# Motion Equation Problem

1. Sep 14, 2013

### harujina

1. The problem statement, all variables and given/known data
Sam is running at 3.8 m/s and is 75m behind John who is running at a constant velocity of 4.2m/s. If Sam accelerates at 0.15m/s^2, how long will it take him to catch John?

2. Relevant equations

d = (vf+vi/2)t
vf = vi + at
d = vit + 1/2at^2
d = vft - 1/2at^2

3. The attempt at a solution

I honestly do not know how to solve this problem...
I think I should probably find Δd of John first? His acceleration = 0, his initial and final velocity is 4.2m/s... correct? and for Sam, his initial velocity = 3.8m/s, his acceleration = 0.15m/s^s and would his Δd = -75m ....

I really don't understand this question or what I should begin to solve first and how...
Any help would be appreciated!

2. Sep 14, 2013

### yands

For Sam to catch John both of them should have the same point in space

3. Sep 14, 2013

### bp_psy

Start by defining a distance d from the initial position of Sam to the point they will meet. Then look at the formulas.

4. Sep 14, 2013

### harujina

So initial position or Δd of Sam = -75m + 3.8m/s... no?
and point they will meet is 4.2m/s + 75m? since they're both in constant motion?
I'm still really confused.

5. Sep 14, 2013

### bp_psy

Always look at your units to see if the answer makes sense!!!

The initial position of Sam can be taken as 0 and the point where they will eventually meet is a distance d from it.Now you use what you know about Sam and John plus the formulas to find two equations with the time it takes them to reach the point d .Solve them to find t.
Try drawing a diagram if you still have problems understanding.