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CAIE A-Level Physics 5.1.7 Using P = Fv

Practise deriving and applying P = Fv, resolving angled forces and checking that force and velocity refer to the same direction in propulsion, lifting or resistance problems.

Syllabus
2028–2030
Course
Physics 9702
Level
AS

Exam points

  • derive P = Fv for a force acting in the direction of steady velocity from P = W/t and W = Fs
  • calculate power, force or velocity using P = Fv, resolving an angled force where required
  • apply P = Fv to propulsion, lifting or resistive-force problems and check that the force and velocity refer to the same direction

5.1.7—P = Fv and use it to solve problems question 1

[Maximum number: 2]

A car travels in a straight line up a slope, as shown in Fig. 3.1.

Fig. 3.1

Fig. 3.1

The car has mass 850 kg and travels with a constant speed of 9.0 ms−19.0 \mathrm{~ms}^{-1}. The car's engine exerts a force on the car of 2.0 kN up the slope.

A resistive force FDF_{\mathrm{D}}, due to friction and air resistance, opposes the motion of the car.
The variation of FDF_{\mathrm{D}} with the speed v of the car is shown in Fig. 3.2.

Fig. 3.2

Fig. 3.2

Show that the power output of the car is 1.8×104 W1.8 \times 10^{4} \mathrm{~W}.

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