Unit notes and practice questions
Everything here is for unit 2 of the Grade 12 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 12 Physics page. Open one at a time.
When you use it
Use when relating the orbital period of a planet or satellite to its average orbital radius around a central mass.
Watch out
The mass M in the denominator is the mass of the central body being orbited, not the orbiting satellite.
Drafted from Grade 12 Physics, pages 25-68, then checked twice before it went up
When you use it
Use to compute the gravitational attraction force acting between any two bodies with mass.
Watch out
The separation distance r is measured between the centers of the bodies, not between their surfaces.
Drafted from Grade 12 Physics, pages 25-68, then checked twice before it went up
When you use it
Use when finding the horizontal range of a projectile launched from and landing on the same level ground.
Watch out
This equation applies only when the launch elevation and landing elevation are identical.
Drafted from Grade 12 Physics, pages 25-68, then checked twice before it went up
| Linear motion | Rotational motion |
|---|---|
When you use it
Use to solve rotational motion problems with constant angular acceleration by mapping directly from linear kinematics.
Watch out
All angular displacements must be converted to radians before substituting into these kinematic equations.
Drafted from Grade 12 Physics, pages 25-68, then checked twice before it went up
When you use it
Use to connect the net rotational force with rotational inertia and angular acceleration of a rigid body.
Watch out
Angular acceleration must always be in radians per second squared, never revolutions per second squared.
Drafted from Grade 12 Physics, pages 25-68, 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
In ideal projectile motion where air resistance is negligible, which of the following quantities remains constant throughout the flight?
Answer: B Horizontal velocity
Why
Gravity acts only in the vertical direction. Because there is no horizontal force acting on the projectile, the horizontal acceleration is zero and the horizontal velocity vx remains constant.
Question 2
At what launch angle with respect to the horizontal is the horizontal range of an inclined projectile maximum on level ground?
Answer: B 45 degrees
Why
The range formula is R = (v02 sin 2θ) / g. The factor sin 2θ reaches its maximum value of 1 when 2θ = 90°, which gives θ = 45°.
Question 3
What is the SI unit of moment of inertia (I)?
Answer: B kg m^2
Why
The moment of inertia for a point mass is given by I = m r2. The SI unit of mass is kg and distance is m, giving the unit kg m2.
Question 4
According to Kepler's first law of planetary motion, what is the shape of a planet's orbit around the Sun?
Answer: C An ellipse with the Sun at one focus
Why
Kepler's first law, also known as the law of ellipses, states that the orbit of a planet around the Sun is an ellipse with the Sun located at one of the two foci.
Question 5
An airplane flying horizontally at 50 m/s over Dire Dawa drops a food relief package from a height of 180 m. Taking g = 10 m/s2, how long does it take for the package to hit the ground?
Answer: A 6.0 s
Why
The initial vertical velocity is zero. Using Δy = (1/2) g t2, we have 180 = (1/2)(10) t2, which gives 5 t2 = 180, so t2 = 36 and t = 6.0 s.
Question 6
Two football players in Addis Ababa kick identical balls with the same initial speed v0. Ball A is launched at an angle of 37° and Ball B is launched at 53°. Neglecting air resistance, which statement is true about their horizontal ranges on level ground?
Answer: C Both balls achieve the same horizontal range.
Why
Projectiles launched at complementary angles (angles that add up to 90°, such as 37° and 53°) with the same initial speed have the same horizontal range because sin(2 × 37°) = sin 74° and sin(2 × 53°) = sin 106° = sin 74°.
Question 7
A bicycle wheel in Hawassa accelerates uniformly from rest to an angular speed of 20 rad/s in 4.0 s. What is the angular acceleration of the wheel?
Answer: B 5 rad/s^2
Why
Angular acceleration is defined as α = (ωf - ω0) / Δt. Substituting the given values: α = (20 rad/s - 0) / 4.0 s = 5 rad/s2.
Question 8
A technician applies a force of 150 N perpendicular to a beam at a distance of 0.4 m from the pivot point. What is the magnitude of the torque produced?
Answer: B 60 N m
Why
Torque is calculated using τ = r F sin θ. Since the force is perpendicular, θ = 90° and sin 90° = 1. Thus, τ = 0.4 m × 150 N × 1 = 60 N m.
Question 9
A projectile is launched from level ground in such a way that its horizontal range R is equal to 3 times its maximum height H. What is the launch angle θ?
Answer: D 53 degrees
Why
The textbook gives the relation H = (R tan θ) / 4. Setting R = 3H gives H = (3H tan θ) / 4, so 1 = (3 tan θ) / 4, which leads to tan θ = 4/3 ≈ 1.33. Therefore, θ = 53°.
Question 10
A wheel with a moment of inertia of 2 kg m2 changes its angular speed from 3 rad/s to 8 rad/s under the action of a constant net torque of 50 N m. How long does the wheel take to make this change?
Answer: A 0.20 s
Why
First, use τ = I α to find the angular acceleration: α = τ / I = 50 / 2 = 25 rad/s2. Next, using ωf = ω0 + α t, we get t = (ωf - ω0) / α = (8 - 3) / 25 = 5 / 25 = 0.20 s.
The app holds 12 questions on this unit, marked as you go. These are the first 10.
Unit 2 runs from page 25 to page 68 of the Ministry of Education edition.
Ministry of Education edition, English, 186 pages
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