WTMA Wheels, Axles, and Rotation 3 — Questions and Answers
Question 1: A compound gear train has three gears in sequence: Gear 1 (40 teeth) drives Gear 2 (10 teeth), which shares an axle with Gear 3 (30 teeth), which drives Gear 4 (15 teeth). What is the overall speed ratio (Gear 4 speed / Gear 1 speed)?
- 2:1
- 4:1
- 8:1 (Correct answer)
- 16:1
Correct answer: 8:1
Stage 1 ratio: 40/10 = 4; Stage 2 ratio: 30/15 = 2; overall = 4 × 2 = 8, so Gear 4 spins 8 times faster than Gear 1.
Question 2: A crossed belt connects two pulleys. If the driving pulley rotates clockwise, the driven pulley rotates:
- Clockwise
- Counterclockwise (Correct answer)
- The same direction depends on pulley size
- It doesn't move
Correct answer: Counterclockwise
A crossed belt reverses the direction of rotation, so a clockwise driver produces counterclockwise motion in the driven pulley.
Question 3: Which change would MOST increase the torque output of a wheel-and-axle system when effort is applied at the wheel?
- Decreasing the wheel's radius
- Increasing the axle's radius
- Increasing the wheel's radius (Correct answer)
- Decreasing the axle's length
Correct answer: Increasing the wheel's radius
Torque output increases when the effort arm (wheel radius) is larger relative to the load arm (axle radius), so increasing wheel radius boosts mechanical advantage.
Question 4: A bicycle has a front chainring with 48 teeth and a rear sprocket with 16 teeth. How many times does the rear wheel rotate for each pedal revolution?
- 0.33
- 1
- 3 (Correct answer)
- 48
Correct answer: 3
Speed ratio = driver teeth / driven teeth = 48/16 = 3, so the rear wheel makes 3 revolutions per pedal revolution.
Question 5: A wheel rolling uphill loses speed compared to rolling on flat ground (same force applied). This is primarily because:
- The wheel diameter increases on hills
- Gravity adds a resisting force component along the slope (Correct answer)
- The axle friction increases
- The wheel circumference decreases
Correct answer: Gravity adds a resisting force component along the slope
On an incline, gravity acts against motion along the slope, requiring additional force to maintain speed, which slows the wheel if force is unchanged.
Question 6: Two identical wheels are connected by their axles via a rigid rod (like a car axle). When the vehicle turns left, which wheel must rotate faster?
- The left wheel
- The right wheel (Correct answer)
- Both rotate at the same speed
- Neither — they slide instead
Correct answer: The right wheel
In a left turn, the right (outer) wheel travels a longer arc and must rotate faster than the inner left wheel.
Question 7: A pulley system uses a fixed pulley only. What mechanical advantage does it provide for lifting a load?
- 2:1
- 3:1
- 1:1 (Correct answer)
- 4:1
Correct answer: 1:1
A single fixed pulley only changes the direction of force, not its magnitude, providing a mechanical advantage of 1:1.
WTMA Wheels, Axles, and Rotation 3
A compound gear train has three gears in sequence: Gear 1 (40 teeth) drives Gear 2 (10 teeth), which shares an axle with Gear 3 (30 teeth), which drives Gear 4 (15 teeth).
What is the overall speed ratio (Gear 4 speed / Gear 1 speed)?