Unit notes and practice questions
Everything here is for unit 4 of the Grade 11 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 11 Physics page. Open one at a time.
When you use it
Use this when a question asks for the resultant of an action force and its reaction, or when you add forces to find the net force on one body.
Watch out
Action and reaction are equal and opposite but act on two different bodies, so they cannot be added to give zero. Sum only the forces acting on the same body.
Drafted from Grade 11 Physics, pages 95-174, then checked twice before it went up
When you use it
Use this to decide whether a body at rest will stay at rest, or to find the largest friction force before it starts to slide.
Watch out
Static friction is only at most mu_s F_N, not always equal to it. Once the body slides, switch to kinetic friction, F_k = mu_k F_N, which is smaller than the static maximum.
Drafted from Grade 11 Physics, pages 95-174, then checked twice before it went up
When you use it
Use this when a body accelerates under one or more forces and you need either the acceleration or the net force.
Watch out
Use the net force, the vector sum of all forces including friction, not the applied force alone. Example 4.7 subtracts friction before dividing by the total mass.
Drafted from Grade 11 Physics, pages 95-174, then checked twice before it went up
When you use it
Use this when a block rests on or slides down a slope at angle theta and you need the surface's push on it, for example before calculating friction.
Watch out
Normal force equals weight only on a level surface. On a slope it is mg cos theta, which is smaller, so friction on the slope must be calculated from this value.
Drafted from Grade 11 Physics, pages 95-174, then checked twice before it went up
When you use it
Use this to find the weight of a body near Earth's surface or on another planet or moon.
Watch out
Weight is a force in newtons while mass is in kilograms. The value of g changes from place to place, so the same mass has a different weight on the Moon (g = 1.6 m/s^2 in Example 4.1).
Drafted from Grade 11 Physics, pages 95-174, 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
According to Newton's first law of motion, which property of an object quantitatively measures its inertia?
Answer: A Mass
Why
Inertia is the tendency of a body to oppose any change in its state of rest or uniform motion. As the textbook explains, larger masses have larger inertia than small masses.
Question 2
Which of the following is one of the four fundamental forces in nature?
Answer: A Strong nuclear force
Why
The four fundamental forces in nature are the strong nuclear force, electromagnetic force, weak nuclear force, and gravitational force. Friction, tension, and normal forces are contact forces.
Question 3
In the centimeter-gram-second (cgs) unit system, work is measured in ergs. How many ergs are equal to 1 joule?
Answer: A 10<sup>7</sup> erg
Why
By definition, 1 joule = 107 erg, where 1 erg = 1 dyne . cm.
Question 4
A 1 kg package of teff flour is taken to the surface of the Moon, where the acceleration due to gravity is one-sixth of its value on Earth (gm = 1.6 m/s2). What is the weight of the package on the Moon?
Answer: A 1.6 N
Why
Weight is given by W = mgm = (1 kg)(1.6 m/s2) = 1.6 N.
Question 5
In Hawassa, a man pushes a stalled vehicle with a constant force F = (150i - 40j) N through a displacement S = (14i + 11j) m. How much work does the man do on the car?
Answer: A 1660 J
Why
Work is the dot product of force and displacement: W = F . S = (150)(14) + (-40)(11) = 2100 - 440 = 1660 J.
Question 6
Two blocks with masses m1 = 4 kg and m2 = 8 kg are connected by a light string on a frictionless level surface. A horizontal force of 36 N pulls m1 to the right. What is the acceleration of the system and the tension in the connecting string?
Answer: A a = 3 m/s<sup>2</sup>, T = 24 N
Why
The total mass is m1 + m2 = 4 kg + 8 kg = 12 kg. Acceleration a = F / (m1 + m2) = 36 N / 12 kg = 3 m/s2. The tension pulling the rear mass m2 is T = m2 a = (8 kg)(3 m/s2) = 24 N.
Question 7
A 6 kg block rests on top of a ramp inclined at 37° above the horizontal. Taking g = 9.8 m/s2 and cos(37°) = 0.8, what is the normal force acting on the block?
Answer: A 47 N
Why
The normal force on an inclined plane is FN = mg cos θ = (6 kg)(9.8 m/s2)(cos 37°) = (58.8 N)(0.8) = 47 N.
Question 8
A student pulls a wooden box across a floor by applying a force of 20 N at an angle of 30° above the horizontal. If the box moves a horizontal distance of 5 m, what is the work done by the applied force? (Take cos 30° = 0.866)
Answer: A 86.6 J
Why
Work done by a force at an angle is W = F S cos θ = (20 N)(5 m)(cos 30°) = (100 Nm)(0.866) = 86.6 J.
Question 9
In an Atwood machine, two masses m1 = 4 kg and m2 = 6 kg are connected by a light cord over a frictionless pulley. Taking g = 9.8 m/s2, what are the acceleration of the masses and the tension in the cord?
Answer: A a = 1.96 m/s<sup>2</sup> and T = 47 N
Why
Acceleration a = g(m2 - m1)/(m1 + m2) = 9.8(6 - 4)/(6 + 4) = 1.96 m/s2. The tension in the cord is T = m1(g + a) = 4(9.8 + 1.96) = 47 N.
Question 10
A 6 kg crate rests on a ramp inclined at 30° above the horizontal. If the crate remains at rest, what is the magnitude of the static friction force preventing it from sliding, and what is the coefficient of static friction? (Take g = 9.8 m/s2)
Answer: A F<sub>s</sub> = 29.4 N and µ<sub>s</sub> = 0.58
Why
At rest, the static friction balances the component of gravity parallel to the incline: Fs = mg sin 30° = (6 kg)(9.8 m/s2)(0.5) = 29.4 N. The coefficient of static friction is µs = tan 30° = 0.58.
The app holds 12 questions on this unit, marked as you go. These are the first 10.
Unit 4 runs from page 95 to page 174 of the Ministry of Education edition.
Ministry of Education edition, English, 338 pages
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