The ratio of useful work output to work input
a. principle
b. efficiency
c. effort
d. load
Answers
Answer:
B.Efficiency
Explanation:
Efficiency can be expressed as the ratio of useful work output to total work input.
Answer:
Fixed pulley: A pulley which has its axis of rotation fixed in position, is called a fixed pulley.
Single fixed pulley is used in lifting a small load like water bucket from the well.
Question 1 (MCQ)
A single fixed pulley is used because it:
(a) Has a mechanical advantage greater than 1
(b) Has a velocity ratio less than 1
(c) Gives 100% efficiency
(d) Helps to apply the effort in a convenient direction.
Solution 1 (MCQ)
It helps in applying effort in a convenient direction.
Explanation: A single fixed pulley though does not reduce the effort but helps in changing the direction of effort applied. As it is far easier to apply effort in downward direction, the single fixed pulley is widely used.
Question 1 (Num)
A woman draws water from a well using a fixed pulley. The mass of bucket and water together is 6 kg. The force applied by the women is 70 N. calculate the mechanical advantage. (Take g = 10 m s-2).
Solution 1 (Num)
The force applied by the women is= 70 N
The mass of bucket and water together is = 6 kg
Total load = 6 x 10 = 60 N
Mechanical advantage Selina Solutions Icse Class 10 Physics Chapter - Machines
Question 2
What is the ideal mechanical advantage of a single fixed pulley? Can it be used as a force multiplier?
Solution 2
The ideal mechanical advantage of a single fixed pulley is 1.
It cannot be used as force multiplier.
Question 2 (MCQ)
The mechanical advantage of an ideal single movable pulley is:
(a) 1
(b) 2
(3) less than 2
(4) less than 1.
Solution 2 (MCQ)
The mechanical advantage of an ideal single movable pulley is 2.
Derivation: Consider the diagram given below:
Selina Solutions Icse Class 10 Physics Chapter - Machines
Here the load L is balance by the tension in two segments of the string and the effort E balances the tension T at the free end, so
L = T + T = 2T and E = T
Assumption: Weight of the pulley is negligible.
We know that,
Selina Solutions Icse Class 10 Physics Chapter - Machines
Thus, a single movable pulley has a M.A. equal to 2.
Question 2 (Num)
A fixed pulley is driven by a 100 kg mass falling at a rate of 8.0 m in 4.0 s. It lifts a load of 75.0 kgf. Calculate:
(a) The power input to the pulley taking the force of gravity on 1 kg as 10 N.
(b) the efficiency of the pulley, and
(c) the height to which the load is raised in 4.0 s.
Solution 2 (Num)
(a) Effort driving the pulley is, E = mg = 100 × 10 = 1000 N
Input power is
Selina Solutions Icse Class 10 Physics Chapter - Machines
(b) Load pulled by the pulley is L = 75 × 10 = 750 N
Therefore, M.A. is
Selina Solutions Icse Class 10 Physics Chapter - Machines
When the effort moves by a distance d downwards, the load moves by the same distance upwards. So, V.R. = 1
Hence, efficiency is
Selina Solutions Icse Class 10 Physics Chapter - Machines
(c) When the effort moves by a distance d downwards, the load moves by the same distance upwards. So, height to which the load moves is 8 m.
Question 3
Name the pulley which has no gain in mechanical advantage. Explain, why is such a pulley is then used?
Solution 3
There is no gain in mechanical advantage in the case of a single fixed pulley. A single fixed pulley is used only to change the direction of the force applied that is with its use, the effort can be applied in a more convenient direction. To raise a load directly upwards is difficult.
Question 3 (MCQ)
A movable pulley is used as :
(a) a force multiplier
(b) a speed multiplier
(c) a device to change the direction of effort
(d) an energy multiplier
Solution 3 (MCQ)
Force multiplier
Explanation: The mechanical advantage of movable pulley is greater than 1. Thus, using a single movable pulley, the load can be lifted by applying an effort equal to half the load (in ideal situation), i.e. the single movable pulley acts as a force multiplier.
Question 3(Num)
A single fixed pulley and a movable pulley both are separately used to lift a load of 50 kgf to the same height. Compare the efforts applied.
Solution 3(Num)
In case of a single fixed pulley, the effort (Ef needed to lift a load) is equal to the load itself.
Thus, Ef = L
→ Ef = 50 kgf
In case of a single movable pulley, the effort needed to lift a load is equal to half the load.
→ Em =Selina Solutions Icse Class 10 Physics Chapter - Machines
→ Selina Solutions Icse Class 10 Physics Chapter - Machines
Thus, the ratio of efforts applied by the respective pulley is
Selina Solutions Icse Class 10 Physics Chapter - Machines
Ef:Em = 2:1
Question 4
What is the velocity ratio of a single fixed pulley?
Solution 4
The velocity ratio of a single fixed pulley is 1.
Question 4 (Num)
In a block and tackle system consisting of 3 pulleys, a load of 75 kgf is raised with an effort of 25 kgf. Find: (i) the mechanical advantage, (ii) the velocity ratio, and (iii) the efficiency.
Solution 4 (Num)
Load = 75 kgf
Effort=25kgf
n = 3
MA = Load/Effort = 75/25 = 3
or MA = n = 3
velocityratio VR= n = 3
Efficiency Selina Solutions Icse Class 10 Physics Chapter - Machinesor 100%
Explanation: