Friday
April 18, 2014

Homework Help: Physics

Posted by Danny on Saturday, January 11, 2014 at 10:29pm.

A train traveling at a speed of 110 km/h uniformly decelerates for 20 seconds. During that time it travels 450m.
A) What is the acceleration? (m/s^2)
B) What is its speed after deceleration? (km/h)
A ANS = -.805m/s^2
B ANS = 52km

s = 450m
t = 20 seconds

u = 110km/h*1000m/km /60s/min*60 min/h
u = 110000/ 3600
u = 30.55

A) s = ut + 1/2 at^2

a = s - ut/ 1/2 * t^2
a = 450m - 30.55 * 20 sec/ 1/2 * 20^2
a = 450 - 611/ 1/2 * 400
a = -161/ 200
a = -.805m/s^2

I get stuck trying to figure out how to use the formula to get the answer to B which is 52 km.

I believe I would start with the formula: s = v + u/ 2 *t.

I would really appreciate the help with the second part of the question.
Thank you.

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