## Can you please help me with this easyPhysics Question?

Question

If an equation for a moving object is

X(t)= -9 meters + (3.5 meters per second)*t

How much time would it take for an object to reach a position of +40 meters?

I keep getting 20.43 m/s=t which does not make much sense to me. Do I plug it in or am I doing it wrong?

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General Physics
4 years
1 Answer
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## Answer ( 1 )

I can’t see how you could get 20.43 in any units at all.

Wait a minute. Yes I can. You have forgotten your second form BODMAS rules

40 = – 9 + 3.5 * t

Having seen the multiplication you divide by 3.5 but incorrectly.

40/3.5 = -9/3.5 + t is correct because every single term is divided by 3.5

But what YOU did was

40/3.5 = -9 + t

that is incorrect because you failed to divide ALL terms by 3.5

I always taught the SADMOB rules as well. When deconstructing anything you MUST do it in the reverse order of construction.

So that if you have an addition in the equation then to DECONSTRUCT the equation that should be removed first

40 = -9 + 3.5* t

add 9 to both sides

49 = 3.5 * t

NOW divide by 3.5

49/3.5 = t

Your units should have given you the clue because 40/3.5 = m/(m/s) = s

but the -9 is in m so at this point you can see that you have made an error

both sides now have different units which is impossible.