# Homework Help: 1st year engineering physics problem

1. Jan 19, 2008

### Vash

Im having troubles with this question for my assignment...

Determine the time required for a car to travel 1km along a road if the car starts from rest, reaches a max speed at some intermediate point, and then stops at the end of the road. The car can accelerate at 1.5 m/s^2 and decelerate at 2 m/s^2.

I thought about sing the constant acceleration formulas (s= s0 +v0t+1/2at^2 V=v0+at) and letting v=v1, s=s1, t1 to be the time at which acceleration switches. make 4 equations first 2 using s1 and v1. second 2 using 1000m and v2=0 while v0=v1 (using t2-t1 for this part). I did the subbing in and it all turned into a mess. Anyone know of a easier way to approach this problem?

2. Jan 19, 2008

### Integral

Staff Emeritus
That sounds like the right approach. Try again. Look for ways to simplify, try different routes. It is not uncommon to have to go back and try again. Don't give up.

If you show us your algebra, we may be able to find errors.

3. Jan 19, 2008

### Vash

my equations are:

s1=1/2at1^2 v1=at1
1000m=s1+v1(t2-t1)+1/2a(t2-t1)^2 v2=0=v1+a(t2-t1)

Those are my equations...I think i know what to do, but setting it up is the problem. Before I move on, do they seem correct?

4. Jan 19, 2008

### plutoisacomet

Hello there. You have 2 parts to your solution. It is stated in the question itself.
Phase 1 is acceleration phase from rest to a certain x
phase 2 you have deceleration from a new initial velocity and a remaining x

5. Jan 19, 2008

### Vash

Yes that is true, Im just waiting on someone to check if my equations are correct.

6. Jan 19, 2008

### plutoisacomet

7. Jan 19, 2008

### Vash

ok i figured it out, thanks for your replies.

8. Jan 20, 2008

### plutoisacomet

Great Job Vash

9. Jan 20, 2008

thanks :)