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Physics Multiple Choice Exam (Internal) Errors

Topics to work on

  • Pressure
  • Thermal physics
  • Optics
  • Mechanics (forces, moments, etc.)
  • Elasticity
  • Dynamics (momentum, impulse, etc.)

Multiple choice questions

  1. Which quantities, in addition to the acceleration of free fall, g, affect the pressure at the bottom of a pond of water?

    • Correct: A (the density of the water and the depth of the pond only)
      • Why it’s CORRECT: The pressure exerted by a fluid at a certain depth is calculated using P = ρgh, where:
        • P is the pressure
        • ρ is the density of the water
        • g is the acceleration of free fall
        • h is the depth of the pond
    • Chose D (the depth of the pond, the density of the water, and the surface area of the pond).
      • Why it’s WRONG: Surface area does not matter. The total force on the bottom of the pond depends on the area, but pressure is force per unit area (P = F/A). Since the area cancels out in the derivation, the surface area or shape of the pond has no effect on the pressure at the bottom.

  2. What is the temperature difference between the fixed points on the C temperature scale?

    • Correct: B (100°C)
      • Why it’s CORRECT: The Celsius scale is defined by two reference points, the temperature at which pure water freezes/pure ice melts, set at 0°C, and the temperature at which pure water boils/steam condenses, set at 100°C. To find the difference between these two fixed points, it’s simply 100°C - 0°C = 100°C.
    • Chose A (10°C).
      • Why it’s WRONG: It does not match the answer to the difference between the temperature where pure water boils/steam condenses and the temperature where pure water freezes/pure ice melts.

  3. Four thermometers, with their bulbs painted different colors, are placed at equal distances from a radiant heater. Which thermometer shows the slowest temperature rise when the heater is first switched on?

    • Correct: D (shiny white)
      • Why it’s CORRECT: When the heater is switched on, it emits thermal radiation (infared radiation). The rate at which the thermometers heat up depends on how well their bulbs absorb this radiant energy. The two main factors that affect absorption are color (dark = good absorber, light = good reflector) and texture (dull = good absorber, shiny = good reflector).
    • Chose B (dull white).
      • Why it’s WRONG: Even though white is a good reflector of heat, it has a dull texture which is good at absorbing heat. Therefore, it does not have the slowest temperature rise.

  4. An object is placed 30cm in front of a plane mirror. Which statement describes the image of the object?

    • Correct: B (The image is the same size and 60cm from the object.)
      • Why it’s CORRECT: Plane mirrors create images with specific properties:
        • The object distance is the same as the image distance. The virtual image appears just as far behind the mirror as the object is in front of it.
        • The magnification is 1. The image is always the exact same size as the original object.
      • The question asks for the distance between the OBJECT and its IMAGE, NOT the distance from the MIRROR.
        • Total dist. = Dist. from obj. to mirror + Dist. from mirror to image
        • Total dist. = 30cm + 30cm = 60cm.
    • Chose A (The image is the same size and 30cm from the object.)
      • Why it’s WRONG: The fundamental rule of plane mirrors is that the image is formed the exact same distance behind the mirror as the object is in front of it. You have to add the two distances of the object from the mirror and the image from the mirror together.

  5. A wheelbarrow has a weight W of 140N. Which vertical force F is needed to support the wheelbarrow in the position shown?

    • Correct: A (60N)
      • Why it’s CORRECT: To find the correct vertical force F, you take moment about the wheel (the pivot). For the wheelbarrow to be supported in equilibrium, the clockwise moment must equal the anticlockwise moment.
      • First, find the clockwise moment. Force = 140N, Perpendicular distance from pivot = 1.4m - 0.8m = 0.6m. Clockwise moment = 140N x 0.6m = 84Nm
      • Second, find the anticlockwise moment. Force = F, Perpendicular distance from pivot = 1.4m, Anticlockwise moment = F x 1.4m
      • Third, equate and solve. Anticlockwise = Clockwise. **F x 1.4m = 84Nm. F = 84/1.4 = 60N.
    • Chose B (80N).
      • Why it’s WRONG: When finding the clockwise moment, you used the wrong distance, 0.8m. You were supposed to use 1.4m, which yielded 80N.

  6. Which property of an object is a consequence of the effect of a gravitational field acting on it?

    • Correct: D (weight)
      • Why it’s CORRECT: Weight is the gravitational force acting on an object’s mass. It is calculated using the formula, W = mg. Since weight directly depends on the strength of the gravitational field, g, an object’s weight will change depending on where it is in the universe.
    • Chose A (density)
      • Why it’s WRONG: Density is the mass per unit volume, ρ = m/V. Since both mass and volume are independent of gravity, density is as well.

  7. A spring, which obeys Hooke’s Law, has an unstretched length of 10cm. A load of 20N is suspended from the spring. The new length of the spring is 36cm. What is the spring constant, k, of the spring?

    • Correct: B (0.77N/cm)
      • Why it’s CORRECT: Hooke’s Law states that the force F applied to a spring is directly proportional to its extension x. The formula is F = kx. where:
        • F is the load/force applied (in this case 20N)
        • k is the spring constant
        • x is the extension (how much the spring stretched and NOT its total length)
      • First, find the extension, x. The extension is final length - initial length. x = 36cm - 10cm = 26cm.
      • Second, rearrange the formula to solve for k. k = F/x.
      • Third, plug in the values into the formula. k = 20N/26cm = 0.77N/cm.
    • Chose A (0.56N/cm).
      • Why it’s WRONG: You have to calculate the extension, NOT the total length. Error is found when calculating k: k = 20N/36cm.

  8. A cricket ball has a mass of 0.16kg. The ball travels at 30m/s. The ball is hit by a bat with a force of 10,800N. After being hit, the ball moves off at 30m/s in the opposite direction. For how long was the ball in contact with the bat?

    • Correct: B (0.00089s)
      • Why it’s CORRECT: The problem uses the relationship between force, impulse, and momentum. The formula relating these quantities is F = Δp/t. Rearrange this to t = Δp/F to solve for time.
      • First, calculate the change in momentum, Δp. Because momentum is a vector quantity, direction matters immensely. Let the ball’s initial direction be positive (+30m/s) and the opposite return direction be negative (-30m/s). Δp is calculated as Δp = m(v - u).
        • Δp = 0.16kg x (-30m/s - 30m/s)
        • Δp = 0.16kg x (-60m/s)
        • Δp = -9.6kg • m/s (Negative sign indicates direction of the force; the magnitude 9.6kg • m/s can be used for the time calculation)
      • Second, solve for contact time, t.
        • t = Δp/F
        • t = 9.6kg • m/s / 10,800N
        • t = 0.00089s
    • Guessed A.