$A$ weightlifter lifts $240 \,kg$ from the ground to a height of $2.5 \,m$ in $3 \,s$. The average power is:

Calculate the electricity bill amount for the month of April if $4$ bulbs of $40 \, W$ for $5 \, hrs$, $4$ tube lights of $60 \, W$ for $5 \, hrs$, a $TV$ of $100 \, W$ for $6 \, hrs$, and a washing machine of $400 \, W$ for $3 \, hrs$ are used per day. The cost per unit is ₹ $1.80$.

Given below are a few situations. Study them and state in which of the given cases work is said to be done. Give a reason for your answer:
$(i)$ $A$ person pushing hard a huge rock,but the rock does not move.
$(ii)$ $A$ bullock pulling a cart up to $1 \, km$ on the road.
$(iii)$ $A$ girl pulling a trolley for about $2 \, m$ distance.
$(iv)$ $A$ person standing with a heavy bag on his head.

$(a)$ State the law of conservation of energy. Explain it with one example.
$(b)$ Two girls,each of weight $400 \, N$,climb up a rope to a height of $8 \, m$. Girl $A$ takes $20 \, s$ while Girl $B$ takes $50 \, s$ to accomplish the task. What is the power expended by each girl?
$(c)$ An electric heater is rated $1500 \, W$. How much energy does it use in $10 \, h$?

What are the factors on which the work done depends?

State whether the following statement is true or false:
Work done by centripetal force is zero.