# Homework Help: Acceleration, force

1. Jul 26, 2009

### dance_sg

1. The problem statement, all variables and given/known data
A 50.0 kg object is pulled along a horizontal surface with a horizontal net force of 250 N. A second object with a mass of 75.0 kg travels 4.00 m while changing its velocity from rest to 4.00 m/s. How many times greater is the acceleration of the 50.0 kg object than that of the 75.0 kg object?

2. Relevant equations
a=Fnet/m, v=d/t, a=v/t

3. The attempt at a solution
i tried finding acceleration of the 50 kg object by a=f/m which was 5m/s2. then to find acceleration of the 75kg object i used t=d/v to find the time so that i can plug it into the a=v/t formula. I got 4.00m/s2 for the 75 kg object. What am i doing wrong?because i cannot find the answer.

2. Jul 26, 2009

### Staff: Mentor

That equation only applies for constant velocity motion, but this is accelerated motion. Hint: Use the average velocity.

3. Jul 26, 2009

### dance_sg

but vf-vi still equals 4 doesnt it/

4. Jul 26, 2009

### Staff: Mentor

To find the time, use t = d/vave (instead of t = d/v). What's the average velocity?

5. Jul 26, 2009

### dance_sg

ok, but is the average velocity not 4?? cuz vf is 4 and vi is 0...

6. Jul 26, 2009

### dance_sg

could i use the formula (vf + vi)/2 to find it? then divide 5m/s2 by 2m/s2 to give me 2.5??

7. Jul 26, 2009

### Staff: Mentor

Right--that gives you the average velocity. Then use that to find the time, then the acceleration.

8. Jul 26, 2009

### dance_sg

alrite. thanks =D
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