Complete the following by choosing the correct answer from the bracket:

(a) Bottle opener is an example of …………… [class I/class II/class III] lever.

(b) If there are n movable pulley with one fixed pulley, the mechanical advantage is given by …………… $[2^n/2^{n+1}/2^{n-1}]$

(c) According to work energy theorem, the increase in the kinetic energy of a moving body is equal to …………… by a force acting in the direction of moving body [velocity/acceleration/work done].

(d) If a machine does a given amount of work in less time, …………… power is spent by it. [less/more/same]

(e) If the length l of wire is increased to 3l by stretching its resistance R increases to …………… [3R/6R/9R]

(f) If a coil is viewed from one end, and the current flows in clockwise direction, then the end is a ………….. pole. [north/south]

(a) Bottle opener is an example of class II lever.

(b) If there are n movable pulley with one fixed pulley, the mechanical advantage is given by 2ⁿ.

(c) According to work energy theorem, the increase in the kinetic energy of a moving body is equal to work done by a force acting in the direction of moving body.

(d) If a machine does a given amount of work in less time, more power is spent by it.

(e) If the length I of wire is increased to 31 by stretching its resistance R increases to 9R.

(f) If a coil is viewed from one end, and the current flows in clockwise direction, then the end is a south pole.

Reason —

(a) In a bottle opener, the fulcrum and the effort are at the two ends of it and the load is somewhere in between the effort and the fulcrum in such a way so that the load and effort are on the same side of the fulcrum but acting in opposite directions same as class II lever so here the effort arm is always longer than the load arm.

(b) As each movable pulley doubles the mechanical advantage by halving the effort required, and these effects multiply as more movable pulleys are added like a single movable pulley which divides the load between 2 rope segments which make the effort needed only half the load and has mechanical advantage equal to 2 so similarly n movable pulleys divide the load between 2n rope segments which make mechanical advantage equal to 2ⁿ.

(c) Work energy theorem states that the increase in kinetic energy of a moving body is equal to the work done by a force acting in the direction of the moving body.

(d) Power spent by a source is given by the amount of work done by the source per unit time taken by the source to do the said work i.e., $\text{Power} = \dfrac{\text {Work done}}{\text {Time taken}}$ So, if a machine does a given amount of work in less time then more power is spent by it.

(e) The resistance of a wire is directly proportional to its length and inversely proportional to the area of cross section of the wire and is given as :

$\text R = \text ρ\dfrac{\text l}{\text a}$

When length of a conductor is increased by 3 times then it's cross sectional area decreases to 1/3 to keep volume constant therefore, resistance increases by 3 × 3 = 9 times.

(f) The polarity at the two faces of a coil depends on the direction of current in the coil so while looking at the face of the coil, if the current in wire around that face is in anticlockwise direction, the face has the north polarity, while if the current at that face is in clockwise direction, the face has the south polarity.