Page 207 - Physics

Work, energy and power

Example 9.2 So, 140 J of work was required to lift

How much work is done in lifting a 7 the object at a height of 2 m, but no
kg object through a height of 2 m and additional work was done in holding
then holding it at that height for 10 s? the object at that height for 10 s.
(Assume, g = 10 N/kg)
Solution Exercise 9.1
Mass (m) ) = 7 kg, distance(d) = 2 m,
Force(F
g = 10 N/kg Answer all questions (where necessary
use g = 10 N/kg).
To lift an object or hold it in the air, an 1. If a man pushes a van with a force
upward force equal to its weight must
be exerted. of 300 N for a distance of 10 m, how
much work does he do?
Weight (N) of an object = mass (kg) of

an object × acceleration due to gravity 2. A man lifts a load of 20 kg through
(N/kg) a height of 3 m. Calculate the work
done.
w = mg
3. Zenge lifts a brick of mass 10 kg
But, weight = Force from the fl oor to a shelf that is 3 m

Then, high. How much work does she do?

Force = mg = 7 kg × 10 N/kg = 70 N 4. In each of the following cases, state
whether work is done or not.
So, you must exert on the object an
upward force of 70 N. (a) A cow pulling a 50 kg cart of
maize along a rough road.
To lift the object a distance of 2 m, the (b) A teacher holding a book in his
work done is calculated as follows: hand while talking to his friend.

W = F × d
(c) An aeroplane fl ying upward at
= 70 N × 2 m
take-off.
∴W = 140 J
(d) An aeroplane moving

To hold the object for 10 s: horizontally 3 000 m above sea
F = 70 N, d = 0 m level
W = F × d 5. A sack of rice which weighs 900 N
= 70 N × 0 m is lifted to a height of 2 m. Calculate
the work done against gravity.
= 0 J

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Physics Form 1 Final.indd 201 16/10/2024 20:58