1. ## A-Level Physics Question

Hi all, been set a question and really not too sure on the answer

Question is: if every force has an equal but opposite reaction then how can forces be unbalanced - i.e acceleration?

I believe the answer is to do with newtons second law, every object will remain stationary or at constant velocity unless acted upon by an external force.

So acceleration is caused by an external force.

Now that could be totally wrong but seems to make sense. BUT what is the external force?

Anyone got any ideas?
Thanks
Ben  Reply With Quote

2. Originally Posted by BenW
Question is: if every force has an equal but opposite reaction then how can forces be unbalanced - i.e acceleration?
I was under impression they where balanced. If I accelerate a ball forwards by throwing it, I am also being accelerated backwards with equal force.  Reply With Quote

3. The external force depends on what it is. Say you push something, the external force is your hand on the object. External usually being something outside the system, unless you think about somehting like a car, which uses chemical energy to propel itself, generating heat etc.

I really should be able to explain better than that being as im on year 4 of an mphys, but i cant. Thats not really physics question anyway, sounds suspicously like philosophy to me. Who set it?  Reply With Quote

4. although every force has an equal and opposite reaction force, the weight of the force also matters. For instance, although there is an equal and opposite force when you push on a wall while seated on a skateboard, because the wall is so much heavier than you, the wall does not move. so i think that it is to do with weight  Reply With Quote

5. Its set by my physics teacher. Currently doing mechanics module.

Its possible that it could be to do with F=ma but not sure.

Maybe its because im thinking of cars/planes in simulations instead of say a ball. In that case gravity would be the external force causing acceleration?

Maybe the answer is simply 'an external force'  Reply With Quote

6. Originally Posted by mp3c
although every force has an equal and opposite reaction force, the weight of the force also matters. For instance, although there is an equal and opposite force when you push on a wall while seated on a skateboard, because the wall is so much heavier than you, the wall does not move. so i think that it is to do with weight
This is really a pointless question i've been set......right?

I mean the external force totally depends on the environment and if its a theoretical answer or real life simulation answer  Reply With Quote

7. My first question is, why is acceleration considered 'unbalanced'?  Reply With Quote

8. Because the velocity is changing

If something is changing then it can't be balanced (I thought)  Reply With Quote

9. Originally Posted by BenW
Because the velocity is changing

If something is changing then it can't be balanced (I thought)
Yes, but in changing it's velocity you're getting an equal and oposite reaction elsewhere.

e.g. If you drop a ball, it accelerates towards earth due to gravity. But the earth accelerates towards the ball an equall amount.  Reply With Quote

10. Ok so i'f i roll a ball along a level floor it deaccelerates do to friction. are you saying that friction is actually the ball deaccelerating the floor/earth?  Reply With Quote

11. This may explain it (from http://www.geocities.com/CapeCanaver...on/part1.html).....

NEWTON (Ingenious non-sense)

Newton created the concept of 'mass' because he failed to recognize the significance of Galileo's demonstrated proof that for every 'applied' force, there always exists some natural form of an exactly equal but opposite 'resistance force'. Newton therefore assumed that 'unbalanced' forces exist. He then created a new imaginary mathematical concept of 'mass inertia' that duplicated the naturally existing resistance force. Natural reality involving balanced 'forces' did not change - only the way that man explained the reality in mathematical terms changed. Explain this

If Newton had more carefully considered the results of Galileo's experiments, then he might have recognized that his new mathematical concept of F=MA should have been stated as:

"For every activating force (F) an equal but opposite naturally occurring resistance force (M) exists, such that net resultant force is zero. The location where the activating and resisting forces meet experiences 'stress'. The magnitude of the stress will coexist with some form of change in equal proportion to that stress. That portion of the stress which is not otherwise attributable to a recognized resistance force such as structure, friction, pressure, or temperature may appear in the format of a change in the rate of motion of the stressed object. If so, the magnitude of the change in rate of motion (A) will be directly proportional to the magnitude to the 'unbalanced' portion of the opposing forces. Let us refer to that portion of the total naturally existing resistance force that corresponds with the motion as inertia."

After creating a false concept of mass inertia, Newton created a false concept of 'mass attraction'. He failed to recognize that the 'centrifugal' force he associated with orbital motion of a planet around an absolutely fixed location in space (ie, the Sun) was already counter-balanced by a corresponding orbital motion of the Sun with the planets, and that both Sun and planet (like all pairs of 'orbiting' celestial bodies) rotate at equal angular rate of motion on opposite sides of a specific location in space now referred to as a 'barycenter'. Explain this

The centrifugal force associated with the rotation of a planet around the barycenter is always exactly equal but opposite in direction to the centrifugal force associated with the rotation of the Sun around that same barycenter. The ratio of the 'mass' of the two mutually rotating bodies is simply the inverse ratio of the radii of rotation around that barycenter.

Newton may have been the first person to advise that 'for every action there is an equal but opposite reaction', and he may have been the first to advise that celestial planets move in unison around a common center located between the planets. But evidently he must have recognized these things sometime after he had postulated his concepts of 'mass' and 'mass attraction', because these are the very concepts that nullify his prior concepts about 'unbalanced' forces, mass inertia, and mass attraction.  Reply With Quote

12. The bit I'm not sure about is the equal but opposite reaction... take weight as an example of force. When that acts on a object at rest (in air) there is no equal and opposite force, therefore causing the acceleration of 9.81ms^-2. Eventually, you get an equal and opposite force in the form of air resistance, which is when you reach terminal velocity... perhaps it is that there is an equal and opposite reaction, just not acting on that object, or not at that time?  Reply With Quote

13. I was talking about Gravity. You're now talking about kenetic friction Still if we assume the energy from the ball is all lost to heat due to friction, the amount of energy will still have been conserved.  Reply With Quote

14. Ahhhh....maybe it is to do with F=ma then and not newtons first law

In air F=mass*9.81

at terminal velocity F=mass*0

both are balanced but acceleration is different in both  Reply With Quote

15. Originally Posted by mike_w
When that acts on a object at rest (in air) there is no equal and opposite force
Yes there is. Just as the object has the potential energy to accelerate towards the earth (in this case) the earth will be acclerated towards the object.  Reply With Quote

16. Hugely confussed now, think i'll just get back to you with the answer tomorrow  Reply With Quote