Exercise 7.1
State whether True or False. (i) Work is said to be done when a force is applied, even if the object does not move. (ii) Lifting a bucket vertically upward results in positive work done on the bucket. (iii) The SI unit for both work and energy is joule (J). (iv) A motionless stretched rubber band has kinetic energy. (v) Energy can change from one form to another.
Not cross-checked
NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.
NCERT’s answer
(i)
F (ii) T (iii) T (iv) F (v) T
(i) False — work needs displacement. \(\displaystyle W = F \times s \), and if \(\displaystyle s = 0 \) then \(\displaystyle W = 0 \), however hard you push.
(ii) True — the applied force is upward and the bucket's displacement is upward, so force and displacement are in the same direction and the work done is positive.
(iii) True — both work and energy are measured in the joule (J), where \(\displaystyle 1\ \text{J} = 1\ \text{N} \times 1\ \text{m} \).
(iv) False — it is motionless, so \(\displaystyle K = \frac{1}{2}mv^{2} = 0 \). What it stores, because of its deformation, is potential energy.
(v) True — for example electrical energy becomes light energy in a bulb, and chemical energy of food becomes mechanical energy in our muscles.