# Homework Help: Work and energy question

1. Mar 26, 2015

### snav96

1. A cross-country skier ascends a peak and rises 300 m vertically during the ascent. Her mass including clothing skis, poles, shoes and backpack is 65 kg.
1. (a) Find the minimum mechanical work needed to be done by the skier (neglect friction).

2. (b) Why do we not need to consider the skier’s kinetic energy in part (a)?

3. (c) What is the metabolic rate needed, if the efficiency for this activity is 11% and it took 30 min to climb up that 300 m? (‘rate’ here means energy/time, i.e. power).

4. (d) If the mean sliding friction force opposing motion of the skis when slid forward with each stride is 4.5 N, how much energy is dissipated by friction in a distance of 1200 m measured along the path?

2. Relevant equations

3. The attempt at a solution

a) Potential Energy = 300 m x 9.8 x 65 Kg = 191100 J

b) because the speed stays constant. there's no change in kinetic energy

c) amount of efficient energy = 191100 x 0.11 = 21021 J
Power = 21021 / 30 x 60 = 11.68 w

d) ?

can anyone check to see if my answers are correct? and help me with part d? cause i have no idea how to answer part d

thanks

2. Mar 27, 2015

### vela

Staff Emeritus
Efficiency is (useful output)/input. The useful output in this problem is the work needed to ascend the 300 m.

Part d is asking you to calculate the work done by friction.

3. Mar 31, 2015

### snav96

so it is it just 4.5N x 1200m ?

4. Mar 31, 2015

### vela

Staff Emeritus
Yup.

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