Physics is all about understanding concepts!

Classical Mechanics Level 3

If a car travels 120 kilometres in 2 hours, what is the magnitude of its mean velocity?

See also Physics is all about understanding concepts! (2)

90 km/h 60 km/h It requires more information to be answered. 240 km/h 120 km/h

Miguel Vásquez Vega by Miguel Vásquez Vega , 29, Colombia

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1 solution

The mean velocity is defined as

v = Δ x Δ t . \mathbf{v}=\frac{\Delta\mathbf{x}}{\Delta t}.

Since it doesn't specify the direction in which the car is travelling, the magnitude of the mean velocity could vary from 0 (in the case in which it comes back to the same point after the 2 hours) to 60 km/h (in the case in which it travels in a straight line).

2 Helpful 0 Interesting 0 Brilliant 0 Confused

That is not the mean velocity. That is the instantaneous velocity. The mean velocity is related to the secant line, and the derivative is related to the tangent line, which is the instantaneous velocity.

Leonardo de Araujo - 5 years, 9 months ago

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You are absolutely right. Do you agree with the change I have made?

Miguel Vásquez Vega - 5 years, 9 months ago

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Yes, but I believe the problem is not well written. What you are referring in the problem is the speed, which is the velocity magnitude, not velocity itself, which is a vector. If you are referring to the mean velocity, than the answer of the problem is right. But as it is, it is wrong, because it seems like it is referring to the speed, which then the answer would be b) and not e).

Leonardo de Araujo - 5 years, 9 months ago

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@Leonardo de Araujo – I understand what you want to say, but speed is only the magnitude of the instantaneous velocity, not the magnitude of the mean velocity. Besides, the problem clearly states "mean velocity" and not "speed", so I don't see ambiguity there. Do you maybe think it should be specified "what is the magnitude \textbf{magnitude} of the mean velocity"?

Miguel Vásquez Vega - 5 years, 9 months ago

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@Miguel Vásquez Vega – I agree, that would be clearer.

Leonardo de Araujo - 5 years, 9 months ago

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@Leonardo de Araujo – Done. Thanks!

Miguel Vásquez Vega - 5 years, 9 months ago

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