help with prelim question... (1 Viewer)

[littleboy]

A brand new Rolls-Royce rolls off the back of a truck as it is beling delivered to tis owner. It lands on its wheels. The truck is travelling along a straight road at a constant speed of 72km/h (20m/s). The Rolls-Royce slows down at a constant rate, coming to a stop over a distance of 240m. It is a full minute beofre the truck driver realises that the precious load 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?

i keep getting nine hundred something metres but the actual answer is 793 metres. Can someone give the working out for this?

Thanks in advance

 

[Stan..]

I Cannot seem to get this one out.
The Distance when the driver of the truck stops is 985m From the Rolls-Royce.
Unless you use the A=V-U/T Formula to find out the velocity of the Truck and find out how fast the truck travelled over that distance.

Remember that the Intact Distance is 985m. Dosen't include Time Taken For Braking.
A(Car)= 20m/s
A(Truck)= 10m/s
For the Truck, I got 493m In Distance.
493+240= 733 (Car's Breaking) :. Use 20m/s
50+733=783 (Truck Breaking) :. Use 10m/s To find out how long for the Truck And 20m/s For how far. (Because it is the trucks breaking in terms of distance from the car, So use the Car's Acceleration to find it)
I have probably made a mistake in my working, So the 10 isn't there. But the Working Is.

 

[BlackJack]

That's ridiculous, how do you get a 3 at the end?
|..........60.5 sec travel......|.25.|
|----------|--------------------|-----|
....240....|

Note the truck travels at 20m/s constantly for the 60.5 sec.
That's 1210 metres. It takes 25 more metres to stop.
The truck travels 1235 meters in total, after the RR leaves the truck.

The RR starts at 20 m/s and linear decrease in speed. It takes only 24 seconds to stop, so the RR has stopped by the time the driver notices. (area of triangle; v vs t)
HENCE RR travels 240m.

== 995.

 

[Stan..]

I gotta agree with blackjack. You must have the wrong answer, Cause It makes no sense.

 

[littleboy]

well i got it from the Jacaranda Textbook 1

can we trust its answers?

 

[Stan..]

Maybe. Anyway, You got two answers. Take your pick on which one you want to take as the valid one, Cause textbooks can have human error.