Unit notes and practice questions

Grade 9 Physics unit 5: Simple Machines

Everything here is for unit 5 of the Grade 9 Physics textbook: what the chapter covers, the short notes, practice questions with their answers, and the lesson video where one exists.

4
Cards: 10
Practice questions on unit 5: 82–113
The textbook pages for unit 5

What unit 5 covers

1.Simple Machines and their Purposes
2.Simple Machines at Home
3.Simple Machines at Work Place
4.Classification of Simple Machines
5.Mechanical Advantage, Velocity Ratio and Efficiency of Simple Ma
6.Designing Simple Machine

Short notes for unit 5

The cards filed under this chapter on the Grade 9 Physics page. Open one at a time.

Machine Efficiency, Mechanical Advantage and Velocity RatioFormula

\eta = \frac{\text{MA}}{\text{VR}} = \frac{\text{Work output}}{\text{Work input}}

\eta = \frac{\text{MA}}{\text{VR}} = \frac{\text{Work output}}{\text{Work input}}

$\eta$: Efficiency of the machine $\text{dimensionless}$
$\text{MA}$: Mechanical Advantage, ratio of Load to Effort $\text{dimensionless}$
$\text{VR}$: Velocity Ratio, distance moved by Effort over distance moved by Load $\text{dimensionless}$
$\text{Work output}$: Useful work done by the machine on the Load $\text{J}$
$\text{Work input}$: Work put into the machine by the Effort $\text{J}$

When you use it

Use when finding the efficiency of any simple machine or relating output work and input work to mechanical advantage and velocity ratio.

Watch out

Mechanical Advantage, Velocity Ratio, and Efficiency are ratios and have no units. Efficiency is always less than 1 (or less than 100 percent) in real machines.

Drafted from Grade 9 Physics, pages 82-113, then checked twice before it went up

Machines Do Not Multiply Energy or WorkCommon mistake

When you use it

Use when evaluating true or false statements and conceptual questions about whether machines create energy or decrease total work.

Watch out

Machines never multiply work or energy. A machine can multiply force or multiply speed and distance, but never both at the same time. Due to friction, work output is always less than work input, so AMA is always less than IMA.

Drafted from Grade 9 Physics, pages 82-113, then checked twice before it went up

Three Classes of LeversReference table1st Class · 2nd Class · 3rd Class

Class	Middle Component	IMA Characteristic	Examples
1st Class	Fulcrum	$\text{IMA} > 1$ , $\text{IMA} = 1$ , or $\text{IMA} < 1$	Seesaw, scissors, crowbar
2nd Class	Load	$\text{IMA} > 1$	Wheelbarrow, nutcracker, bottle opener
3rd Class	Effort	$\text{IMA} < 1$	Tweezers, broom, fishing pole, tongs

When you use it

Use to identify the class of a lever based on which point lies in the middle: Fulcrum, Load, or Effort.

Watch out

Remember the middle component order: 1st class has Fulcrum in the middle, 2nd class has Load in the middle, and 3rd class has Effort in the middle.

Drafted from Grade 9 Physics, pages 82-113, then checked twice before it went up

Velocity Ratio and IMA of Simple MachinesReference tableInclined Plane · Wedge · Screw · +2

Simple Machine	Velocity Ratio (VR) Formula	Symbols
Inclined Plane	$\text{VR} = \frac{l}{h}$	$l = \text{slope length},\; h = \text{height}$
Wedge	$\text{VR} = \frac{l}{t}$	$l = \text{penetration length},\; t = \text{thickness}$
Screw	$\text{IMA} = \frac{2\pi r}{p} = \frac{\pi d}{p}$	$r = \text{radius},\; d = \text{diameter},\; p = \text{pitch}$
Wheel and Axle	$\text{VR} = \frac{R}{r}$	$R = \text{wheel radius},\; r = \text{axle radius}$
Pulley System	$\text{VR} = N$	$N = \text{number of rope strands supporting load}$

When you use it

Use to calculate the theoretical Velocity Ratio or Ideal Mechanical Advantage directly from the physical dimensions of the machine.

Watch out

For a screw, pitch is the distance between consecutive threads. For wheel and axle, ensure the effort radius is divided by the load radius.

Drafted from Grade 9 Physics, pages 82-113, then checked twice before it went up

Practice questions on unit 5

10 multiple-choice questions written against this chapter, each with its answer and the reason. Try one before you open the answer.

Question 1

What is the main purpose of using a simple machine?

A.To create energy out of nothing
B.To make mechanical work easier
C.To reduce the amount of total work required to zero
D.To increase the mass of an object

Show the answer and why

Answer: B To make mechanical work easier

Why

A simple machine is a device that helps us do mechanical work more easily. Simple machines do not create energy; they only transfer it.

Question 2

Simple machines are classified into two main groups: inclined planes and levers. Which of the following belongs to the inclined plane group?

A.Pulley
B.Wheel and axle
C.Wedge
D.Crowbar

Show the answer and why

Answer: C Wedge

Why

The inclined plane group consists of the ramp (inclined plane), wedge, and screw. Pulleys and wheel and axles belong to the lever group.

Question 3

What term is used for the force applied to a simple machine by an external body, such as a person?

A.Effort
B.Load
C.Friction
D.Work output

Show the answer and why

Answer: A Effort

Why

Effort is the force exerted on a machine by an external body like a human being, while load is the force exerted by the machine on the object being moved.

Question 4

In a second-class lever, which part is located in the middle between the other two?

A.The fulcrum
B.The load
C.The effort
D.The axle

Show the answer and why

Answer: B The load

Why

In a second-class lever (such as a wheelbarrow), the load is placed in the middle, between the fulcrum and the effort.

Question 5

A person in Gojjam uses a claw hammer to pull out a nail. An effort of 20 N is applied to pull a load force of 100 N. What is the mechanical advantage (M.A) of the hammer?

A.0.2
B.5
C.80
D.120

Show the answer and why

Answer: B 5

Why

Mechanical advantage is calculated as: M.A = Load / Effort. Here, M.A = 100 N / 20 N = 5.

Question 6

An inclined plane in Addis Ababa is used to slide heavy goods into a warehouse. The slope has a length of 30 m and raises objects to a vertical height of 5 m. What is the velocity ratio (V.R) of this inclined plane?

A.0.17
B.6
C.25
D.150

Show the answer and why

Answer: B 6

Why

For an inclined plane, V.R = length of inclined surface (l) / height of inclined plane (h). Thus, V.R = 30 m / 5 m = 6.

Question 7

A simple machine provides a work output of 80 J when 400 J of work is supplied to it. What is the efficiency of this machine?

A.20%
B.50%
C.80%
D.320%

Show the answer and why

Answer: A 20%

Why

Efficiency is defined as: η = (work output / work input) × 100%. So, η = (80 J / 400 J) × 100% = 0.2 × 100% = 20%.

Question 8

A lever has its effort located 40 cm from the fulcrum and its load located 10 cm from the fulcrum. What is the velocity ratio (V.R) of this lever?

A.0.25
B.4
C.30
D.50

Show the answer and why

Answer: B 4

Why

For a lever, V.R = distance from effort to fulcrum (d_E) / distance from load to fulcrum (d_L). Here, V.R = 40 cm / 10 cm = 4.

Question 9

A machine has a mechanical advantage (M.A) of 3 and a velocity ratio (V.R) of 6. What is the efficiency of this machine?

A.18%
B.20%
C.50%
D.200%

Show the answer and why

Answer: C 50%

Why

Efficiency expressed in terms of M.A and V.R is: η = M.A / V.R. Here, η = 3 / 6 = 0.5 = 50%.

Question 10

A wheel and axle is used to lift a bucket of water from a deep well near Hawassa. The radius of the wheel is 30 cm and the radius of the axle is 5 cm. What is the velocity ratio (V.R) of this wheel and axle?