# Homework Help: Time when two cars have equal speeds

1. Nov 17, 2009

### MechaMZ

1. The problem statement, all variables and given/known data

At t = 0, one toy car is set rolling on a straight track with initial position 16.0 cm, initial velocity -3 cm/s, and constant acceleration 2.90 cm/s2. At the same moment, another toy car is set rolling on an adjacent track with initial position 10.5 cm, an initial velocity of 6.30 cm/s, and constant zero acceleration.
(a) At what time, if any, do the two cars have equal speeds?

3. The attempt at a solution

My answer is 1.1379s by using the method below,
Vf = Vi +at
6.3 = 3 + (2.9)t
t = 1.1379s

However, the answer is 3.21s, with the equal speeds as 12.309cm/s(I suppose by using Vf = Vi +at).

I don't understand why the final common speed is not 6.30 cm/s since the one of the object(with initial velocity of 6.30 cm/s) is moving under zero acceleration. Shouldn't it moves under a constant velocity throughout the whole journey?

2. Nov 17, 2009

### ehild

The initial velocity of the first car is -3cm/s: vf=-3+2.9t. The question is when the cars have the same speed, that is the same magnitude of their velocity. This means either v1=v2 or v1=-v2.

ehild

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3. Nov 17, 2009

### MechaMZ

yes, I know this clearly. unfortunately, I couldn't get the correct answer.

4. Nov 18, 2009

### Redbelly98

Staff Emeritus
Can you show your re-work when you use the correct initial velocity, -3 cm/s?

5. Nov 19, 2009

### donhubix

Plug in the minut(-) before the 3. Remember that the car is moving the other way, so it has to stop and start moving in the opposite direction before it can catch up with the other one(velocity wise.)

Try:

6.3=-3+2.9(t)

Find t.

Hubert