# Homework Help: Newton's second law

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1. May 28, 2015

### Abhijith S. Raj

We all know that f = m*a. But why can't we write this as f = m * s/t^2 ???
I know this is wrong. But what is the reason?? Need opinion on this.
Thankyou.

2. May 28, 2015

### SammyS

Staff Emeritus
You can't do it because $\displaystyle \ a\ne \frac{s}{t^2} \$ .

3. May 28, 2015

### Abhijith S. Raj

a = dv/dt. Ya now i get it.

4. May 28, 2015

### SammyS

Staff Emeritus
If initial velocity is zero, and acceleration is constant, then the final velocity is v = at, but the average velocity is v/2 . The average velocity multiplied by time gives distance, so that s = (1/2)at*t = (1/2)at2, a = 2a/t2 .

Therefore, you could write f = 2ma/t2, provided that initial velocity is zero, and acceleration is constant.

5. May 28, 2015

### Abhijith S. Raj

Thanks

6. May 29, 2015

### akashpandey

if accelaration is not constant then???

7. May 29, 2015

### SammyS

Staff Emeritus
Then, ... all bets are off. That's also true if the initial (or final) velocity isn't zero.

8. May 30, 2015

### akashpandey

sorry but i don't understand.
please can u tell this mathematically. plzzx