Simple Machines: Exercise 4 Complete Solutions
Learn work, energy, mechanical advantage, efficiency, levers, pulleys, inclined planes, wedges, screws, and wheel-and-axle systems through interactive questions, diagrams, and verified numerical solutions.
Chapter Overview
A quick revision before solving the exercise.
Work
Work is done when a force produces displacement in its direction. If the object does not move, no mechanical work is done on it.
Energy
Energy is the capacity of a body or system to do work.
Machine
A machine makes work more convenient by changing the magnitude or direction of force. It does not create energy or reduce the total ideal work required.
Six Simple Machines
Example: beam balance
Example: well pulley
Example: door knob
Example: ramp
Example: needle or axe
Example: bottle cap
Three Classes of Levers
Class I Lever: Fulcrum between effort and load
Arrangement: Effort - Fulcrum - Load. Examples: see-saw, beam balance, scissors, pliers.
Class II Lever: Load between fulcrum and effort
Arrangement: Fulcrum - Load - Effort. Examples: wheelbarrow, nutcracker, lemon squeezer. Mechanical advantage is greater than 1 in the ideal lever.
Class III Lever: Effort between fulcrum and load
Arrangement: Fulcrum - Effort - Load. Examples: forceps, sugar tongs. Mechanical advantage is less than 1.
Mechanical Advantage Calculator
Efficiency Calculator
Exercise 4 - Objective Type Questions
Select an option, check it, and then read the explanation.
For an ideal machine, the efficiency is:
Reason: For an ideal machine, useful work output equals work input, so efficiency is 1 or 100%.
Mechanical advantage of a machine is defined as:
Reason: Mechanical advantage tells us how many times a machine multiplies the applied effort.
The mechanical advantage of an ideal lever is equal to:
Reason: By the principle of moments, Effort x Effort arm = Load x Load arm.
A single fixed pulley is commonly used because it:
Reason: A fixed pulley changes the direction of effort, allowing a downward pull to raise a load.
A wheel is used with an axle because:
Reason: Rolling surfaces offer much less resistance than sliding surfaces, making movement easier.
Question 2: Fill in the Blanks
(a) The useful work done by an actual machine is always ______ than the work done on the machine.
(b) In Class II levers, the ______ is between the fulcrum and the ______.
(c) The mechanical advantage of a Class ______ lever is always less than 1.
(d) A fixed pulley is used to change the ______ of effort.
(e) The mechanical advantage of an ideal inclined plane is always ______ than 1.
(a) less
(b) load; effort
(c) III
(d) direction
(e) greater
Question 3: True or False
(a) A boy does mechanical work on a wall while pushing it, even if the wall does not move.
(b) A machine performs work by itself without an energy input.
(c) In an ideal machine, work done on the load equals work done by the effort.
(d) All levers are force multipliers.
(e) A single fixed pulley changes the direction of force.
(f) An ideal non-vertical inclined plane has mechanical advantage greater than 1.
(a) False. The wall has no displacement, so no mechanical work is done on it.
(b) False. A machine requires energy or work input.
(c) True. An ideal machine has no energy loss.
(d) False. Class III levers are speed or distance multipliers and have MA less than 1.
(e) True. A fixed pulley reverses the direction of effort.
(f) True. Ideal MA = length of slope / vertical height, which is greater than 1 for a non-vertical plane.
Question 4: Match the Columns
| Column A | Correct Match | Machine Type |
|---|---|---|
| Needle | (v) | Wedge |
| Door knob | (iv) | Wheel and axle |
| Ramp | (ii) | Inclined plane |
| Lemon crusher | (i) | Class II lever |
| See-saw | (iii) | Class I lever |
Exercise 4 - Short Answer Type
Open each question to reveal the corrected answer.
Work is said to be done when a force acting on a body produces displacement in the direction of the force. If no displacement occurs, no mechanical work is done.
Energy is the capacity or ability to do work.
A machine is a device that makes work more convenient by changing the magnitude or direction of an applied force. It cannot create energy.
A machine follows the principle of conservation of energy. Energy supplied to a machine is transformed into useful output energy and, in actual machines, some unwanted forms such as heat and sound.
A machine can:
- Multiply force, so a smaller effort can move a larger load.
- Change the direction of the applied force to a more convenient direction.
Work input: Work done on a machine by the applied effort.
Work output: Useful work done by the machine on the load.
Mechanical advantage is the factor by which a machine multiplies the applied effort.
MA = Load / Effort
Efficiency is the ratio of useful work output to work input.
Efficiency = (Useful work output / Work input) x 100%
An ideal machine is a machine with no energy loss. Its work output equals its work input, and its efficiency is 1 or 100%.
No. In real machines, some input energy is lost mainly as heat due to friction, and sometimes as sound or deformation. Therefore, useful output is less than input.
It means 75% of the input work becomes useful output work, while 25% is lost in forms such as heat due to friction.
A lever is a rigid bar that can turn about a fixed point called the fulcrum.
MA = Load / Effort = Effort arm / Load arm
This follows from the principle of moments: Effort x Effort arm = Load x Load arm.
An ideal Class II lever always has mechanical advantage greater than 1 because its effort arm is longer than its load arm. Example: nutcracker.
A Class III lever always has mechanical advantage less than 1 because its effort arm is shorter than its load arm. Example: forceps or sugar tongs.
(i) MA greater than 1: a pair of pliers when the load is near the fulcrum.
(ii) MA equal to 1: a beam balance with equal arms.
(iii) MA less than 1: a pair of scissors when cutting near the tips of long blades.
| Object | Lever Class |
|---|---|
| Pair of scissors | Class I |
| Lemon squeezer | Class II |
| Nutcracker | Class II |
| Sugar tongs | Class III |
| Beam balance | Class I |
| Oar used for rowing | Class I in the usual school model |
| Wheelbarrow | Class II |
| See-saw | Class I |
| Pair of pliers | Class I |
| Crowbar | Usually Class I when the fulcrum lies between effort and load |
(a) Class II lever: wheelbarrow.
(b) Class I lever: pair of scissors.
(c) Class III lever: forceps.
Increase the effort arm, decrease the load arm, or do both.
Friction uses part of the applied effort, so less force is transmitted to the load. Therefore, the actual mechanical advantage decreases.
| Feature | Class I | Class II | Class III |
|---|---|---|---|
| Middle component | Fulcrum | Load | Effort |
| Arm lengths | Effort arm may be shorter, equal to, or longer than load arm | Effort arm is longer than load arm | Effort arm is shorter than load arm |
| Ideal MA | Less than, equal to, or greater than 1 | Greater than 1 | Less than 1 |
A pulley is a grooved wheel over which a rope or chain passes. A single fixed pulley changes the direction of effort and helps raise a load conveniently.
For an ideal single fixed pulley, the effort equals the load, so MA = 1. Other pulley systems can have MA greater than 1.
A screw is an inclined plane wound around a cylindrical rod. Examples: screw jack and threaded bottle cap.
A wheel and axle consists of a large wheel rigidly fixed to a smaller axle so both rotate together. It can multiply turning effect. Examples: steering wheel and door knob.
Effort applied at the rim of the larger wheel acts through a greater radius and produces a larger turning effect on the smaller axle. The wheel and axle rotate together.
A wedge is formed by two inclined planes meeting at a sharp edge. It is used for cutting, splitting, or piercing. Examples: axe and needle.
(a) Beam balance: Class I lever
(b) Lemon crusher: Class II lever
(c) Sugar tongs: Class III lever
(d) Ramp: inclined plane
(e) Door knob: wheel and axle
(f) Needle: wedge
- Keep the machine clean and free from dust.
- Protect iron parts from rust by painting or coating them.
- Lubricate moving parts regularly to reduce friction and wear.
- Use the machine within its safe load limit and service it on time.
Correct statement: Friction in the moving parts of a machine reduces its efficiency.
(a) A wheelbarrow is Class II, not Class I.
(b) Actual machines have efficiency below 100%.
(d) Class II levers can have MA greater than 1.
(e) An inclined plane allows a load to be raised with less effort over a longer distance.
(f) A wedge is made from two inclined planes; a screw is an inclined plane wound around a cylinder.
Exercise 4 - Long Answer Type
Clear explanations with text-based diagrams that remain responsive in Blogger.
| Simple Machine | Example |
|---|---|
| Lever | Beam balance |
| Pulley | Well pulley |
| Wheel and axle | Steering wheel |
| Inclined plane | Ramp |
| Wedge | Nail or axe |
| Screw | Bottle cap |
Class I Lever
The fulcrum lies between the effort and the load. Its mechanical advantage may be less than, equal to, or greater than 1, depending on the arm lengths. Example: see-saw.
Diagram: Effort - Fulcrum - Load
Class II Lever
The load lies between the fulcrum and the effort. The effort arm is longer than the load arm, so its ideal mechanical advantage is greater than 1. Example: nutcracker.
Diagram: Fulcrum - Load - Effort
Class III Lever
The effort lies between the fulcrum and the load. The effort arm is shorter than the load arm, so mechanical advantage is less than 1. Example: forceps.
Diagram: Fulcrum - Effort - Load
(a) See-saw: Class I. The fulcrum is between load and effort. In a balanced see-saw, the two arms are equal.
(b) Beam balance: Class I. The central fulcrum lies between the load and balancing effort; the arms are equal.
(c) Nutcracker: Class II. The load lies between the hinge fulcrum and the effort applied at the handles.
(d) Forceps: Class III. The effort is applied between the joined end acting as fulcrum and the load at the tips.
In an actual fixed pulley, friction at the axle and bending of the rope use part of the applied effort. Therefore, the effort is slightly greater than the load and actual MA is less than 1.
It is still useful because it changes the direction of effort. We can pull downward in a convenient direction to lift the load upward.
Support
O
Load upward on one side | Effort downward on the other side
For an ideal single fixed pulley:
Effort = Load and MA = 1.
An inclined plane is a sloping surface used to raise or lower a load with less effort over a longer distance.
For an ideal inclined plane:
MA = Length of the plane / Vertical height
Since the length of a non-vertical plane is greater than its height, its ideal MA is greater than 1.
Examples: A hospital ramp for wheelchairs and stretchers; a wooden plank used to roll a heavy drum onto a truck.
Across: Machine, Pulley, Force.
Down: Friction, Wheel, Axle, Lever.
Exercise 4 - Numericals
All numerical answers have been checked step by step.
Frequently Asked Questions
Fast revision for Class 6 Physics.
A simple machine is a basic device that makes work more convenient by changing the magnitude or direction of force.
Mechanical advantage = Load / Effort.
Some input energy is lost as heat, sound, and deformation, mainly because of friction.
An ideal Class II lever always has MA greater than 1 because the effort arm is longer than the load arm.
Its mechanical advantage is 1 because effort equals load.
It allows the same height to be reached with a smaller force applied over a longer distance.
Follow Speed Up Science on Instagram
Get quick science explanations, revision posts, quizzes, and learning updates.