Kinematic Equations for Acceleration: Solving Problems with Changing Velocities

[krazykaci]

Im a 1st year physics student in the 11th grade. I haven't 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 I am doing wrong I would appreciate it! =)

~kaci

 

[tony873004]

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

 

[Tomsk]

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

 

[krazykaci]

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

 

[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?

 

[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.

 

[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.