1. Calculation of Gravitational P. E
Example: Suppose the mass of an object is m which is lifted vertically through a height h m
Weight of object = m  g
= mg h
Calculation of G.P.E:
Force needed to lift object = Weight of object
=mg
Work done = force  distance lifted = mg  h
But we know
Energy = Work done
P.E = mgh

2. A package of 5kg is lifted vertically through a distance of 10m at a constant speed. Calculate P.E gained by the package.
We know
P.E = mgh
= 5  10  10
= 500J
Here,
m =5 kg
g =10 m/s2
h =10 m
P.E = ?
A package of 250 kg is lifted vertically through a distance of 3m at a constant speed. Calculate P.E gained by the package.
We know
P.E = mgh
= 250  10  3
= 7500J
Here,
m =250 kg
g = 10 m/s2
h =3 m
P.E =?

3. Calculation of Kinetic Energy (K.E.)
A bullet of mass 0.02 kg travels at a speed of 1200 m/s. Calculate its K.E
We know
Here,
m = 0.02 kg
v = 1200 m/s
Ek=?

4. A bullet can break one object
A bullet can break one object. If the velocity of the bullet make twice of its initial velocity then find the number of the object it be can able to break.
Suppose, mass of bullet = m
Initial velocity = v
For 1st case
For 2nd case ;
need for object “
“ “ 1
“ “
= 4 object ans “

5. Here, m = 2 kg t = 2 sec g =10 m/s2 u = 0 Ek=? Ep=?
A packet of 2kg mass is dropped from 50m height. Find P.E and K.E after 2 sec.
Here,
m = 2 kg
t = 2 sec
g =10 m/s2 u = 0
Ek=?
Ep=?
50 m above total energy = P.E
= mgh
= 2  50  10
= 1000J
After 2 sec velocity
v - u = g  t
v = u + g  t
=  2 = 20 m/s
After 2 sec K.E
ans
But here
P.E + K.E = 1000J
P.E = 1000 – K.E = 1000 – 400 = 600J ans

6. Suppose you see two electric kettles which are rated at 1000W and 500W respectively. Then which one is more powerful and why?
Ans: The 1000W electric kettle is more powerful. If we put a similar amount of water into these two electric kettle to boil then the 1000W electric kettle will take less time than 500W electric kettle.
Calculate the power of a pump which can lift 200kg of water through a vertical height of 6m in 10S
We know
w = mgh
= 20010 6
= 12000J
Here
m = 200kg
h = 6m
g = 10N
again we know

7. A boy whose mass is 40kg finds that he can run a flight of 45 steps each of 16cm high, in 5.2s. Find the power walking upstairs.
We know
w = mgh
= 4010 7.2
= 2880J
Here
m = 40kg
h =(4516)cm =720cm =7.2m
g = 10N
again we know
An electric heater is rated at 250W. Calculate the quantity of heat generated in 10 minutes.
E = P  t =250  600=1.5  105Jans

8. An electric motor in a washing machine has a power output of 1. 0 kW
An electric motor in a washing machine has a power output of 1.0 kW. Find the work done in half an hour.
W = Pt
= (1.0103)(30  60)
= 1.8 106Jans
Explain the statement:
“A 100W lamp is more powerful than a 60W lamp”