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The question is the following:

An object accelerates from rest to $100\,\mathrm{km}$ per hour in $4.0\,\text{seconds}$. What fraction of the acceleration due to gravity is the car's acceleration?

From the question above, I can work out the acceleration (change in velocity over change in time) of the object using the given quantities: initial velocity ($0\,\mathrm{kph}$), final velocity ($100\,\mathrm{kph}$), and time ($4.0\,\mathrm s$).

But what does gravitational acceleration have to do with this? Is the answer related to friction created by the downward acceleration due to gravity?

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  • $\begingroup$ To get a feel for the acceleration here, that's roughly zero to sixty mph in $4$ seconds. So that will push you back in your seat, but not squish you to jelly. $\endgroup$
    – gandalf61
    Oct 10, 2019 at 15:20

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They are just asking you to find the car's acceleration and then to divide it by the well-known gravitational acceleration $g$.

If the result of $a_\text{car}/g$ is, say, $\frac13$, then gravity accelerates stuff 3 times faster than the car is accelerating. The fraction $\frac13$ is the answer they are asking for. Maybe they want to compare with gravity, because acceleration in general can feel a bit intangible.

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  • $\begingroup$ Thank you. This makes a lot of sense! $\endgroup$
    – Sam
    Oct 5, 2019 at 4:14
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But what does gravitational acceleration have to do with this?

As far as I can tell, nothing.

It sounds like only a, "well, how would this compare to, say, the acceleration due to gravity?". If that's the intent, the wording is pretty awful!

Hope this helps.

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Acceleration due to gravity has no effect on the horizontal (x) velocity of an object as it operates in the vertical (y) direction. Both vectors are perpendicular to each other and therefore have no effect on the other.

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