LAB POWER

INTRODUCTION, PLEASE READ
REmember: Work = force x distance. Force in newton, distance in meters and work in joules.

The concept or work is important in Physics. It allows us to measure the rate at which work is done:
Power is the rate of doing  work - That is, the work done in a unit time interval.
Mathematically, power can be found by the following equation:
P = W /t . Power is measured in Watts (W)
A Watt is a joule per second/ A watt is small. The power needed to lift a
glass of water in a normal way is about one watt.
So power is usually measured in kilowatts (kW).
1 kilowwat = 1,000 watts.

PROBLEMS
1) An electric motor hoists an elevator (1.20 10⁴ N) 9.00 m in 15.0s. A) What is the power of the motor in watts ? B) in kilowatts ?

2)A box that weights 1.00 10³N is lifted a distance of 20.0 m straight up by a rope and pulley system.
THe work  is done in 10s. What is the power developed in watts and in kilowatts ?

3) A rock climber wears a 12.0 kg knapsack while scaling a cliff. After 30.0 minutes, the climber is 8.2m above the starting point.
A) How much work is joules is done on the knapsack ?
B) If the climber weights 6.00 10²N, how much total work is done against gravity?


4) An electric motor develops 65KW of power as it lifts a loaded elevator  18.0m in 40.0s . How much force does the motor deliver?


5) A gardener applies a force of 150N to push a wheelbarrow 50.0m at a constant speed for 2.0 10¹s.
A) what is the gardener's power in watts?
B) What is the gardener 's power if the speed is doubled?


6) A pump raises 35 liters of water per minute from a depth of 110m. What is the wattage expended ?
hint : a liter of water has a mass of 1kg.

7) A boat's engine propels it through water at a constant speed of 15m/s. The force of friction that the boat needs to overcome to maintain this speed is
6.0 10³N. What is the powe of the engine in kilowatts?

8) Neglecting frictional effects, how much work is done to accelerate a 1.0 10³ kg car from an initial speed of 25m/s to 35m/s, if the acceleration given to it by its engine is 1.25m/s/s ?
hint: YOU NEED TO USE ALL YOUR INDEX CARDS
(F = ma  , V2² = V1² + 2 a d,   W = F d )

9) A horizontal force of 500N is needed to drag a crate across a horizontal floor with a constant speed.
A) How much work do you do on the crate when moving it 22 m?
B) If you complete the job in 8.0 s , what power is developed ?

10) A student performs a laboratory activity in which a constant 15 newton force acts on a 2kg mass.
THe work done over time is summarized in the data table. (Remember P = W / t or W = P t

time (s)	work (J)
0	0
1	32
2	59
3	89
4	120

A) plot the data points. work an approriate scale for work. title the graph. label the axes. don't forget to write the units.
B) Draw the line of best fit. (don't connect the dots)
C) Find the slope. (don't forget the units)
D) What is the pysical significance of the slope of the graph ?
E) This analysis shows you that even if the work done increases over time, the rate at which the work is done is _____________.
THat means, the student perform the same amount of worh every ____________.

11) Dr. Lankar, a 700 newton physics teacher, runs at a constant speed up a flight of stairs rising 6.0 meters in 7.0 seconds. Explain why the teacher can claim more powerful than five 100-watt light bulbs ?
(hint : first find the power delivered by the teacher. P = W / t. with W = F x d)




watch that video about power
Simply Science: Rate of Energy Conversion