newtons laws contradiction question
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hi, thanks for reading this
what im not sure is that in my textbook, they gave an eg of a book resting on the table. then they say it has a foece weight acting down on the table then there is a contact force acting against the object. so when i read this, im thinkin this is e3rd law as its an equal n opposite reaction and they act on different bodies. so how can this be, eg a boy kicks off a wall in a pool with 10N then he was sent away with 10N by contact force with the wall. i know this would defy everything, but then y wouldnt the book just fly up! thanks for helping me
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gravity
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wow that was fast, but gravity is a contributing factor to weight as W=mg n weight is a force acting downwards. btw since W=mg, n an object of 10kg falls with an acceleration of 5m/s then what would be the force acting downwards? is it like both Weight plus what we know or something else? thanks
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Originally posted by SgStudentStressed:
hi, thanks for reading this
what im not sure is that in my textbook, they gave an eg of a book resting on the table. then they say it has a foece weight acting down on the table then there is a contact force acting against the object. so when i read this, im thinkin this is e3rd law as its an equal n opposite reaction and they act on different bodies. so how can this be, eg a boy kicks off a wall in a pool with 10N then he was sent away with 10N by contact force with the wall. i know this would defy everything, but then y wouldnt the book just fly up! thanks for helping me
3rd law applies.
The free body diagram of the book is as follows:
Downwards force: Weight of book
Upwards force: Contact force by tableThe free body diagram of the table is as follows:
Downwards force: Weight of table + contact force by book (by N3L)
Upwards force: Contact force from groundWhen the book is at rest or travelling at constant velocity, resultant force = 0, or downwards force = upwards force. Thus, the contact force by the table is equal to the weight of the book. BUT this is not an application of newton's 3rd law.
Newton's 3rd law is applied when we say the
upwards force due to contact force by table ON the book
equal
downwards force due to contact force by book ON table. -
hi eagle, i dont quite understand your last statement isnt it still 3rd law i mean the forces weight and contact forces are acting on 2 different bodies. sorry for ky impudence but can u pls help me explain that? thanks
also, since W=mg, then when a box weighing 5kg fall with 0 accelation, would the downwards force be 0 ot Weight aka 50N? or in a case where it has 4m/s acceleration n we take net F=ma it would be 20N then in this case what should i do? thanks for the help
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Originally posted by SgStudentStressed:
hi eagle, i dont quite understand your last statement isnt it still 3rd law i mean the forces weight and contact forces are acting on 2 different bodies. sorry for ky impudence but can u pls help me explain that? thanks
also, since W=mg, then when a box weighing 5kg fall with 0 accelation, would the downwards force be 0 ot Weight aka 50N? or in a case where it has 4m/s acceleration n we take net F=ma it would be 20N then in this case what should i do? thanks for the help
If we were to apply newton's 3rd law on weight, it would be:
the force of the earth on the book (weight of book)
equal
the force of the book on the earth (Book exerts a gravitational force on earth as well, and this is covered in greater detail at A levels)Weight and contact forces are forces of two different nature. Newton's 3rd law is about action reaction forces, and the two forces must be of the same nature/type.
It's quite tough to explain based on text. You have to draw out the free body diagram, or get with your teacher or tutor to draw the free body diagram in front of you for explanation.
also, since W=mg, then when a box weighing 5kg fall with 0 accelation, would the downwards force be 0 ot Weight aka 50N? or in a case where it has 4m/s acceleration n we take net F=ma it would be 20N then in this case what should i do? thanks for the help
0 acceleration means 0N resultant force.
4m/s2 acceleration means 20N resultant force.This is always correct. Downwards force is not zero. It is the resultant (or vector sum) of the different forces that is zero.
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oh ok thanks for the help. i think ill get my tutor to explain the first one. for the second one when calculating the forces y dont we include weight as a downwards force?
n so sorry to bother u bt i have one last qns about tension. for eg, load of 20N is hung on a wall with a string and a spring balance. the load will cause a downwards force all the way tp the top of the string, then will there be a tensioal force which has the same magnitude at the top of the string at the ceiling place? then becquse the tensional force n the downwards force cancel each other out, there eould be one extra downwards force which is the load which causes net force of 20N downwards. then if 2 men pull at each end with 20N would it be 0N becauae when they pull there would be 2 opposite force that would cancel each other but also tenaional force at the two opposite side too. so the force pulling is 40N at both ends which totally cancel each other out so if there wqs a spring balance it would record 0N?
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When the book is resting on the table, the weight / force of the book is acting on the table. You sketch an arrow down. A resultant forces is acting on the book, you sketch an arrow upward.
At the corner of your free body diagram, you sketch the sign +ve ... We get 5 marks for this in those days.
Therefore, the arrow you sketch as "down" is plus, while the resultant is minus.
When the book is removed from the table, the force from the book is gone. The resultant force is also gone.
The sign is important as you need to sum up the expression.
Note also that Newton law sate that all object FALLs (therefore propelled by something is not included) at the SAME accelaration. Therefore, there is no such thing as "or in a case where it has 4m/s acceleration ..... "
1st year 1st semester is always stressful. Where 1st semester work is understood, 2nd semester is easier. They are related.
"Be a monk" for few years.
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but if force is acting on the table n a contact force acts on the book then wouldnt it be third law? cos its 2 different objects. in the third law, it is stated that with every firce there is an equal and opposite reaction on TWO MUTUAL BODIES. for a third law eg, a boy kicks the wall with 10N there is a reaction/contact force of 10N as its on 2 different bodies, the boy would move away. so this is what i dont understand the forve of the book on the table weight causes a reaction force which is contact force on the book which has the same magnitude. then its a third law case so y doesnt itfly upwards. thats all im askin pls hrlp me answer this thanka
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What law I do not remember. Those lecture notes are no longer with me.
If the book is +10 N, the opposite reaction will be -10 N. The book will not be lifted up. In the free body diagram, sign is as shown in my previous sentence.
Therefore, all the sign MUST be indicated clearly. If the lecturer have a hard time figuring out which is plus, which is minus ..... Well, that question is a gone case.
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Originally posted by SgStudentStressed:
but if force is acting on the table n a contact force acts on the book then wouldnt it be third law? cos its 2 different objects. in the third law, it is stated that with every firce there is an equal and opposite reaction on TWO MUTUAL BODIES. for a third law eg, a boy kicks the wall with 10N there is a reaction/contact force of 10N as its on 2 different bodies, the boy would move away. so this is what i dont understand the forve of the book on the table weight causes a reaction force which is contact force on the book which has the same magnitude. then its a third law case so y doesnt itfly upwards. thats all im askin pls hrlp me answer this thanka
Weight balances contact force, hence resultant force = 0
It appears that you do not know how to draw a free body diagram, nor do you understand the significance. All these begin from knowing how to draw the free body diagram, so I see no point in continuing to ponder over these questions without first understanding the significance of the free body diagram and it's applications.
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Look at f(ii). That is the free body diagram on the block
NOTE that the force of the book on the surface is not drawn. This is because the force of the book on the surface is not a force acting on the book.
Your application of newton's 3rd law is wrong because we have to consider a force acting on an object itself. A pair of action reaction forces acts on two different objects, but in determining whether there is a resultant force on an object, we consider merely the forces on that object only.
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Originally posted by SgStudentStressed:
oh ok thanks for the help. i think ill get my tutor to explain the first one. for the second one when calculating the forces y dont we include weight as a downwards force?
n so sorry to bother u bt i have one last qns about tension. for eg, load of 20N is hung on a wall with a string and a spring balance. the load will cause a downwards force all the way tp the top of the string, then will there be a tensioal force which has the same magnitude at the top of the string at the ceiling place? then becquse the tensional force n the downwards force cancel each other out, there eould be one extra downwards force which is the load which causes net force of 20N downwards. then if 2 men pull at each end with 20N would it be 0N becauae when they pull there would be 2 opposite force that would cancel each other but also tenaional force at the two opposite side too. so the force pulling is 40N at both ends which totally cancel each other out so if there wqs a spring balance it would record 0N?
Tension is the force of the string pulling back at each man. In this case, tension of string is still 20N if the pulling force on both sides is 20N.
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Dear Eagle,
You shared a wonderful resource :) Thanks!
Cheers,
Wen Shih -
so does tht mean that there is a net downwards force on the table n does it mean that when i hold something, its heavy becauae of the contact force on my hand n not because of weight since weight is acting on the object? cos of that when n object is hung on the ceiling, there us no net force, but when i put my hand over to touch the base of the object i experience contact force but its not heavy? im assuming this as what u r sayin is that weight acts on the object, n the floor has a contact force on the weight thus cancellin them out but then there is still a contact force acting on the ground by the object so naturally id think that thungs are heavy because of the contact force and not weight. but in the object hanging on the ceiling case, im not sure if weight is cancelled by tensional force because i dont know anything much about tensional force. so the main questions im askin is whether things are heavy cos of contact force on the ground? and since there is a contact force on the ground, would there be a net downwards force? finally if the reason y someone can kick off a pools wall is because there is only an action n reaction force but no weight to cancel the reaction force on the person n the reaction force is contact force on the boy? thanks so much for the help eagle u understood my question the best
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btw eagle, r u a tuition teacher?
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so does tht mean that there is a net downwards force on the table n does it mean that when i hold something, its heavy becauae of the contact force on my hand n not because of weight since weight is acting on the object?
cos of that when n object is hung on the ceiling, there is no net force, but when i put my hand over to touch the base of the object i experience contact force but its not heavy? im assuming this as what u r sayin is that weight acts on the object, n the floor has a contact force on the weight thus cancellin them out but then there is still a contact force acting on the ground by the object so naturally id think that thungs are heavy because of the contact force and not weight.
so the main questions im askin is whether things are heavy cos of contact force on the ground? and since there is a contact force on the ground, would there be a net downwards force?
I did some paragraphing in the quote.
When you find (or rather, you feel) it heavy, yes, it is the contact force acting on your hand.
The weight of the object acts on the object, and not on your hand.
Here's a question to test your understanding. If a book is on your hand and both your hand and the book are accelerating downwards at 10m/s2, what is the contact force of the book on your hand?
In physics, we don't talk about whether things are heavy. Heavy is a subjective word. Instead, we talk about forces. What is the force of something acting on a body, etc.
When something is on the ground, and not moving, by newton's second law, there is zero net force. There is no net downward force as there is no resultant downwards acceleration. You may treat the ground as a totally immovable object for O levels.
but in the object hanging on the ceiling case, im not sure if weight is cancelled by tensional force because i dont know anything much about tensional force.
Yes. You have to draw the free body diagram of the object with it's weight and being supported by the tension in the ropes.
finally if the reason y someone can kick off a pools wall is because there is only an action n reaction force but no weight to cancel the reaction force on the person n the reaction force is contact force on the boy?
Yes, the reaction force pushes the person off the pool's wall. This is a horizontal force.
Weight is acting downwards, and this is balanced by upthrust from the pool's water. Hence, there is no net movement in the vertical direction, and the net force on the person is horizontal. You may treat upthrust as an upwards force due to principles of floatation. The real explanation and idea of upthrust is in A levels.
btw eagle, r u a tuition teacher?
Yes. I teach mainly A levels physics and give junior physics olympiad training to 2 secondary schools.
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oh thanks eagle u helped me so much n im sec 3 takin pure physics n chem n a maths, saw ur webaite no classes for o level. aww man
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oh thanks eagle u helped me so much n im sec 3 takin pure physics n chem n a maths, saw ur webaite no classes for o level. aww man
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Sorry to hijack this post. @Eagle, is your answer 0N for contact force?
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hi eagle so what u r sayin is that weight does not exert any downwards force but when it hits someyhing the reason y it hurts is because there is a net force downeards of contct force on the hand? so there is always a net downwards force when its on something? lastly is that contact force the same aa weight i think it is as weight is countered by contact force on the liad itself n there is one contact force on the ground so if weight was gone it would be a 3rd law case n the object will fly up so is that true? eagle is this the correct working taking the object to be 10kg.
F weight=10x10=100N F contact:=100N
so all the forces on the object would be weight and contact force on hand is contact force on it
so is the reason on y it falls unlike earth the hand is a movable object and thus it move down with a force of 100N which qas exerted by contact force on the hand by the book. as its a third law case as now its 2 objects. the objects are the hand and the book but because the cointact force on the book is cancelled out by the weight so there is a net force downwards by contact force from book on hand?
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Originally posted by CaiHongRainx:
Sorry to hijack this post. @Eagle, is your answer 0N for contact force?
Correct
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Originally posted by SgStudentStressed:
hi eagle so what u r sayin is that weight does not exert any downwards force but when it hits someyhing the reason y it hurts is because there is a net force downeards of contct force on the hand? so there is always a net downwards force when its on something? lastly is that contact force the same aa weight i think it is as weight is countered by contact force on the liad itself n there is one contact force on the ground so if weight was gone it would be a 3rd law case n the object will fly up so is that true? eagle is this the correct working taking the object to be 10kg.
F weight=10x10=100N F contact:=100N
so all the forces on the object would be weight and contact force on hand is contact force on it
so is the reason on y it falls unlike earth the hand is a movable object and thus it move down with a force of 100N which qas exerted by contact force on the hand by the book. as its a third law case as now its 2 objects. the objects are the hand and the book but because the cointact force on the book is cancelled out by the weight so there is a net force downwards by contact force from book on hand?
No, weight exerts a downward force.
But if an object on a table is stationary, then a upwards contact force (with magnitude equal to weight) has to be applied to the object by the table, so as to make resultant force = 0.
By newton's third law, because there is a force by the table on the object, there is also a force on the table by the object.
F weight=10x10=100N F contact:=100N
so all the forces on the object would be weight and contact force on hand is contact force on it
When in equilibrium, yes. This is correct.
so is the reason on y it falls unlike earth the hand is a movable object and thus it move down with a force of 100N which qas exerted by contact force on the hand by the book. as its a third law case as now its 2 objects. the objects are the hand and the book but because the cointact force on the book is cancelled out by the weight so there is a net force downwards by contact force from book on hand?
I think you need to improve on your English. Not very sure what you mean here.
The reason why things fall at increasing speed is either due to zero upwards force or an upwards force that is much smaller than its weight. This results in a resultant downwards force, and hence things accelerates downwards.
However, things can still fall at constant speed (terminal velocity) with downwards force (weight) equals to upwards force (air resistance). There is zero resultant force here. The upwards force of the air on the falling object equals to the downwards force of the falling object on the air (Newton's third law).
The key point here to take note is resultant force. Only with a resultant force would you have acceleration (Newton's 2nd Law). With zero resultant force, you have zero acceleration (Newton's 2nd Law). As a result, objects would either remain stationary or continue in the same velocity as before (Newton's 1st law).
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LOL is this the explaination? These 2 forces arent opposite to each other, which N3L say that the forces must be equal and opposite. So its 0N.