Newton's Second Law - Vertical Motion

[waterwater]

Where required take g=9.8ms-2

7)A post of mass 3kg is pushed downwards by a force 300 N and experiences resisting forces of 325N form the ground. Find the accelration of the post.

Heres how i tried to work it out.

using f=ma

300+9.8=309.8
325-309.8=3xa
15.2=3 x a
15.2/3=a
a=5.06ms-2 wrong answer.


The right answer is 1.47ms-2.

I am not getting close to that answer

 

[drkitten]

Where required take g=9.8ms-2

7)A post of mass 3kg is pushed downwards by a force 300 N and experiences resisting forces of 325N form the ground. Find the accelration of the post.

Heres how i tried to work it out.

using f=ma

300+9.8=309.8

I think here's your problem. The acceleration of gravity (g) is an acceleration, not a force. So the post is pushed downward both by the acceleration force (300N), but also by the force due to gravity, which is 3kg times 9.8 m/s^s, which I think is 29.4N.

The total downward force is therefore 329.4N.

We now return you to your previously scheduled problem.

 

[RenaissanceBiker]

Gravity is pulling the post down at 3kg * 9.8MS-2 = 29.4 N.
Another downward force is exerting 300 N (if I understand correctly).
The ground is resisting with a force of 325 N.

The net downward force is 300 N + 29.4 N - 325 N = 4.4 N
The net acceleration then is A = F/M = 4.4 N / 3 kg = 1.467 MS-2 downward.

Edit: Is this is a school problem?

 

[waterwater]

thanks

 

[drkitten]

The gravity is pulling the post down at 3kg * 9.8MS-2 = 29.4N.
Another downward force is exerting 300 N (if I understand correctly).
The ground is resisting with a force of 325 N.

The net downward force is 300 N + 29.4 N - 325 N = 4.4 N
The net acceleration then is A = F/M = 4.4 N / 3 kg = 1.467 MS-2 downward.

Ha! Wrong! (As drkitten leaps out from hiding as upon a ball of yarn carelessly left within paw-reach.....)

The net acceleration is then 1.47 m/s^2, not 1.467. You don't have enough significant figures in any of your data to justify an answer correct to four places.

In fact, since the mass of the post is only known to one significant figure, the answer shoud be 1 m/s^2.

(Returns triumphantly to hiding behind the entertainment center, tail twitching....)

 

[RenaissanceBiker]

I had an introductory physics professor give me a 99% on an exam once because I said his iceberg weighed 12.3e6 lbs. He said the unit is just lb, not lbs. It pissed me off because I always thought the plural of pound (lb) was pounds (lbs). I was so pissed off that I was very strict about units and significant figures on the next exam and got a 100%. No one else in the class knew of anyone who had ever gotten a 100% from him. Thanks, drkitten, for reminding me of that.

/damn cats. I do all the work, they get their ear scratched.