physics conceptual test qn
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When a person is in the motion of jumping, the normal force acting on the person: i)reduces as he is leaving the ground
ii)increases as he is leaving the ground
iii)remains the same throughout -
remain ? I assume normal forces to be gravity, you go up , gravity will still be 10/ms-1
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what do u mean by normal force
if it is gravity, it is what as has been said above
if it is the force acting on the person to lift him up, the force is used up the moment the feet break contact with the ground. What helps the person to move upwards after that will be inertia... -
lol normal force is the force acting perpendicular to the surface of the ground and towards the mass in context. gravity is gravitational force.Originally posted by alexlau:remain ? I assume normal forces to be gravity, you go up , gravity will still be 10/ms-1 -
but u require an acceleration to increase the person's velocity upwards before a motion can occur from stationary position. that would imply a resultant force in the upward direction. the question now is what is this upward force due to.Originally posted by eagle:what do u mean by normal force
if it is gravity, it is what as has been said above
if it is the force acting on the person to lift him up, the force is used up the moment the feet break contact with the ground. What helps the person to move upwards after that will be inertia...
n i meant while he bends down n pushes himself upwards -
oh n btw, what helps him move up is not because of inertia, its coz it has a starting velocity which is constantly decreasing due to gravity.Originally posted by eagle:what do u mean by normal force
if it is gravity, it is what as has been said above
if it is the force acting on the person to lift him up, the force is used up the moment the feet break contact with the ground. What helps the person to move upwards after that will be inertia... -
please write out your qn concisely so we can help
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as a person is jumping(before he leaves the ground) there is a normal force acting on him. in the process of pushing his legs against the ground to jump, the magnitude of the normal force increases, decreases or remains the same?Originally posted by unclebutcher:please write out your qn concisely so we can help
clear enough anot? -
That is called inertia =DOriginally posted by darkhour:oh n btw, what helps him move up is not because of inertia, its coz it has a starting velocity which is constantly decreasing due to gravity.
If there's no gravity, he would have continued in the same speed =D -
Originally posted by darkhour:
in the process: he has not jumped yet...so normal force which is equal to his weight is same
as a person is jumping(before he leaves the ground) there is a normal force acting on him. in the process of pushing his legs against the ground to jump, the magnitude of the normal force increases, decreases or remains the same?
clear enough anot?
if he has started jump: there is an upward force countering the downward weight, normal force decreases -
eh i tot inertia is the reluctance of an object/body to move or stop moving
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this upward force is due to Newton's 3rd lawOriginally posted by darkhour:but u require an acceleration to increase the person's velocity upwards before a motion can occur from stationary position. that would imply a resultant force in the upward direction. the question now is what is this upward force due to.
n i meant while he bends down n pushes himself upwards
Firstly, the person will exert a force on the ground. By Newton's 3rd law, the ground exerts an equal but opposite force on the person. This force (after discounting all your gravity rubbish) will accelerate the person upwards.
Once you leave the ground, this force will naturally be absent. -
provided no external forceOriginally posted by uaremyfayth:eh i tot inertia is the reluctance of an object/body to move or stop moving
In our case, gravity is the external force
It's my mistake for giving too brief an explanation
The person leaving the ground will continue at a constant speed (inertia)
But then he will slow down because of gravity (the external force acting on him) -
yea but how can juz discount away gravity since its a constant force always acting on the body? the normal force will disappear the instant the person leaves the ground that i know. the thing is an acceleration can onli be caused by a resultant force. in this case, if normal force is the onli force acting on the body besides gravity, then it shld increase to a level higher than gravity to cause a resultant force on the body to make it accelerate.Originally posted by eagle:this upward force is due to Newton's 3rd law
Firstly, the person will exert a force on the ground. By Newton's 3rd law, the ground exerts an equal but opposite force on the person. This force (after discounting all your gravity rubbish) will accelerate the person upwards.
Once you leave the ground, this force will naturally be absent. -
the force that accelerates only acts when the feet are still touching the groundOriginally posted by darkhour:yea but how can juz discount away gravity since its a constant force always acting on the body? the normal force will disappear the instant the person leaves the ground that i know. the thing is an acceleration can onli be caused by a resultant force. in this case, if normal force is the onli force acting on the body besides gravity, then it shld increase to a level higher than gravity to cause a resultant force on the body to make it accelerate.
After leaving the ground, only gravity comes into action, which means after leaving the ground, you decelerate. -
so which is the stronger force that causes a resultant force that is responsible for the body's acceleration upwards?Originally posted by unclebutcher:in the process: he has not jumped yet...so normal force which is equal to his weight is same
if he has started jump: there is an upward force countering the downward weight, normal force decreases -
yea so im asking what is responsible for the resultant force. isit bcoz the normal force increase larger than the weight?or what?anyways i tink i figured out that is shld be due to an increase in normal force as when a person jumps, part of his body accelerates downwards, then he creates a force upwards to stop his top part of his body from continuing to accelerate downwards, thus causing the ground to increase its normal force against the body. in pushing upwards, the person will maintain the normal force or increase it by continuing to push his body upwards. this will cause the normal force to be larger than its weight therefore allowing the body to accelerate upwards.Originally posted by eagle:the force that accelerates only acts when the feet are still touching the ground
After leaving the ground, only gravity comes into action, which means after leaving the ground, you decelerate.
comment if u think its wrong bah but i feel that ur answers r rather brief n not reali answering the qn
no offence yea?juz feel this way bah...appreciate ur help anyways 
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yea i know...the thing now is i wanna know mainly is what happen during the process at which ur legs r in contact with the ground...what happens to the normal force of it...i provided an explanation of how i think le, u go see if its right can?Originally posted by eagle:the force that accelerates only acts when the feet are still touching the ground
After leaving the ground, only gravity comes into action, which means after leaving the ground, you decelerate. -
Yes of course.Originally posted by darkhour:yea so im asking what is responsible for the resultant force. isit bcoz the normal force increase larger than the weight?or what?
You must see the body as one whole unit by itself. Internal forces within the body do not affect the motion. Only external forces acting on the body do.Originally posted by darkhour:anyways i tink i figured out that is shld be due to an increase in normal force as when a person jumps, part of his body accelerates downwards, then he creates a force upwards to stop his top part of his body from continuing to accelerate downwards, thus causing the ground to increase its normal force against the body.
Strictly speaking, the person pushes downwards.Originally posted by darkhour:in pushing upwards, the person will maintain the normal force or increase it by continuing to push his body upwards. this will cause the normal force to be larger than its weight therefore allowing the body to accelerate upwards.
What you have just said were what we have explained. Perhaps u did not understand the terms used. What we understand from your definition of normal force is the reaction force from the ground, which of course reduces to zero when you are not touching the ground. It does not remain there. Because it is present for such a short time, you may also call it an impulse. Mathematically, an impulse is the multiplication of the force times the time of application of force.
Yes, the resultant force is what causes the person to accelerate, and it is due to the reaction force from the ground being greater than the weight of the person.
If you are asking about the magnitude of the resultant force during the whole course of the jumping motion, it cannot be answered properly. -
yep i know its impulse, the thing is what causes the normal force to increase. if u say internal forces dun matter, then the body cant juz push downwards since it has no other external forces acting on it what.Originally posted by eagle:Strictly speaking, the person pushes downwards.
What you have just said were what we have explained. Perhaps u did not understand the terms used. What we understand from your definition of normal force is the reaction force from the ground, which of course reduces to zero when you are not touching the ground. It does not remain there. Because it is present for such a short time, you may also call it an impulse. Mathematically, an impulse is the multiplication of the force times the time of application of force.
Yes, the resultant force is what causes the person to accelerate, and it is due to the reaction force from the ground being greater than the weight of the person.
If you are asking about the magnitude of the resultant force during the whole course of the jumping motion, it cannot be answered properly.
ok in the first place, all u guys mentioned was inertia, y the normal force is absent, n what is a normal force etc...what i wanna know is how the normal force changes from the point in time when he bends down up till the time when he is about to leave the ground. in that sense u guys din mention any part of it.
btw im asking abt how the magnitude of the normal force changes with time during the process of him bending down n abt to leave the ground. not resultatnd force. -
The normal force should not change when u r still bending down. Only at the point of leaving would the force be greater than your weight, else you would have been lifted up off the ground before you even finish the jumping motion. =DOriginally posted by darkhour:yep i know its impulse, the thing is what causes the normal force to increase. if u say internal forces dun matter, then the body cant juz push downwards since it has no other external forces acting on it what.
ok in the first place, all u guys mentioned was inertia, y the normal force is absent, n what is a normal force etc...what i wanna know is how the normal force changes from the point in time when he bends down up till the time when he is about to leave the ground. in that sense u guys din mention any part of it.
btw im asking abt how the magnitude of the normal force changes with time during the process of him bending down n abt to leave the ground. not resultatnd force. -
so the graph will b like __/ then go to 0 rite?Originally posted by eagle:The normal force should not change when u r still bending down. Only at the point of leaving would the force be greater than your weight, else you would have been lifted up off the ground before you even finish the jumping motion. =D -
yup =DOriginally posted by darkhour:so the graph will b like __/ then go to 0 rite?
and also since we have agreed that it is an impulsive force =D -
can u pls jump from 10th storey