Calculating work and power

Question

Aggh just can't do this assignment:

1. What is the work required to raise a 10. kg object from the surface of the Earth to a height of 2.0 m?

2. A Net Force of 9.0 N acts through a distance of 3.0 m in a time of 3.0 s. The work done is

3. As the time required to run up a stairs increases, the power developed by that person:

decreseases or remains the same?

Answer 1

E = m*g*h = 10*9.8*2 = 196 J

E = Fd = 9*3 = 27 J

power is rate of doing work. So if he runs faster, work stays constant per flight, and time goes down, so power goes up.

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Answer 2

<< What is the work required to raise a 10. kg object from the surface of the Earth to a height of 2.0 m? >>

Work is multiplying the height at which a body is moved.

W = mgh

where

W = work

m = mass of the body = 10 kg (given)

g = acceleration due to gravity = 9.8 m/sec^2

h = height = 2 m (given)

Substituting values,

W = (10)(9.8)(2) = 196 joules

<<  A Net Force of 9.0 N acts through a distance of 3.0 m in a time of 3.0 s. The work done is >>

Work = Force * distance

Work = 9 * 3 = 27 joules

<<  As the time required to run up a stairs increases, the power developed by that person: decreseases or remains the same? >>

Power = Work/Time

The work done by the same person going up the stairs whether he/she is running, walking or crawling REMAINS CONSTANT.

Thus being so, if the time going up the stairs is increased, then the power is decreased.

Example, if a 100 kg person walks up to the top of the stairs (height of stairs, for example, is 2 meters, then the work done by that person is

Work = 100(9.8)(2) = 1960 joules

If, for example, it took the person 10 seconds to go up this flight of stairs, then

Power = Work/Time = 1960/10 = 196 joules/sec.

If, however, it took the person 20 seconds to go up this flight of stairs, then

Power = Work/Time = 1960/20 = 98 joules/sec.

  

Answer 3

The equation for work is Force*Distance. The equation for force is mass*acceleration. So, for number 1 we first need to find the force it takes  to lift a 10kg object on earth. The acceleration of gravity on earth is 9.8m/s/s, so the force it takes to lift is 98 Newtons. To get the work done you multiply the force by the distance you are applying that force. Therefore the work done is 196 Joules because 98 Newtons*2 meters is 196 Joules. For the second problem you are given the force so you simply multiply the 9 Newtons by 3 meters, giving 27 Joules. The 3 seconds is not needed to calculate work. The equation for power is Work/time. So as the divisor (time) gets bigger the quotient (Power) gets smaller if the dividend (work) stays the same.