1. Machines:

(i) Machine is a device which helps us to do the work at one point and deliver it at another point with a view of accomplishing the work easily.

(ii) Machines can be classified into a lever and inclined plane.

(iii) The lever is further classified into pulley, gear and wheel and axle. Similarly, Inclined plane is classified into wedge and screw.

(iv) Functions performed by machines are

(a) as a force multiplier

(b) to increase or decrease the speed

(d) to change the point of application of effort.

2. Technical Terms Related to a Machine:

(i) Load is the resistive or opposing force against which the machine does the work. It is denoted by $L$ .

(ii) Effort is the force applied to the machine to overcome the load. It is denoted by $E$ .

(iii) Mechanical Advantage $(M A)$ : It is the ratio of the load to the effort. $M A = \frac{L}{E} = \frac{E A}{L A}$ where $E A$ and $L A$ are the length of effort arm and load arm.

(iv) Efficiency $(η)$ : It is the ratio of the work done on the load by the machine to the work put into the machine by the effort. $η = \frac{W_{o}}{W_{i}} \times 100$

(v) Velocity Ratio $(V R)$ : It is the ratio of the distance moved by the effort $d_{E}$ to the corresponding distance moved by the load $d_{L}$ .

(vi) For an ideal machine, $η$ is $100 %$ .

(v) In an actual machine, the output energy is always less than the input energy.

(vi) Relation between $η, M A$ and $V R$ : If $η$ is expressed as efficiency percentage, then $η = \frac{M A}{V R} \times 100$ .

3. Simple machines:

(i) A lever is a straight or bent rod which can be turned or rotated about a fixed point called fulcrum.

(ii) In class I lever, the effort $(E)$ and the resistance (load, $L$ ) are situated on either side of the fulcrum $(F)$ . e.g., A pair of scissors, handle of a water pump, a catapult and the nodding of the human head, etc.

(iii) In class II lever, the load is situated between fulcrum and effort. e.g., A bottle opener, a wheelbarrow, a paper cutter, raising the weight of the human body on toes, etc.

(iv) In class III lever, an effort is situated between the load and the fulcrum. e.g., Sugar tongs, foot treadle, etc.

(v) Three classes of levers are found in the human body (i) class I lever in the action of nodding of head (ii) class II lever in raising the weight of the body on toes (iii) class III lever in raising a load by the forearm.

(vi) Pulley is a wheel on an axle that is designed to support movement and change the direction of a cable or belt along its circumference. A set of pulleys assembled so that, they rotate independently on the same axle to form a block is called pulley system.

(vii) A pulley which has its axis of rotation fixed in position is called a single fixed pulley. For this pulley $M A = 1, V R = 1, η = 100 %$ .

(viii) A pulley whose axis of rotation is not fixed in position is called a movable pulley. For this pulley $M A = 2, V R = 2, η = 100 %$ .

(ix) For combination of pulleys using one fixed pulley and other movable pulley $M A = 2^{n}, V R = 2^{n}, η = 100 %$ .

(x) For block and tackle system $M A = n, V R = n$ with weight of pulley, $M A = n - \frac{W}{E}, V R = n, η = 1 - \frac{W}{n E}$ .

(xi) A gear is a wheel with teeth around its rim. A gear wheel closer to the source of power is called the driver (or the driving gear), while the gear wheel which receives motion from the driver is called the driven gear. For this, gear ratio $= \frac{N_{A}}{N_{B}}$ , gain in torque $= \frac{N_{B}}{N_{A}} = \frac{r_{B}}{r_{A}}$ and gain in speed $= \frac{n_{B}}{n_{A}} = \frac{N_{A}}{N_{B}}$

(xii) The inclined plane is a sloping surface that behaves like a simple machine whose mechanical advantage is always greater than $1$ . $M A = \frac{1}{\sin θ}, V R = \frac{l}{h}$ where $l, h$ are the length and height of the inclined plane.

What is the expression to find the efficiency of a machine?

Important Questions on Machines

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