# Physics question on motion

1. Jul 11, 2012

### einstein101

1. The problem statement, all variables and given/known data
An object that is acclerating uniformly travels 22.5 m in the eighth second of its motion.
a) what is the acceleration of the object?

b) how far does the object travel in 6 seconds?

2. Relevant equations
I'm not exactly sure what equation would be necessary-
perhaps -- a=v-u/t
v=u+at
x=ut+1/2at^2

3. The attempt at a solution

i attempted to use the a=v-u/t equation, however, it wasn't successful. I don't even think you can use that equation since we dont have the values for velocity;initial nor final.

2. Jul 11, 2012

### PhanthomJay

The object has no motion at t = 0. How far does it travel in 8 seconds? How far does it travel in 7 seconds? Its displacement between the. 7th and 8th seconds is given as 22.5 m. Use your last equation.

3. Jul 11, 2012

### Staff: Mentor

Hi einstein101. The eighth second is that time difference between seven seconds and eight seconds.

travel during first 7 seconds= u.7 + (1/2)a.72

travel during first 8 seconds= ????

4. Jul 12, 2012

### einstein101

But how do I find the initial velocity for the equation?
And how would I exactly go about finding the acceleration?

5. Jul 12, 2012

### e^(i Pi)+1=0

It says nowhere that the object started from rest.

6. Jul 12, 2012

### Staff: Mentor

It says "in the eighth second of its motion". I would interpret that to mean that it started from rest.

7. Jul 12, 2012

### Staff: Mentor

Can you finish the equation I left uncompleted?

8. Jul 12, 2012

### e^(i Pi)+1=0

Hmm, I suppose you're right.

9. Jul 12, 2012

### einstein101

so travel during first 8 seconds=u x 8 +1/2(a)(8)^2

then what do i do?

10. Jul 13, 2012

### Staff: Mentor

Good. Now subtract that from the companion equation I gave you, viz., of distance travelled during first 7 seconds.

11. Jul 13, 2012

### PhanthomJay

It has been established that the object starts from rest and thus u = 0 and the distance traveled in 8 seconds is 32(a). In a similar manner, find the distance traveled in 7 seconds. Subtract this 2nd equation from the first equation and set the result equal to 22.5 to solve for a.