A sports technician tests an electric resistance sled on an indoor track. Fig. 1 shows the sled pulled by a student using a handle. The student pulls with a...
Question 1 Report
A sports technician tests an electric resistance sled on an indoor track. Fig. 1 shows the sled pulled by a student using a handle. The student pulls with a horizontal force of 165 N for 240 m. The sled travels at constant speed and takes 48 s. Its motor and control box draw an electrical power of 1.1 kW while the sled is moving. The force sensor sends data to a computer.
(a) Calculate the work done by the student on the sled. [3]
(b) Calculate the useful power transferred to the sled. [2]
(c) Calculate the efficiency of the motor and control box. [3]
(d) Explain why the horizontal resultant force on the sled is zero while its speed is constant. [2]
(e) Describe the main energy transfer as the student does work on the sled. [1]
(f) State the unit of power. [1]
Answer Details
(a) Work done is force multiplied by distance moved in the force direction:
\[W=Fs=165\times240=39\,600\text{ J}\]
39 600 J. [3 marks]
(b)
\[P=\frac{W}{t}=\frac{39\,600}{48}=825\text{ W}\]
825 W. [2 marks]
(c) Convert the input power first:
\[1.1\text{ kW}=1100\text{ W}\]
\[\text{efficiency}=\frac{825}{1100}=0.75=75\%\]
0.75, or 75%. [3 marks]
(d) The sled moves at constant speed, so it has no acceleration and therefore no resultant force. The pulling force must equal the resistive force. [2 marks]
(e) Chemical energy in the student's energy store is transferred mechanically to the sled. [1 mark]
(f) The unit of power is the watt, W. [1 mark]
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