Calculating New Acceleration with F=ma: A Homework Problem

[juju1]

Homework Statement

If the acceleration of an object is 2.00 m/s2 and the force on it is divided by 3 and its mass is divided by 7, what is the new acceleration?

Homework Equations

The Attempt at a Solution

so I know the equation we use is F=ma, and so i did (F/3)=(m/7) 2a
i divided by (m/7) on both sides to get (7/3)(F/m)=2a
then i divided by 2 on both sides to get 1.17 m/s/s = a
but it is showing that I am wrong, where did I go wrong?[/B]

 

[TSny]

so I know the equation we use is F=ma, and so i did (F/3)=(m/7) 2a

I don't see how you got this last equation, especially the 2a on the right side.

 

[juju1]

Newton's equation is F=ma
so i substituted the givens into that

 

[TSny]

Okay, the new force is F/3, where F is the original force. And m/7 is the new mass, where m is the original mass.

Why did you write 2a for the acceleration?

 

[juju1]

because the acceleration is 2?

 

[TSny]

because the acceleration is 2?

That's the original acceleration.

Originally, you have F_old = m_olda_old, where the subscripts indicates the old (original) values.

After the changes, you would have F_new = m_newa_new. You need to solve for a_new.

 

[juju1]

oh okay...so solving for a new then, i got 2.33 m/s/s

 

[TSny]

oh okay...so solving for a new then, i got 2.33 m/s/s

I don't think that's right. Can you show your steps to see how you got 2.33 m/s²?

 

[juju1]

OH WAiiitttt i believe i got it now. I was dividing by 2 not multiplying, so i got 4.67 m/s/s!

 

[TSny]

Yes, that's the right answer. I hope it makes sense.