Share this thread: 
#1
Mar2812, 03:11 PM

P: 18

Newton's third law says that every force X exerts onto Y, Y exerts and equal and opposite force onto X. Ergo, if I'm standing on my linoleum floor, I'm exerting a force equal to my weight (mass x force of gravity) onto the floor and it's exerting an equal and opposite force on to me.
However, Newton also told us F=mA and that would mean that the force I am exerting on the floor, my weight, is equal to 0. (Assuming my downward acceleration is zero.) Though, as I mentioned, I am exerting my weight, w=mG and F=mA; Ergo if w=F then mG=mA and G=A. Can gravity really just be thought of as acceleration? 


#2
Mar2812, 03:19 PM

Mentor
P: 41,555




#3
Mar2812, 03:39 PM

P: 18

I thought my weight was cancelled out by the force exerted on the floor, but not nonexistent.;
w= mG & F=mA > w+F=0 w=F mG=(mA) G=A And since negative acceleration is simply called acceleration... G=A I must have phrased part of my question wrong, but what I'm concerned with is the ultimate implication I just showed in these two posts, can gravity be thought of as acceleration? 


#4
Mar2812, 04:05 PM

Mentor
P: 41,555

Newton's laws of motion...
ƩF = ma The forces on you are your weight (acting down) and the normal force from the floor (acting up). They are equal and opposite: mg + N = 0 (since you are not accelerating) If you were in free fall, with the only force acting on you being gravity, then you would have a downward acceleration equal to g = 9.8 m/s^2. 


#5
Mar2812, 04:21 PM

P: 343

I think a part of the problem comes from wanting to apply the second law, which really only applies to dynamics, in a statics problem where the acceleration is zero.
The other part of the difficult is the failure to write the force SUMMATION, writing instead simply F. It is extremely important to recognize that it is the sum of all of all forces that is equal to m*a, not any one force. With a few simple oversights, it is easy to get confused. 


#6
Mar2812, 04:27 PM

P: 172

If you're standing stationary on the ground, your net force is the vector sum of your weight and the normal force (which equals 0). This net force is the only force that Newton's second law applies to. 


#7
Mar2812, 04:29 PM

P: 172




#8
Mar2812, 04:30 PM

P: 18

OK, thanks for that.
I knew I was too dumb to be onto something. :P 


#9
Mar2812, 05:05 PM

P: 343




#10
Mar2812, 05:33 PM

P: 172




#11
Mar2812, 07:21 PM

P: 343

Split many hairs? If you really want to get picky, we could say that statics is only a subset of dynamics, and that the first law is only a special case of the second law.



#12
Mar2812, 09:44 PM

P: 172




#13
Mar2812, 09:55 PM

Sci Advisor
P: 2,470




#14
Mar2912, 08:39 AM

Engineering
Sci Advisor
HW Helper
Thanks
P: 7,279




#15
May1012, 06:20 AM

P: 82

Dear All,
I have a question about newton's third law. As you know , by pushing your hand through the wall the force F is done on the wall. But a small amount of this force is converted to heat. So let's say the force which the wall is experiencing is Fe. Does the wall exert F on your hand or Fe? 


#16
May1012, 06:28 AM

Mentor
P: 41,555

Whatever force you end up exerting on the wall will be equal and opposite to the force that the wall exerts on you. 


#17
May1012, 08:17 AM

P: 459

So, you were wrong, but there are profound relationships between gravity and acceleration. 


#18
May1012, 08:19 AM

P: 82




Register to reply 
Related Discussions  
Newton's Laws and Motion  Introductory Physics Homework  1  
Newton's Laws of Motion  Introductory Physics Homework  14  
NEWTON's laws of motion  Introductory Physics Homework  1  
Newton's Three Laws of Motion  General Physics  12  
Newton's Laws of Motion  Introductory Physics Homework  1 