POWER
THE RATE AT WHICH WORK IS BEING DONE
POWER
• THE RATE OF DOING WORK
• WORK = FORCE * DISTANCE
IS
Power  Work / time
Power  Fd / t
Power  Force  velocity
SO WHAT IS THE UNIT FOR POWER?
Joules/sec
or Watts
1 watt is the power produced by
a force of 1N moving an object
1ms-1
CALCULATE & COMPARE POWER
• DURING THE ASCENT PHASE OF A REP OF THE BENCH PRESS, TWO
LIFTERS EACH EXERT AN AVERAGE VERTICAL FORCE OF 1000 N
AGAINST A BARBELL WHILE THE BARBELL MOVES 0.8 M UPWARD. BUT
EACH TAKES A DIFFERENT AMOUNT OF TIME. CALCULATE THE POWER
OF EACH.
LIFTER A: 0.50 SECONDS
LIFTER B: 0.75 SECONDS
CALCULATE & COMPARE POWER
LIFTER A
LIFTER B
TABLE OF VARIABLES
TABLE OF VARIABLES
F = 1000 N
F = 1000 N
D = 0.8 M
T = 0.50 S
Fd
t
1000 N  0.8m
Power 
0.50 s
800 J
Power 
 1600 w
0.50 s
Power 
D = 0.8 M
T = 0.75 S
Fd
t
1000 N  0.8m
Power 
0.50 s
800 J
Power 
 1067 w
0.75s
Power 
POWER ON A STATIONARY BIKE
P Fd / t
GIVEN A FORCE OF 3N AND
DISPLACEMENT OF 6M PER
REVOLUTION, WHAT IS THE
POWER PER MINUTE IF A
REVOLUTION TAKES 1 SEC?
P Fd  rev / min
P  18 Nm *60 / min
P 1080 Nm / min
P  1080 J / min
RUNNING UP STAIRS
• DURING THE POWERHOUSE LAB, JEROME RUNS UP THE STAIRS, ELEVATING HIS
102 KG BODY A VERTICAL DISTANCE OF 2.29 METERS IN A TIME OF 1.32
SECONDS AT A CONSTANT SPEED.
A. DETERMINE THE WORK DONE BY JEROME IN CLIMBING THE STAIR CASE.
B. DETERMINE THE POWER GENERATED BY JEROME.
RUNNING UP STAIRS
• DURING THE POWERHOUSE LAB, JEROME RUNS UP THE STAIRS, ELEVATING HIS
102 KG BODY A VERTICAL DISTANCE OF 2.29 METERS IN A TIME OF 1.32
SECONDS AT A CONSTANT SPEED.
A. W = 102 * 10 * 2.29 = 2335.8J
B. 2335.8 / 1.32 = 1769.5W
•
• THE SKI SLOPES AT BLUEBIRD MOUNTAIN MAKE USE OF TOW ROPES
TO TRANSPORT SNOWBOARDERS AND SKIERS TO THE SUMMIT OF
THE HILL. ONE OF THE TOW ROPES IS POWERED BY A 22-KW
MOTOR WHICH PULLS SKIERS ALONG AN ICY INCLINE OF 14° AT A
CONSTANT SPEED. SUPPOSE THAT 18 SKIERS WITH AN AVERAGE
MASS OF 48 KG HOLD ONTO THE ROPE AND SUPPOSE THAT THE
MOTOR OPERATES AT FULL POWER.
• A. DETERMINE THE CUMULATIVE WEIGHT OF ALL THESE SKIERS.
B. DETERMINE THE FORCE REQUIRED TO PULL THIS AMOUNT OF
WEIGHT UP A 14° INCLINE AT A CONSTANT SPEED.
C. DETERMINE THE SPEED AT WHICH THE SKIERS WILL ASCEND THE
HILL.
• THE SKI SLOPES AT BLUEBIRD MOUNTAIN MAKE USE OF TOW
ROPES TO TRANSPORT SNOWBOARDERS AND SKIERS TO THE
SUMMIT OF THE HILL. ONE OF THE TOW ROPES IS POWERED
BY A 22-KW MOTOR WHICH PULLS SKIERS ALONG AN ICY
INCLINE OF 14° AT A CONSTANT SPEED. SUPPOSE THAT 18
SKIERS WITH AN AVERAGE MASS OF 48 KG HOLD ONTO THE
ROPE AND SUPPOSE THAT THE MOTOR OPERATES AT FULL
POWER.
• A. DETERMINE THE CUMULATIVE WEIGHT OF ALL THESE SKIERS.
B. DETERMINE THE FORCE REQUIRED TO PULL THIS AMOUNT OF
WEIGHT UP A 14° INCLINE AT A CONSTANT SPEED.
C. DETERMINE THE SPEED AT WHICH THE SKIERS WILL ASCEND
THE HILL.
a. Weight = 48*18*10*cos(14)
= 8383N
b. Force = 8383 * sin(14)
= 2028N
c. 22000 = 2028 * v
v = 22000/2028
= 10.8ms-1