Need help on Acceleration Calculations?

Question

A car accelerates from a standstill to 60 km/hr in 10.0 seconds. What is its acceleration?

A train is accelerating at a rate of 2.0 km/hr/s. If its initial velocity is 20km/hr, what is its velocity after 30 seconds?

Equation: a= vf-vi/ t a=velocity
vf= final velocity
vi=initial velociy
t=time

Answer 1

1)

First you needto make common units of time. you have a velocity in hours and a change in time in seconds. Either make both in hours or both in seconds to be consistent.

Since the SI units for acceleration is m/s^2, we'll go with that.

To convert 60 km/hr to m/s, do the following conversion:

(60 km / 1 hr)(1000 m / 1 km)(1 hr / 3600 sec)

Solving, you get 60 km/hr = 16.67 m/s

Since acceleration = change in velcoity / change in time and initial velocity is 0 at time 0,

Acceleration = (16.67 - 0)/(10 - 0) = 1.67 m/s^2

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2)

Rewriting your equation to solve for v_f, you get:

v_f = a*t + v_i

Again, I would convert to SI units for simplicity:

(2 km/hr/s)(1000 m / 1 km)(1 hr / 3600 sec) = 0.56 m/s^2
(20 km/hr)(1000 m / 1 km)(1 hr / 3600 sec) = 5.56 m/s

Plugging in your known values, solve for v_f:

v_f = (0.56)(30) + 5.56 ----------> v_f = 22.36 m/s

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Hope this helps

Answer 2

(1) v=u+at 60=0+a*10 a=6km/hr/sec

(2) v=u+at, v=20+30*2 = 80km/hr
vf=v
vi=u

Answer 3

You have the equations. Start plugging in the values and DO your homework.