# Homework Help: How to calculate Time

1. Jul 10, 2013

### wangwang2626

Ok so the car is 25 m behind a truck which suddenly braked. I put it as decelerating at -6.5 m/s it was travelling at 110km/h . I would like to know how long it takes to collide with the truck , and if it dosent what would I have to change the deceleration to be so that it does collide with the truck?
What I did was
v=vo+at
t=(v-vo)/a
t=30.5556/(-6.5)
but that's obviously wrong
I don't know what to do next
or what other formula to use

Last edited: Jul 10, 2013
2. Jul 10, 2013

### rude man

How fast was the car going and how fast is it decelerating, starting with how long after the truck is decelerating?

3. Jul 10, 2013

### wangwang2626

the car was travelling at 110km/h when they noticed that the truck was not moving , it was 25 meters behind , I said it was decelerating at 6.5 m/s so how long would it take till it got to the truck and at what speed would it hit the truck. If the car does not reach the truck can you please suggest a different deceleration?

4. Jul 10, 2013

### rude man

In the absence of the truck, how much distance is covered by the car from the moment it starts to decelerate until its speed = 0? Is that distance < or > 25m? (Hint: you first need to compute the time to decelerate to zero, which you have already done except for the wrong sign).

When you get this part we can continue with the second part of the question.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted