8.4 CHAPTER 8. WORK, ENERGY AND POWER
watch. Note: the human body is not very efficient in these activities, so your
actual power will be much greater than estimated here.Exercise 8 - 3
- [IEB 2005/11 HG] Which of the following isequivalent to the SI unitof power:
(a) V·A
(b) V·A−^1
(c) kg· m·s−^1
(d) kg· m· s−^2- Two students, Bill and Bob, are in the weightlifting room of their local gum. Bill
lifts the 50 kg barbell over his head 10 times inone minute while Boblifts the
50 kg barbell over his head 10 times in 10 seconds. Who does the most work?
Who delivers the most power? Explain your answers. - Jack and Jill ran up the hill. Jack is twice asmassive as Jill; yet Jill ascended the
same distance in half thetime. Who did the mostwork? Who delivered the most
power? Explain your answers. - Alex (mass 60 kg) istraining for the Comrades Marathon. Part of Alex’s training
schedule involves push-ups. Alex does his push-ups by applying a force toelevate
his centre-of-mass by 20cm. Determine the number of push-ups that Alex must
do in order to do 10 J of work. If Alex does all this work in 60 s, then determine
Alex’s power. - When doing a chin-up, a physics student liftsher 40 kg body a distance of 0.25 m
in 2 s. What is the power delivered by the student’s biceps? - The unit of power that is used on a monthlyelectricity account is kilowatt-hours
(symbol kWh). This is aunit of energy deliveredby the flow of l kW of electricity
for 1 hour. Show how many joules of energy you get when you buy 1 kWh of
electricity. - An escalator is usedto move 20 passengers every minute from the first floor of a
shopping mall to the second. The second floor islocated 5-meters abovethe first
floor. The average passenger’s mass is 70 kg. Determine the power requirement
of the escalator in order to move this numberof passengers in this amount of
time. - Calculate the powerrequired for an electricmotor to pump 20kg ofwater up to
ground level from a borehole of depth 10m in half a minute.
More practice video solutions or help at http://www.everythingscience.co.za(1.) 01tz (2.) 01u0 (3.) 01u1 (4.) 01u2 (5.) 01u3 (6.) 01u4
(7.) 01u5 (8.) 01u6