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Newton's 3rd Law

10

Ok I get the principles, but i'm always misinterpretin something when I do the questions.. for example:

A door which cannot be opened by pushing steadily on it can often be kicked open. By considering what happens to the foot as it hits the door, explain why the kick is more efefctive. Refer to Newton's 2nd and 3rd Laws.

My answer is: The foot accelerates then decelerates as it hits the door, losing momentum. This process is very rapid therefore by F = mv-mu/t or F=ma, the force is greater The door must be able to provide an equal force against the foot as the door is kicked. If the force is larger, the door may not be able to provide this force.

This is right, but the mark scheme states that the 3rd Law Pair is

Door acts on foot --> so Newton's 3rd Law Pair is the foot acts on door, shouldn't it be the other way round?

As well as this.. if a book is at rest on a table ( the force of the book being the weight ), isn't the 3rd Law pair the reaction force of the table upwards onto the book. Apparently I've read that the 3rd law pair for this force is that the book pulls the Earth up towards it..

err..? Help me out.

- Thanks

Reply 1

gorilla_baby

9

newtons' third law forces must be the same size and the same type
the book's weight is a gravitational force and the opposite to that is the pull of the earth upwards, as it must be the same type i.e. gravitational
the force of the table on the book upwards is a contact force, so the opposite to that is the force of the book on the table, which is also a contact force
i.e. make sure newton 3 pair forces are the same type and are opposite, i.e. if A acts on B with a contact force size X, then B acts on A with a contact force of size X as well
newton 3 pair forces are the same type, size and have the same line of action

Reply 2

Creative

OP

10

Ok thanks, that type thing really helped. Now I understand the book.

Just one more thing, does the order matter?

For example.. boot pushes door, 3rd Law pair door pushes boot with equal force

or is it the other way round.. this one makes sense.. but the Edexcel markschemes are just.. out of this world :tongue:

Reply 3

gorilla_baby

9

no, the order doesn't matter
door pushes foot and foot pushes door
same as foot pushes door and door pushes foot, since they usually occur at the same time, i.e. the newton 3 pair of forces act for the same time

Reply 4

Eau

15

Creative

Ok thanks, that type thing really helped. Now I understand the book.

Just one more thing, does the order matter?

For example.. boot pushes door, 3rd Law pair door pushes boot with equal force

or is it the other way round.. this one makes sense.. but the Edexcel markschemes are just.. out of this world :tongue:

It's all relative.

You can't have this first, that second. It always act in pairs. So both AT THE SAME TIME.

Reply 5

Creative

OP

10

okay. i get it then, thanks for the help both of ya.

Reply 6

Mohit_C

10

So I presume the actual Third Law Pair doesn't matter either, whether it is the door pushing on the foot or the foot pushing on the door?

Reply 7

Eau

15

Mohit_C

So I presume the actual Third Law Pair doesn't matter either, whether it is the door pushing on the foot or the foot pushing on the door?

Nope! If were "very small" and observed from the foot (you can't see the foot), you would see the door coming towards you!

I am not a v. good physicist, but I believe this is an example of relativity, which is arguably the most basic law in physics.

Reply 8

gorilla_baby

9

Mohit_C

So I presume the actual Third Law Pair doesn't matter either, whether it is the door pushing on the foot or the foot pushing on the door?

i don't get what you mean, but it's just a pair of forces, not a sequence of events - i.e. the force pair act at the same time and thus both events occur at the same time

Reply 9

Mohit_C

10

Well Eau answered my question. What I was basically asking was does the "who is the third law pair" matter, i.e. in you situation the door acting on the foot or the foot acting on the door?

Reply 10

phys981

10

Which force comes first?

Their logic is that you've just reasoned by Newton-2 that the foot decelerates quickly as it hits the door, therefore there is a force on the foot, provided by the door. That's why they choose the "door pushes foot" first for Newton-3.

Door pushes foot, so foot pushes door.

Reply 11

Mohit_C

10

^Good way of remembering it. Thanks rsk.

Reply 12

Morbo

17

Arguably you should preserve causality in this question. Indeed, "foot pushes door, so door pushes foot" could, in the context of N3L, be written the other way around and still make sense mathematically, however we must remember that the time ordering of events is still important.

Consider the initial rest frame of the door and foot (i.e. at rest with respect to the Earth's surface). The foot is definitely accelerated first by the muscles in the body - you cannot say that the door is accelerated. Relativistic symmetry is broken here - as a "little person on the end of the foot", you would know that it was you who was accelerated towards the door, and not the door that was acclerated towards you. So you cannot say that "the door is travelling towards the foot".

So, it stands to reason that - in the initial rest frame - the foot provides the "action" force on the door, since without the foot's prior motion, the door would not be able to provide the "reaction" force.

Though, of course, at the time of contact between foot and door, mathematically the forces are equal and opposite, so causality is irrelevant.

Reply 13

Neo1

4

That's well explained worzo. But we havent considered what happens to the reaction force (which is from the door), when the foot kicks the door and it opens..does the door still exert the same force on the foot that the force of the foot exerted on the door to open it?

Thanks

Reply 14

Mohit_C

10

Neo1

That's well explained worzo. But we havent considered what happens to the reaction force (which is from the door), when the foot kicks the door and it opens..does the door still exert the same force on the foot that the force of the foot exerted on the door to open it?

Thanks

That's what making me think. The OP stated that the time in which it took place was very small so the force would be very large but that means that the reaction would also be very large. Can someone explain why there would be a resultant force, I seem to be confused on this now?

Reply 15

Morbo

17

Neo1

does the door still exert the same force on the foot that the force of the foot exerted on the door to open it?

Yes. N3L is "when body A exerts a force on body B, body B exerts an equal and opposite force on body A". Note the "equal" in this.

Mohit_C

That's what making me think. The OP stated that the time in which it took place was very small so the force would be very large but that means that the reaction would also be very large. Can someone explain why there would be a resultant force, I seem to be confused on this now?

I don't understand what you mean by "a resultant force". The resultant force on a body simply the sum of the forces acting on it. In this case, each body only has one force acting on it provided by the other body.

Reply 16

Mohit_C

10

Ah yes, yes! I forgot that N3L acts on different bodies....I somehow thought it was on the foot itself so didn't see where the resultant force came from. Thanks!

Newton's 3rd Law

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