Bennett Mechanical Acoustics and Optics Questions and Answers 1 — Questions and Answers
Question 1: A person stands 100 meters from a large, flat cliff face and shouts. They hear an echo shortly after. If the person then walks 50 meters closer to the cliff and shouts again, what will happen to the time it takes to hear the echo?
- The time will be doubled.
- The time will remain the same.
- The time will be halved. (Correct answer)
- The time will be quartered.
Correct answer: The time will be halved.
An echo is the reflection of sound. The sound must travel from the source to the reflecting surface and back. Initially, the total distance is 100m (to the cliff) + 100m (back) = 200m. After moving 50m closer, the new distance is 50m (to the cliff) + 50m (back) = 100m. Since the total distance the sound travels is cut in half and the speed of sound is constant, the time it takes to hear the echo will also be halved.
Question 2: A person is standing on the bank of a clear, still pond and wants to spear a fish that they see in the water. Due to an optical effect, the fish is not where it appears to be. Where should the person aim to hit the fish?
- Directly at the apparent location of the fish.
- Slightly below the apparent location of the fish. (Correct answer)
- Slightly above the apparent location of the fish.
- To the left or right of the apparent location.
Correct answer: Slightly below the apparent location of the fish.
This phenomenon is due to the refraction of light. When light rays travel from the fish (in the water) to the observer's eyes (in the air), they bend away from the normal. This makes the fish appear to be at a shallower depth than it actually is. Therefore, to hit the fish, one must aim below its apparent image.
Question 3: A mechanic is working in a large garage with concrete walls and floors, and the sound of the tools is creating a loud echo. Which of the following changes would be most effective at reducing the echo?
- Painting the walls with glossy paint.
- Hanging heavy fabric curtains or acoustic foam panels on the walls. (Correct answer)
- Installing large glass windows.
- Replacing the concrete floor with polished tile.
Correct answer: Hanging heavy fabric curtains or acoustic foam panels on the walls.
Echoes are caused by sound waves reflecting off hard, non-porous surfaces like concrete, glass, and tile. To reduce echoes, one must use materials that absorb sound energy. Soft, porous materials like heavy fabric, foam, or carpet are excellent sound absorbers because they trap sound waves and convert their energy into heat, preventing them from reflecting back into the room.
Question 4: You are standing still on a sidewalk as an ambulance with its siren on approaches you, passes you, and then moves away. How does the pitch of the siren you hear change?
- The pitch is high as it approaches and low as it moves away. (Correct answer)
- The pitch is low as it approaches and high as it moves away.
- The pitch continuously gets lower from the moment you first hear it.
- The pitch remains constant the entire time.
Correct answer: The pitch is high as it approaches and low as it moves away.
This is a classic example of the Doppler effect. As the ambulance approaches, the sound waves are compressed from the listener's perspective, leading to a higher frequency and thus a higher perceived pitch. After it passes and moves away, the sound waves are stretched out, resulting in a lower frequency and a lower perceived pitch.
Question 5: Which of the following optical devices is most suitable for concentrating sunlight to a single point to start a fire?
- A concave lens.
- A flat pane of glass.
- A plane mirror.
- A convex lens. (Correct answer)
Correct answer: A convex lens.
A convex lens, which is thicker in the middle than at the edges, converges (brings together) parallel rays of light to a focal point. By focusing the sun's rays onto a small spot, the energy is concentrated enough to generate intense heat and ignite flammable material. A concave lens, in contrast, diverges light rays.
Question 6: If you hold a sign with the word 'PHYSICS' printed on it up to a standard flat mirror, how will the word appear in the reflection?
- PHYSICS
- SↃIƧYHꟼ (Correct answer)
- ԀHʎSIƆS
- SCISYHP
Correct answer: SↃIƧYHꟼ
A plane mirror creates a laterally inverted image, which means it is reversed from left to right. Not only is the order of the letters reversed (S-C-I-S-Y-H-P), but each individual letter is also flipped horizontally. This is why the word 'AMBULANCE' is often written in reverse on the front of the vehicle, so it can be read correctly in a rearview mirror.
