Moving About - problem/question. Help please. :) (1 Viewer)

[~shinigami~]

A brand new Rolls Royce rolls off the back of a truck as it is being delivered to its owner. It lands on its wheels. The truck is travelling along a straight road at a constant speed of 20m.s^-1. The Rolls Royce slows down at a constant rate, coming to a stop over a distance of 240m. It is a full minute before the truck driver realises that the precious cargo is missing. The driver brakes immediately, leaving a 25m long skid mark on the road. The driver's reaction time (time interval between noticing the problem and depressing the brake) is 0.5s. How far back is the Rolls Royce when the truck stops?

Answer:793m

How do I work it out?

I would really appreciate it if someone could help me out with this question from the chapter review of a textbook. I wish they'd put the solutions and not just the answer.

Thanking you in advance.

 

[Riviet]

Just curious, but does this textbook have alot of wrong answers? Because I get 995m.

 

[~shinigami~]

I got 995m as well.

So far the textbook hasn't had many wrong answers, including this, it would have 2 wrong but I haven't done many questions yet.

Textbooks inevitably have wrong answers so I'll take your answer then.

EDIT: Actually, I got 985m...where did I go wrong?

 

[Riviet]

Distance travelled by truck=60x20=1200m
Subtract distance travelled by rolls royce=1200-240=960
Add 25m skidding distance and the (0.5 x 20)m distance in which the driver is realising it:
=960+25+10=995m

 

[~shinigami~]

Distance travelled by truck=60x20=1200m
Subtract distance travelled by rolls royce=1200-240=960
Add 25m skidding distance and the (0.5 x 20)m distance in which the driver is realising it:
=960+25+10=995m

Ok. Thank you once again Riviet. I forgot to add the 0.5x20m...

I've seen you answer quite a few questions on Physics and I'm really grateful for that.

From looking at things, I guess you'll do really well in the HSC as it looks like you have everything down pat.

 

[Riviet]

No problem, it's a pleasure to help.

 

[~shinigami~]

No problem, it's a pleasure to help.

And it's a pleasure to be helped by someone great!

 

[Kanny]

but how do we know that when the truck driver realised that its gone that the rolls royce hasn't yet stopped?

 

[Riviet]

but how do we know that when the truck driver realised that its gone that the rolls royce hasn't yet stopped?

It doesn't matter when the Rolls Royce has stopped, we are given a specfic distance that it has rolled before stopping.

 

[Kanny]

but it did not say it stopped before the truck driver realised yes we are given that it evetually stops but maybe it is still rolling when the truck driver realised its gone

 

[Riviet]

but it did not say it stopped before the truck driver realised yes we are given that it evetually stops but maybe it is still rolling when the truck driver realised its gone

Well the question wasn't very specific with the timing of everything so we have to assume that it's travelled the full 240 metres. If the question had other info, then we would need to reconsider our working. Good observation though.

 

[frank85]

well... assuming the initial velocity of the royce was 20m/s and it decelerated to 0m/s in 240m we can then calculate how long it took to slow down a stop and hence how far it had travelled at any given point in time. Like wise for the truck driver, we know he too 1min to realise it was gone then 0.5s to react and we can then work out how long it took for him to come to a stop. Knowing how long it took him to stop we can then work out if the royce had actually come to a compleat stop, if not.. then it we can work out how far it had actually traveled ad then relate that distance back to the distance where the truck had stopped.