All Free Physics MCQs with Answers

81. Which pair of quantities are both scalars?

• A. Force and acceleration
• B. Velocity and momentum
• C. Work and power
• D. Torque and displacement

Explanation: Work and power have magnitude only, with work being a dot product and power its rate of change with time. Force, acceleration, velocity, momentum, torque and displacement are all vectors. The habit worth building is to ask whether reversing the direction changes the physical meaning, since for work it does not.

82. A ladder rests against a smooth wall. The wall exerts a force on the ladder that is

• A. along the ladder
• B. vertically upwards
• C. perpendicular to the wall
• D. parallel to the wall

Explanation: A smooth surface can exert no friction, so the only force it can apply is the normal reaction, which by definition acts at right angles to the surface. That makes the reaction from a vertical wall horizontal. The ground, being rough, supplies both a normal force and the friction that stops the ladder sliding out.

83. Three forces acting on a body are in equilibrium. When drawn head to tail they must

• A. lie along one straight line
• B. form a closed triangle
• C. all be equal in magnitude
• D. be mutually perpendicular

Explanation: A zero resultant means the head of the last vector coincides with the tail of the first, so the three vectors close a triangle. This triangle of forces lets an unknown force be found by scale drawing or by the sine rule. The three need not be equal, and they cannot be collinear unless two of them point the same way.

84. In a transverse wave, the particles of the medium vibrate

• A. along the direction of propagation
• B. perpendicular to the direction of propagation
• C. in circles of increasing radius
• D. not at all

Explanation: Light, waves on a string and ripples on water are transverse, with the displacement at right angles to the direction the energy travels. In a longitudinal wave such as sound the vibration is along the direction of travel, producing compressions and rarefactions. Only transverse waves can be polarised, which is the standard experimental test of which type a wave is.

85. The relationship between the speed v, frequency f and wavelength of a wave is

• A. v equals f divided by wavelength
• B. v equals f times wavelength
• C. v equals wavelength divided by f
• D. v equals f plus wavelength

Explanation: In one period the wave advances exactly one wavelength, so dividing wavelength by period gives the speed, and since frequency is the reciprocal of period this becomes v equals f multiplied by wavelength. Speed is fixed by the medium, so raising the frequency shortens the wavelength in proportion. That inverse relationship is the point most questions turn on.

86. A wave of frequency 500 Hz travels at 340 m per second. Its wavelength is

• A. 0.68 m
• B. 1.47 m
• C. 170 m
• D. 0.34 m

Explanation: Wavelength is speed divided by frequency, so 340 divided by 500 gives 0.68 m. Multiplying instead of dividing produces the absurdly large 170 m, which a quick sanity check on audible sound would rule out. Middle C at about 262 Hz has a wavelength of roughly 1.3 m, which is a useful reference point.

87. The speed of sound is greatest in

• A. air
• B. water
• C. steel
• D. vacuum

Explanation: Sound travels fastest in solids because their particles are closely bound and pass on a disturbance quickly, giving about 5000 m per second in steel against 1500 in water and 340 in air. It cannot travel through a vacuum at all, since it needs a material medium. Light behaves in exactly the opposite way, travelling fastest in a vacuum.

88. The speed of sound in air increases when

• A. the pressure alone increases at constant temperature
• B. the temperature increases
• C. the humidity falls
• D. the frequency of the sound increases

Explanation: Speed rises by roughly 0.61 m per second for every degree Celsius, because hotter molecules move faster and transmit the disturbance more quickly. Changing pressure alone has no effect, since density changes in the same proportion and the two cancel. Speed is also independent of frequency, which is why a distant band is heard in tune rather than smeared out.

89. The principle of superposition states that when two waves meet, the resultant displacement at any point is

• A. the vector sum of the individual displacements
• B. always zero
• C. always double one of them
• D. the larger of the two displacements

Explanation: Displacements add algebraically, so waves in phase reinforce and waves in antiphase cancel, and afterwards each wave continues unchanged. This single principle underlies interference, beats and stationary waves alike. Cancellation is only complete when the two amplitudes are equal as well as opposite.

90. For two sources to produce a steady, observable interference pattern they must be

• A. of different frequencies
• B. coherent, keeping a constant phase relationship
• C. very far apart
• D. of unequal amplitude

Explanation: If the phase difference between the sources drifts, the positions of the bright and dark fringes shift too fast to be seen and the pattern washes out. Coherence in practice means the same frequency and a fixed phase relationship, which is why Young used a single source split into two rather than two separate lamps. Equal amplitudes are not required, but they give the greatest contrast.