# Homework Help: Kinematic Equations problems!

1. Sep 4, 2006

### krazykaci

Im a 1st year physics student in the 11th grade. I havent had much trouble with the assignment, but for some reason i cannot get the correct solution on this problem.

A certain car is capable of accelerating at a rate of 0.57 m/s2. How long does it take for this car to go from a speed of 53 m/h to a speed of 68 m/h? Anser in units of s.

I would assume that the correct kinematic equation to use would be v=v.+a(t) if .57 is the acceleration/ 53 is v./ and 68 is v........., but when I do that I get the wrong answer, if you could help and tell me what im doing wrong I would appreciate it! =)

~kaci

2. Sep 4, 2006

### tony873004

re-write that equation using algebra to solve for t. Becareful of your units. Your vi and vf are given in meters per hour, but your acceleration is given in meters / s2. You want your answer to be in seconds.

3. Sep 4, 2006

### Tomsk

You have the right equation. What answer are you getting? Maybe you're not converting from mph to m/s?

4. Sep 4, 2006

### krazykaci

im getting 26. 31578947. so would i divide that number by 120? im not wery good at doing unit conversions.

5. Sep 4, 2006

### tony873004

I get that answer if I do (68-53) / .57

So your problem is the units. 68 and 53 are in m/h and .57 is in m/s2

Don't confuse meters with miles. Re-read the question. I'd find it surprising that they'd give you a problem where the car's initial and final velocities are about the speed with which an ant crawls. Should it really take the car a full hour to drive half the length of a football field?

6. Sep 4, 2006

### EP

Check your using the right unit conversions. Is the acceleration in meter per second^2? Is your velocity miles per hour or meters per hour? This would change your answer alot.

7. Sep 4, 2006

### Tomsk

It's best to convert the velocities you're given. 1 mile is 1.6 km. You should be able to get velocities in metres/s, then use v=v.+at.