Chapter 3: MACHINES
Q 1.
(i)State four uses of a machine and give one example of each.
(ii)What is a machine?
ANSWERS:
(i)
(a) In lifting a heavy load by applying less effort, i.e., as a force multiplier. For example, A
jack is used to lift a car.
(b)In changing the point of application of effort to a convenient point, e.g., The rear wheel
of a cycle is rotated by applying the effort on the pedal attached to the toothed wheel which is
joined to the rear wheel with the help of a chain.
(c)In changing the direction of effort to a convenient direction. E.g., using a single fixed
pulley.
(d)Far obtaining a gain in speed. E.g., When a pair of scissors is used to cut the cloth, while
its handles move a little.
(ii)A machine is a device by which we can either overcome a large resistance force (or load) at some
point by applying a small force (or effort) at a convenient point and in a desired direction by which
we can obtain a gain in speed.
Q 2.
Define the following terms related to a simple machine
(i) Load(L)
(ii) Effort€
(iii) Mechanical advantage (M.A)
(iv) Velocity ratio
(v) Work input
(vi) Load input
(vii) Efficiency
ANSWERS:
(i) The resistive or opposing force to be overcome by the machine is called load.
(ii) The force applied on the machine to overcome the load is called effort.
(iii) The ratio of the load to the effort is called mechanical advantage of the machine.
, (iv) The ration of the velocity of effort to the velocity of load is called the velocity ratio of the
machine.
(v) The work done on the machine by the effort is called work input, i.e., work input=work
done by effort.
(vi) The work done by the machine on load is called the work output, i.e., work output =
work done on load.
(vii) Efficiency is the ratio of work done on load by the machine to work done on machine by
effort.
Q 3.
Principle of machine
Ans) When energy is supplied to a machine by applying the effort it overcomes the load by doing
some useful work on it. The useful work done on the machine can never be greater than the
work done on the machine, otherwise it will violate the principle of conservation of energy
therefore, no machine has efficiency greater than 1 (i.e., more than 100%).
Q 4.
(i) What is an ideal machine?
(ii) What is an actual/practical machine?
(iii) State three reasons for loss of energy in a machine due to which no machine can be
ideal?
ANSWERS:
(i) An ideal machine is that in which there is no loss of energy in any manner. Here the
work output is equal to the work input i.e., the efficiency of an ideal machine is 100%.
(ii) In an actual machine, the output energy is always less than the input energy i.e., there is
always some loss of energy during its operation.
(iii) The loss of energy in a machine is due to the following three reasons:
(a) the moving parts in it are neither weightless nor smooth (or frictionless),
(b) the string in it (if any) is not perfectly elastic, and
(c) its different parts are not perfectly rigid.
Note: The most prominent loss in energy is in overcoming the force of friction between
the moving parts of a machine. The energy so lost appears as heat due to which the
moving parts of the machine get heated.
Q 5.
Show that n=MA/VR, where the symbols have their usual meaning.
Let a machine overcome a load L by application of an effort E in time t. Let displacement of effort be
dE and the displacement of load by d L.
Q 1.
(i)State four uses of a machine and give one example of each.
(ii)What is a machine?
ANSWERS:
(i)
(a) In lifting a heavy load by applying less effort, i.e., as a force multiplier. For example, A
jack is used to lift a car.
(b)In changing the point of application of effort to a convenient point, e.g., The rear wheel
of a cycle is rotated by applying the effort on the pedal attached to the toothed wheel which is
joined to the rear wheel with the help of a chain.
(c)In changing the direction of effort to a convenient direction. E.g., using a single fixed
pulley.
(d)Far obtaining a gain in speed. E.g., When a pair of scissors is used to cut the cloth, while
its handles move a little.
(ii)A machine is a device by which we can either overcome a large resistance force (or load) at some
point by applying a small force (or effort) at a convenient point and in a desired direction by which
we can obtain a gain in speed.
Q 2.
Define the following terms related to a simple machine
(i) Load(L)
(ii) Effort€
(iii) Mechanical advantage (M.A)
(iv) Velocity ratio
(v) Work input
(vi) Load input
(vii) Efficiency
ANSWERS:
(i) The resistive or opposing force to be overcome by the machine is called load.
(ii) The force applied on the machine to overcome the load is called effort.
(iii) The ratio of the load to the effort is called mechanical advantage of the machine.
, (iv) The ration of the velocity of effort to the velocity of load is called the velocity ratio of the
machine.
(v) The work done on the machine by the effort is called work input, i.e., work input=work
done by effort.
(vi) The work done by the machine on load is called the work output, i.e., work output =
work done on load.
(vii) Efficiency is the ratio of work done on load by the machine to work done on machine by
effort.
Q 3.
Principle of machine
Ans) When energy is supplied to a machine by applying the effort it overcomes the load by doing
some useful work on it. The useful work done on the machine can never be greater than the
work done on the machine, otherwise it will violate the principle of conservation of energy
therefore, no machine has efficiency greater than 1 (i.e., more than 100%).
Q 4.
(i) What is an ideal machine?
(ii) What is an actual/practical machine?
(iii) State three reasons for loss of energy in a machine due to which no machine can be
ideal?
ANSWERS:
(i) An ideal machine is that in which there is no loss of energy in any manner. Here the
work output is equal to the work input i.e., the efficiency of an ideal machine is 100%.
(ii) In an actual machine, the output energy is always less than the input energy i.e., there is
always some loss of energy during its operation.
(iii) The loss of energy in a machine is due to the following three reasons:
(a) the moving parts in it are neither weightless nor smooth (or frictionless),
(b) the string in it (if any) is not perfectly elastic, and
(c) its different parts are not perfectly rigid.
Note: The most prominent loss in energy is in overcoming the force of friction between
the moving parts of a machine. The energy so lost appears as heat due to which the
moving parts of the machine get heated.
Q 5.
Show that n=MA/VR, where the symbols have their usual meaning.
Let a machine overcome a load L by application of an effort E in time t. Let displacement of effort be
dE and the displacement of load by d L.
