

E \(P_2=\frac{1}{3}P_1\)

Answer/Explanation

Ans:B

Using the equation for power for the first part of the lift, \(P_1=\frac{W}{t}=\frac{mgh}{t}=\frac{mg(3y)}{(4T)}=\frac{3mgy}{4T}\) . For the second part of the lift, \(P_2=\frac{W}{t}=\frac{mgh}{t}=\frac{mg(y)}{(T)}=\frac{mgy}{T}\); thus, \(P1=\frac{3mgy}{4T}=\frac{3}{4}P_2\rightarrow P_2=\frac{4}{3}P_1\)

Question

A wheel with rotational inertia I is mounted on a fixed, frictionless axle. The angular speed 𝜔 of the wheel is increased from zero to 𝜔𝑓 in a time interval T.

What is the average power input to the wheel during this time interval?

A \(\frac{Iω_f}{2T}\)

B \(\frac{Iω_f^2}{2T}\)

C \(\frac{Iω_f^2}{2T^2}\)

D \(\frac{I^2ω_f}{2T}\)

E \(\frac{I^2ω_f^2}{2T^2}\)

Answer/Explanation

Ans:B

Question

Students are collecting data as they control the toy helicopter shown. The helicopter has a mass m and must exert a thrust of power \(P_0\) to initially move upward with a constant speed \(v_0\) . The students then cause the helicopter to exert a thrust so that a net force \(F_{net}=2mg\) accelerates it upward. As the helicopter is accelerating, it has a thruster power \(P_f\) , and as it passes a height h0 it has a speed of \(3v_0\). The ratio \(\frac{P_f}{P_0}\) is most nearly

A 0.11

B 0.17

C 4

D 6

E 9

Answer/Explanation

Ans:E

For a given force and speed, the power of the thrusters is given by the equation \(P=Fv\) . When the helicopter is hovering, it must exert a thrust force equal to its weight. When the helicopter is accelerating, the thrust force Ft is found using an equation for net force, \(F_{net}=F_t−mg\)

\(F_t=F_{net}+mg=3mg\). Substituting this into the equation for the ratio yields \(\frac{P_f}{P_0}=\frac{Fv_f}{Fv_0}=\frac{3mg\times 3v_0}{mg\times v_0}=9\).

Question

In Figure 1 , an engine that delivers power P is attached to a pulley that lifts a box of mass \(m_0\) a vertical distance of \(y_0\) at a constant speed in time \(t_0\). If, as shown in Figure 2 the engine again delivers power P, how long would it take the engine to raise a box of mass \(3m_0\) the same vertical distance \(y_0\) at a constant speed?

A\(\frac{1}{3}t_0\)

B \(t_0\)

C \(\sqrt{3}t_0\)

D \(3t_0\)

E \(9t_0\)

Answer/Explanation

Ans:D

Using the equation for power to calculate the original time yields \(P=Fv=m_0g\frac{y_0}{t_0}\)

\(t_0=\frac{m_0gy_0}{P}\) . To use this equation to solve for the second time yields \(P=Fv=3m_0g\frac{y_0}{t_2}\rightarrow t_2=3\frac{m_0gy_0}{P}=3t_0\).

Question

All of the following are units of power EXCEPT

A watts

B joules per second

C electron volts per second

D newton meters per second

E kilogram meters per second

Answer/Explanation

Ans:E

Power AP Physics 1 FRQ