Page 134 - Understanding NCERT Science 09

Time taken, t = 20 s power, P = Work done/time taken
From Eq. (10.8),
mgh
Power, P = Work done/time taken = t
mgh
= 500 N × 6.75 m
t =
9s
400 N × 8 m = 375 W.
=
20s
Power is 375 W.
= 160 W.
(ii) Power expended by girl B: uestions
Weight of the girl, mg = 400 N
Displacement (height), h = 8 m Q 1. What is power?
Time taken, t = 50 s 2. Define 1 watt of power.
mgh 3. A lamp consumes 1000 J of
Power, P = electrical energy in 10 s. What is
t its power?
400 N × 8 m 4. Define average power.
=
50s

= 64 W. Activity ___________ 10.17
Power expended by girl A is 160 W. • Take a close look at the electric meter
Power expended by girl B is 64 W. installed in your house. Observe its
features closely.
• Take the readings of the meter each
day at 6.30 am and 6.30 pm.
Example 10.8 A boy of mass 50 kg runs • Do this activity for about a week.
up a staircase of 45 steps in 9 s. If the • How many ‘units’ are consumed
height of each step is 15 cm, find his during day time?
power. Take g = 10 m s . • How many ‘units’ are used during night?
–2
• Tabulate your observations.
Solution: • Draw inferences from the data.
• Compare your observations with
Weight of the boy, the details given in the monthly
mg = 50 kg × 10 m s = 500 N electricity bill (One can also estimate
–2
Height of the staircase, the electricity to be consumed by
h = 45 × 15/100 m = 6.75 m specific appliances by tabulating their
Time taken to climb, t = 9 s known wattages and hours of
From Eq. (10.8), operation).

WORK AND ENERGY 123

Rationalised 2023-24