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Maths M1 help - Mechanics

Hey, stuck with a few questions.

1. A freight train 1/4 km long takes 20 seconds to pass a signal. The train is decelerating at a constant rate, and by the time the rear truck has passed the signal it is moving 10 km per hours slower than it was when the front of the train passed the signal. Find the deceleration in kilometre-hour units, and the speed at which the train is moving when the rear truck has just passed the signal.

2. A cheetah is pursuin an impala. The impala is running in a straight line at a constant speed of 16 m/s. The cheetah is 10 m behind the impala, running at 20 m/s but tiring, so that it is decelrating at 1m/s^2. Find an expression for the gap between the cheetah and the impala t seconds later. Will the impala get away?

3. Convert 0.5 m/s into km/h^2

Reply 1

driving_seat

Let the initial speed of the train be x kmh-1
and the final speed = x-10
time in hrs=20/(60*60)=1/180
a= (v-u)/t

a= [(x-10)-x]/(1/180)

a= -1800kmh-1

then use 2as=v²-u² to calculate the initial speed and subtract from 10 to calculate the final speed

Reply 2

rec53

arsenalfan.

Hey, stuck with a few questions.

1. A freight train 1/4 km long takes 20 seconds to pass a signal. The train is decelerating at a constant rate, and by the time the rear truck has passed the signal it is moving 10 km per hours slower than it was when the front of the train passed the signal. Find the deceleration in kilometre-hour units, and the speed at which the train is moving when the rear truck has just passed the signal.

2. Lets deal with one thing at once

1. Draw some diagrams. How far has the front travelled? So write down what you know, and what you want to find out. There should be one equation of motion you know which contains everything you know and the first unknown, and a separate one containing the second unknown. Now its just the case of substitution & solve!

Edit: or read the post above for some of the solution already given!

Reply 3

arsenalfan.

OP

1

thanks for ur replies, i added question3, so please help! :smile:

Thanks!

Reply 4

Morbo

17

You can't do a conversion of m/s into km/h^2.

Reply 5

xx007

2

i think arsenalfan. means m/s into km/h

Reply 6

arsenalfan.

OP

1

Worzo

You can't do a conversion of m/s into km/h^2.

ok thx, I thought u could.

Reply 7

rec53

arsenalfan.

thanks for ur replies, i added question3, so please help! :smile:

Thanks!

2. Start with the Impola having travelled 10m at t=0, Cheeta travelled 0m. Now, work out the position of them both at a time t (normal formulae, but add 10 to the Impola's one). Subtract them, and you have the distance between them. Can this equal 0.

3. They are measuring different things (velocity, accel.), thus this doesn't make sense.

I can't see how we can give much more help until you post your workings so far.

Reply 8

username43524

0

Hey guys, i'm stuck on the same sort of questions, same book i think, but i havn't got as far

v=27, s=40,a= -4 1/2, find t

i've tried using the formula s=vt - 1/2at^2

please could some one just go through it 4 us, thanks

Reply 9

username43524

0

Hey guys, i'm stuck on the same sort of questions, same book i think, but i havn't got as far

v=27, s=40,a= -4 1/2, find t

i've tried using the formula s=vt - 1/2at^2

please could some one just go through it 4 us, thanks

Reply 10

GarethBrown

2

Just plug it in and it should come out right?

40=27t+2.25t^2

2.25t^2+27t-40=0

Then you have a quadratic to solve, so have fun with that.

Reply 11

username43524

0

cheers gareth, i got that far, but didn't spot the quadratic equation

Reply 12

GarethBrown

2

I'm George :wink:

Reply 13

username43524

0

well thanks george lol

stuck again, i was told an A at gcse would be fine to cope with A-Level maths, maybe not lool

but question 3

a milk float moves from the rest with acceleration 0.1ms^-1. find an expression for its speed, vms^-1, after it has gone s meters. illustrate your answer by sketching an (s,v) graph.

we havn't been shown queestions like this, only puttin into a formula, unless there is a formula for this that i cant see

cheers guys

Reply 14

rec53

scottnoplot

well thanks george lol

stuck again, i was told an A at gcse would be fine to cope with A-Level maths, maybe not lool

but question 3

a milk float moves from the rest with acceleration 0.1ms^-1. find an expression for its speed, vms^-1, after it has gone s meters. illustrate your answer by sketching an (s,v) graph.

we havn't been shown queestions like this, only puttin into a formula, unless there is a formula for this that i cant see

cheers guys

You know initial speed (u), acceleration (a) (I'm assuming you meant ms^-2), and you want an equation with final velocity (v) and distance (s) in. What equation has these in? Then substitute in the two items you know to get the required formula.

Reply 15

arsenalfan.

OP

1

Thanks for all replies, it was helpful.

I have both solutions to both questions which are correct, which I will try to post ASAP. :smile:

Maths M1 help - Mechanics

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