Elementary motion

Question

A lorry is travelling at $60km/h$ when the brakes are applied. Assuming that the deceleration is uniform, how far has the lorry travelled when its speed is $36km/h$ after $6$seconds.

What I did:

$s =\frac{(v+u)}2t;$

$s = 80$ $m$

But the answer to the question in the textbook is $40m$. I don't know why this happened.

I tried using the $v/t$ graph but I still arrived at the same answer.

@RobertFrost Yes I did. But negativities will eventually cancel out. — Orestes Dante, Dec 30 '17 at 14:49
This question should be tagged as kinematics not mechanics... — Macrophage, Dec 30 '17 at 14:55
If you are ok, you can accept the answer and set as solved. Thanks! — user, Jan 03 '18 at 17:16

score 2 · Accepted Answer · answered Dec 30 '17 at 14:53

2

I get the same answer as you. Either you read it wrong or there's an error in your textbook.

Average speed is $(60+36)/2 km/h=48km/h$

Time is 6 seconds: $t=6/(60\times60) hours=.0017hours$

Distance travelled equals $d=vt=48\times .0017=.08km=80m$

answered Dec 30 '17 at 14:53

it's a hire car baby

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Elementary motion

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