Newtons law of motion questions

[tyrantboy]

Homework Statement

Tell me what is wrong with this question-

A donkey which has heard of Newtons third law but not understood it very well refuses to pull a cart attached to it because it argues that: ' If i pull on the cart with a certain force, Newton's third law would imply an equal but opposite force, which would then mean that the total force on the cart is zero, and so it will not move. so why waste my effort?i won't pull'. What's wrong with the donkey's argument?

The Attempt at a Solution

Is it because there is no reaction to the cart. for example the space shuttle example is a gud one. the downward thrust produces an equal upward trust. there is only the reaction with the ground the cart has which is friction so the donkey will be able to pull on it since the friction force is very small and there is no reaction with the cart.

 

[badphysicist]

Try to do a free-body diagram of the donkey when it is moving - think about where forces are being applied.

 

[gc2004]

this is a very common question which comes in different forms... since it is against the forum rules, i wouldn't give you a direct answer... but think seriously about who is exerting a force and where it is being felt... and then go ahead to draw individual free body diagrams for the donkey and the cart (as badphysicist tells you to do) and figure out why the donkey moves despite the action-reaction pair...

 

[dynamicsolo]

There is an important conditon that must be met (which we are asking you to recognize).

If the cart were all that the donkey were applying force against, it would be absolutely correct. (See, it's not that dumb...).

But further, if the frictional force between the donkey's hooves and the ground matched the frictional force between the cart's wheels and the ground, it would again get nowhere and be correct. (Think of a comparison between your towing something across dry sand or wet ice and doing the same across dry, firm ground or pavement.)

So what must be true if pulling a cart is to happen at all? As badphysicist says, you want to think about the forces on the donkey, but also look at the forces on the cart.

 

[tyrantboy]

well the forces i know of r the frictional force between the ground and the donkey/cart. there is also the gravitational pull exerted in the cart.

the donkey is exerting a force on the cart as it is pulling it.

please correct me where ever it is incorrect

 

[gc2004]

hi! i think you are making it a bit too complicated... to begin with, gravity doesn't play much of a role out here... it is obviously there, but it doesn't help the cart to move, provided the road is perfectly horizontal... so don't take into account forces which are simply going to clutter things up... simply think about the force the donkey exerts upon the cart... who is applying this force? where is the force being felt? now, as Newton's third law suggests, every force will have a reaction force... now, think about who/what is applying this reaction force and where is it being felt...

 

[Dick]

well the forces i know of r the frictional force between the ground and the donkey/cart. there is also the gravitational pull exerted in the cart.

the donkey is exerting a force on the cart as it is pulling it.

please correct me where ever it is incorrect

Yes, the donkey-cart system has net zero force. The donkey is also exerting a force on the ground. Donkey+cart-ground is also zero. Who suffers? What's holding the Earth in place?

 

[tyrantboy]

thx for making things a bit clearer gc2004.

well come to think about it, the donkey exerts a force on the cart since it is pulling it and thus the cart would exert a force on the wheels which would then in turn move forward. The force is being felt by the wheels and since the donkey force is more than the cart force, the cart moves forward

rite?

god i suck at this:p

 

[tyrantboy]

Oh yea and this is for Dick's question

Yes, the donkey-cart system has net zero force. The donkey is also exerting a force on the ground. Donkey+cart-ground is also zero. Who suffers? What's holding the Earth in place?

the Earth is held together by gravity since all matter attracts all other matter through a force called gravity.

 

[Dick]

Oh yea and this is for Dick's question



the Earth is held together by gravity since all matter attracts all other matter through a force called gravity.

Yes, but that isn't quite what I meant. Picture just the donkey and cart with a stretched spring between them. Nothing else around. The forces are balanced. When you let them go they accelerate towards each other. Having balanced forces doesn't mean nothing can move, just that the center of mass of the whole system can't accelerate. In this case the system has three main parts, donkey, cart and earth. Action may equal reaction but the donkey/cart can still accelerate forward as long as the Earth accelerates backwards.

 

[tyrantboy]

Action may equal reaction but the donkey/cart can still accelerate forward as long as the Earth accelerates backwards.

I quite don't get it what u mean by the Earth moving backwards?

wat does that hav to do with the donkey?

 

[Dick]

Action/reaction is also know as conservation of momentum. If donkey/cart acquire forward momentum something else (the earth) gains momentum in the opposite direction. But rereading the question, I think they may be after a simpler answer. When the donkey exerts a force on the cart, the reaction force acts on the donkey, not the cart. So they don't 'add to zero'. There is only one force on the cart.

 

[tyrantboy]

thx for clearing things up for me but how does the reaction force act in the donkey? wait force is either pull or push rite? so wen the donkey pulls the cart it is exerting a force? tat is the action rite? and so the reaction force (pull) moves the cart forward. ut doesn't this cancel out since the action/ reaction is equal.

So they don't 'add to zero'. There is only one force on the cart.

how come they don't add to zero?

sorry but i hav to clear my doubts or i will never get this :)

 

[Dick]

Think of four forces, A the force the donkey exerts on the cart, B the force the cart exerts on the donkey, C the force the donkey exerts on the ground and D the force the ground exerts on the donkey. A and B are opposite and equal, as are C and D. Only force A acts on the cart. Forces B and D act on the donkey. So as long as D>B the cart/donkey can accelerate forward. A and B don't cancel because they act on different objects.

 

[Razzor7]

thx for clearing things up for me but how does the reaction force act in the donkey? wait force is either pull or push rite? so wen the donkey pulls the cart it is exerting a force? tat is the action rite? and so the reaction force (pull) moves the cart forward. ut doesn't this cancel out since the action/ reaction is equal.how come they don't add to zero?

sorry but i hav to clear my doubts or i will never get this :)

What happens if the forces are balanced?

 

[tyrantboy]

thx dick. u have been of great help to me. thanks for clearing this mess.

 

[tyrantboy]

Delete thread please

 

[tyrantboy]

delete thread please

 

[Dick]

delete thread please

You'll need to contact a moderator if you really want to get rid of this. It's not the usual thing to delete a thread when they are finished. They contents could help someone else.

 

[Parishrut]

the force applied by the donkey is a muscular force using his feet to push himself forward while the reaction force is not. the cart is not alone bieng pulled but along with it the wheel too, which reduces the friction force against the cart and it moves easily.

 

[nikevictory]

The donkey is incorrect because action and reaction forces never act on the same body.
Therefore when the force is applied by the donkey to pull the cart, the ground exerts an equal and opposite force on the donkey to push it forward.

 

[Tomer]

Simply put: Donkey pulls cart < = > cart pulls donkey! So if the donkey exerts a force on the cart and pulls it, the cart will pull the donkey as well (that's why he struggles) - but the cart doesn't pull itself back!
It's tricky, but trivial when you think about it a little :-)

Generally: Particle I exerts a force on particle J < = > Particle J exerts the same force (opposite direction) on particle I. So that the sum of forces on both of them together is 0, but on each separately definitely not!

The donkey needs to get his homework done :-) Or change his life's career... :)

 

[physicist2b]

As per the discussions i went through, i wanted to get a confirmation regarding the crux of the discussion..Is it like.. Donkey applying force on ground ground reverting back to that force in opposite direction and in same amount to the donkey..but the forces don't exactly cancel out each other as they are being applied on different bodies so net force on the anybody is not zero at any point of time. the vertical component becomes the normal to the weight of the donkey at the time of contact and horizontal component helps it move forward and apply some force on cart which further helps move the cart...? Also if it is so then in my view friction acting in the direction opposite to the direction of motion should be equal to the force experiencced by the donkey in horizontal direction but actually it doesn't happen...please comment..

 

[koshal303]

guys
please help me with this question

a stone is dropped in a well is heard to hit the water after 4 sec what will be the depth of the well if the velocity of sound is 350m/s ?

 

[mstud]

guys
please help me with this question

a stone is dropped in a well is heard to hit the water after 4 sec what will be the depth of the well if the velocity of sound is 350m/s ?

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