The work done by the force acting on an object is zero if the displacement of the object
- A.is in the opposite direction of the direction of force
- B.is in the same direction of the direction of force
- C.is in perpendicular direction of the direction of force
- D.none of above
Correct answer
Explanation
The correct answer is C, is in perpendicular direction of the direction of force. Work is W = F × d × cos θ, and when the angle θ between force and displacement is 90°, cos 90° = 0, so the work done is zero.
Work is done only by the part of a force that lies along the displacement. A force at right angles to the motion has no such part. Common examples tested in papers: a porter carrying a load on his head while walking on level ground does no work against gravity, the centripetal force on a stone whirled in a circle does no work, and the Sun's pull on a planet in a circular orbit does no work.
A is wrong because a force opposite to the displacement (θ = 180°) does negative work, as friction does on a sliding block.
B is wrong because a force along the displacement (θ = 0°) does positive work, the largest possible for that force.
D is wrong because option C gives the correct condition.
Exam tip: θ = 0°, positive and maximum work; θ = 90°, zero work; θ = 180°, negative work.
Practice Questions
View allWhich one of the following frequency ranges is sensitive to human ears ?
- A.0-200 Hz
- B.20-20,000 Hz
- C.200-20,000 Hz only
- D.2,000-20,000 Hz only
Show answer
The normal human ear hears sound from about 20 hertz to 20,000 hertz, the audible range. Frequency, measured in hertz, is the number of vibrations per second, and it decides the pitch of a sound: the higher the frequency, the shriller the note.
Read the full explanation →The frequency of ultrasound waves is
- A.less than 20 Hz
- B.between 20 Hz and 2 kHz
- C.between 2 kHz and 20 kHz
- D.greater than 20 kHz
Show answer
Ultrasound is sound with a frequency above 20,000 Hz, the upper limit of human hearing. Humans hear sound between about 20 Hz and 20 kHz.
Read the full explanation →Which one of the following is an example of a clean fuel ?
- A.Coke
- B.Propane
- C.Petrol
- D.Wax
Show answer
Propane is a light gaseous hydrocarbon that burns completely with a blue flame, leaving no smoke, soot or ash. A clean fuel burns completely, gives high heat and releases few pollutants.
Read the full explanation →A sample of 'soft soap' contains
- A.Caesium
- B.Potassium
- C.Calcium
- D.Magnesium
Show answer
Soft soaps are potassium salts of fatty acids, while ordinary hard bar soaps are sodium salts. Soap is made by saponification, heating an oil or fat with an alkali.
Read the full explanation →Which of the following sets of elements has the same valency?
- A.Na, Mg, Ca
- B.Na, Mg, Al
- C.Mg, Ca, K
- D.Mg, Ca, Ba
Show answer
Magnesium, calcium and barium are all alkaline earth metals of group 2, and each has a valency of 2. Elements in the same group of the periodic table have the same number of electrons in their outermost shell, and so the same valency.
Read the full explanation →