Homework Help: Acceleration of a Truck Problem

1. Mar 6, 2010

Paymemoney

1. The problem statement, all variables and given/known data
A truck on a straight road starts from rest, accelerating at 2.00m/s^2 until it reaches a speed of 20.0m/s. Then the truck travels for 20.s at constant speed until the brakes are applied, stopping the truck in a uniform manner in an additional 5.00s.
a) How long is the truck in motion?
b) What is the average velocity of the truck for the motion described?

2. Relevant equations
Constant acceleration equations
average speed formula

3. The attempt at a solution
vfinal=20
vinitial=0
t=?
a=2.00

vfinal=vinitial + at
20=2t
t=10s

vaverage=$$\frac{xfinal=xinitial}{tfinal-tinitial}$$
=$$\frac{100}{10}$$
=10m/s

Now this is incorrect but can someone tell me the correct way of doing this problem?

2. Mar 7, 2010

Staff: Mentor

Are you saying that the truck was in motion for only 10 sec.? How can that be, since the problem states that the truck accelerates for a while, coasts along at a constant speed for 20 seconds, and then brakes to a stop, taking another 5 sec.

3. Mar 8, 2010

Paymemoney

can someone tell me if the information given in the following statement has any impact on the calculations and if so how would i use it.

4. Mar 9, 2010

Staff: Mentor

The total time the truck is in motion is the time it is accelerating plus the time it is moving at a constant speed plus the time it is decelerating to a stop.

The average velocity = (total distance)/(total time)