Unit notes and practice questions
Everything here is for unit 4 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.
The cards filed under this chapter on the Grade 9 Physics page. Open one at a time.
When you use it
Use to find the stored energy of an object raised to a height above the ground.
Watch out
Height h is the vertical distance above the reference point, not the path length along a slope.
Drafted from Grade 9 Physics, pages 60-81, then checked twice before it went up
When you use it
Use to calculate the energy possessed by an object due to its motion.
Watch out
Square only the speed v, not the mass. Doubling the speed quadruples the kinetic energy.
Drafted from Grade 9 Physics, pages 60-81, then checked twice before it went up
When you use it
Use when calculating acceleration, net force, or mass of an accelerating body.
Watch out
Mass must always be converted to kg before calculating. Unbalanced force produces acceleration, not constant velocity.
Drafted from Grade 9 Physics, pages 60-81, then checked twice before it went up
When you use it
Use to calculate the rate at which work is done or energy is transferred.
Watch out
Time t must be converted to seconds before dividing. One watt is one joule per second.
Drafted from Grade 9 Physics, pages 60-81, then checked twice before it went up
When you use it
Use when a constant force moves an object through a displacement parallel to the force.
Watch out
Carrying an object horizontally does no work against gravity because force and displacement are perpendicular.
Drafted from Grade 9 Physics, pages 60-81, then checked twice before it went up
10 multiple-choice questions written against this chapter, each with its answer and the reason. Try one before you open the answer.
Question 1
Which of the following is an example of a non-contact force?
Answer: C Gravitational force
Why
Non-contact forces act without physical contact between objects. Gravitational force and magnetic force are non-contact forces, while friction, normal, and spring forces require direct physical contact.
Question 2
Which property of matter is measured directly by its mass?
Answer: B Inertia
Why
According to Newton's first law, mass is a measure of inertia. Objects with larger masses have larger inertia and resist changes to their state of motion more strongly.
Question 3
The SI unit of power is the watt (W). What is 1 watt equal to?
Answer: A 1 Joule per second (1 J/s)
Why
Power is defined as the rate at which work is done or energy is transferred (P = E / t). Therefore, 1 W = 1 J/s.
Question 4
In physics, in which of the following situations is mechanical work done on an object?
Answer: D A student lifts a heavy box vertically from the floor to a table.
Why
Work is done only when a force causes a displacement of the object in the direction of the force. Lifting a box vertically applies an upward force that moves the box upward.
Question 5
A net horizontal force of 80 N acts on a 20 kg block on a smooth floor in Addis Ababa. What is the acceleration of the block?
Answer: B 4 m/s²
Why
From Newton's second law, F = ma, so acceleration is a = F / m = 80 N / 20 kg = 4 m/s².
Question 6
A farmer pushes a sack of teff across a horizontal store floor with a force of 50 N through a displacement of 8 m in the direction of the force. How much work is done?
Answer: C 400 J
Why
Work is given by W = F * S = 50 N * 8 m = 400 J.
Question 7
A ball of mass 0.5 kg is thrown at a speed of 6 m/s across a field in Hawassa. What is its kinetic energy?
Answer: C 9 J
Why
Kinetic energy is calculated using E_k = (1/2) * m * v² = 0.5 * 0.5 kg * (6 m/s)² = 0.25 * 36 = 9 J.
Question 8
A stone of mass 10 kg is placed on the roof of a house at a height of 15 m above the ground. Taking g = 10 m/s², what is its gravitational potential energy?
Answer: B 1500 J
Why
Gravitational potential energy is given by E_p = m * g * h = 10 kg * 10 m/s² * 15 m = 1500 J.
Question 9
A water pump in Bahir Dar lifts 300 kg of water through a vertical height of 6 m in 10 seconds. Taking g = 10 m/s², what is the power developed by the pump?
Answer: C 1800 W
Why
The potential energy given to the water is E_p = m * g * h = 300 kg * 10 m/s² * 6 m = 18000 J. Power is P = E_p / t = 18000 J / 10 s = 1800 W.
Question 10
How fast must an object of mass 2 kg move in order to have a kinetic energy of 64 J?
Answer: B 8 m/s
Why
From E_k = (1/2) * m * v², we get v² = (2 * E_k) / m = (2 * 64 J) / 2 kg = 64 m²/s². Taking the square root gives v = 8 m/s.
The app holds 12 questions on this unit, marked as you go. These are the first 10.
Unit 4 runs from page 60 to page 81 of the Ministry of Education edition.
Ministry of Education edition, English, 179 pages
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