Please, this simple PHYSICS I don't know how ?

Question

The driver of the train traveling at 30 m/s applies the brakes when he passes an amber signal. The next signal 1.5 km down the track and it reaches 75 secs later. Assuming a uniform motion, find the velocity of train at the second signal?

is it

for the second signal:

d=1.5km

t=75 secs

v=?

v=75/1.5 = 50 km/s?????

Answer 1

When brakes are applied, the train is decelerating at a constant rate.

First use the distance formula

x = v*t+0.5*a*t*t

You know the initial velocity, the distance traveled, and the time it took. Thus, you can find deceleration (negative acceleration).

1500=30*75+0.5*a*75*75

After solving for a, use the velocity formula

vfinal = vinitial+a*t

You know the initial velocity, the acceleration (which should be negative), and the time.

vfinal = 30+a*75

The answer is 10m/s.

Edit:

vik n wrote:

x=V(initial)+.5at^2

1500m=30m/s+.5a(75^2)

a=.5226m/s^2

However, he is missing a 't' next to velocity. You cannot have meters per second equal to meters without multiplying by time.

Answer 2

The guy who gave u the answer IS WRONG...

K first use this equation to find accelation.

V(initial)=30m/s

X= 1500m

t=75s

V(final)=? m/s

a=?m/s^2

So we need to find velocity and acceleration

Frist we need to find acceleration since its slowing down b/c the driver hits the break what ever the answer we get its the opposite of that(decelration just needs a negative sign).  Here i will show you.

x=V(initial)+.5at^2

1500m=30m/s+.5a(75^2)

a=.5226m/s^2

But since its slowing down we have to make the positive acceleration to negative so it is now a=-.5226m/s^2  now we got acceleration(deceleration way)

now to calculate the final velocity as it reaches the second light...

V(final)=V(initial)+at

V(final)= 30m/s+(-.5226)75

V(final)=9.2m/s

NOW THIS IS THE RIGHT METHOD.  the guy upstairs did the problem worng.  

Answer 3

Watch your units! This train is likely not going 50 KILOMETERS a second, especially after hitting the brakes! Convert to meters to make things a little easier. Now, let's work through this.

He goes 1.5 kilometers in 75 seconds.

With a uniform speed:

v = d/t | Velocity Equation

1.5 km / 75 s = 1500 m / 75 s | Convert

v = 20 m/s | Solve

Edit: Disregard this answer. I had assumed that his velocity, not his acceleration, was uniform. I fail.