Mechanical Aptitude Questions and Answers 2 — Questions and Answers

Question 1: What simple machine is a screw an example of?

A modified lever
An inclined plane wrapped around a cylinder (Correct answer)
A type of pulley
A wheel and axle

Correct answer: An inclined plane wrapped around a cylinder

A screw is an inclined plane (thread) wrapped helically around a cylinder. Each thread acts like a ramp, converting rotational motion into linear force with high mechanical advantage.

The screw is one of the six classical simple machines and can be understood as an inclined plane wrapped around a central shaft. The thread forms a continuous helical ramp — if you were to unroll one full revolution of thread, you would see a right triangle where the base equals the circumference (2πr) and the height equals the pitch (distance between threads). The mechanical advantage equals circumference divided by pitch: MA = 2πr/p. This is why fine-threaded screws (small pitch) can generate enormous clamping forces from modest input torque. Wood screws, machine screws, bolts, clamps, jack screws, and Archimedes' screw pumps all exploit this principle.

Question 2: If two gears are meshed and the driver has 15 teeth while the driven has 45 teeth, how many times does the driver rotate for each rotation of the driven gear?

1 time
2 times
3 times (Correct answer)
4 times

Correct answer: 3 times

The gear ratio is 45/15 = 3. The smaller driving gear must rotate 3 times to turn the larger driven gear once. This arrangement provides speed reduction and torque multiplication.

When gears mesh, the number of teeth determines the speed relationship. Since both gears must advance the same number of teeth per unit time at the mesh point, the gear with fewer teeth must complete more revolutions. With 15 teeth driving 45 teeth: the ratio is 45/15 = 3, meaning the 15-tooth gear makes 3 complete revolutions for each single revolution of the 45-tooth gear. This 3:1 reduction means the output turns at 1/3 the input speed but with 3 times the torque (ignoring friction losses). This is a common arrangement in gear reducers used in conveyor systems, winches, and industrial machinery where high torque at lower speed is needed.

Question 3: What is the difference between speed and velocity?

They are identical concepts
Speed includes direction, velocity does not
Velocity includes direction, speed does not (Correct answer)
Speed is faster than velocity

Correct answer: Velocity includes direction, speed does not

Speed is a scalar quantity (magnitude only — how fast something moves), while velocity is a vector quantity (magnitude and direction — how fast and in what direction). An object moving at 50 km/h north has the same speed as one at 50 km/h south, but different velocities.

Speed and velocity are related but fundamentally different. Speed (|v|) measures the rate of distance covered and is always positive. Velocity (v) measures the rate of displacement change and includes direction. A car driving in circles at a constant 60 km/h has constant speed but constantly changing velocity (because direction changes continuously). Average speed = total distance / time, while average velocity = displacement / time. A round trip has an average velocity of zero (displacement = 0) but a positive average speed. The distinction matters in physics because Newton's laws involve velocity (vectors), not speed. Acceleration is the rate of change of velocity, so any direction change involves acceleration even at constant speed.

Question 4: What keeps a satellite in orbit around Earth?

Rocket engines firing continuously
The absence of gravity in space
The balance between gravity pulling it inward and its forward velocity (Correct answer)
Magnetic forces from Earth

Correct answer: The balance between gravity pulling it inward and its forward velocity

A satellite orbits because its forward velocity creates a tendency to fly off in a straight line, while gravity constantly pulls it toward Earth. These two effects balance perfectly so the satellite continuously falls around Earth without getting closer or farther.

Orbital mechanics combine Newton's First Law (an object moves in a straight line unless acted on by a force) with gravitational attraction. A satellite has sufficient tangential velocity that as gravity curves its path downward, it moves forward enough that Earth's surface curves away at the same rate. The satellite is in constant free fall — it experiences weightlessness because everything inside falls together. At low Earth orbit (~400 km), the required velocity is about 7.7 km/s (27,600 km/h). No engines are needed to maintain orbit in the vacuum of space (no air resistance). At higher altitudes, gravity is weaker, so satellites orbit more slowly — geostationary satellites at 35,786 km orbit once per day, appearing stationary relative to Earth.

Question 5: What happens to the volume of a gas when its pressure doubles (at constant temperature)?

Volume doubles
Volume stays the same
Volume halves (Correct answer)
Volume quadruples

Correct answer: Volume halves

According to Boyle's Law, at constant temperature, the volume of a gas is inversely proportional to its pressure (P₁V₁ = P₂V₂). Doubling the pressure halves the volume.

Boyle's Law (1662) states that for an ideal gas at constant temperature: PV = constant, or P₁V₁ = P₂V₂. If pressure doubles (P₂ = 2P₁), then V₂ = P₁V₁/2P₁ = V₁/2 — volume halves. This inverse relationship occurs because compressing gas into a smaller space increases the frequency of molecular collisions with the container walls, producing higher pressure. Boyle's Law is an idealization that works well for most gases at moderate pressures and temperatures but breaks down at very high pressures (molecules occupy significant volume) or near condensation. Applications include scuba diving (air volume in lungs changes with depth/pressure), syringes, pneumatic systems, and understanding atmospheric pressure changes with altitude.

Question 6: What is mechanical resonance?

The ability of a material to absorb vibration
When an external frequency matches an object's natural frequency, causing large oscillations (Correct answer)
The sound produced by metal objects
The breaking point of a brittle material

Correct answer: When an external frequency matches an object's natural frequency, causing large oscillations

Mechanical resonance occurs when the frequency of an external vibration matches an object's natural frequency, causing increasingly large oscillations. This can be destructive if not controlled, as demonstrated by the famous Tacoma Narrows Bridge collapse.

Every object has natural frequencies at which it vibrates freely. When an external periodic force matches one of these frequencies (resonance), energy transfers efficiently into the object, and oscillation amplitude builds dramatically. The amplitude is limited only by damping (energy dissipation). With little damping, resonance can produce destructive vibrations. The Tacoma Narrows Bridge (1940) collapsed when wind-induced vortices matched its torsional natural frequency. Soldiers break step when crossing bridges to avoid resonance. However, resonance is useful in musical instruments (amplifying sound), radio tuning (selecting specific frequencies), MRI machines, and quartz clocks. Engineers design systems with natural frequencies far from expected excitation frequencies, or add damping to limit resonant response.

Mechanical Aptitude Questions and Answers 2

What simple machine is a screw an example of?