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Work-done previously= F*s= (8-4)J=4J
Work-done previously= F*s= (8-4)J=4J
When the force applied is 2F and distance=2s then total workdone will be 2F*2s= 4Fs=4*4J= 16J.
So, original 4J will become 4+16=20J.
*disclaimer: the process maybe totally wrong. :/
Haha. Thanks. I checked the answer first and then devised a way to get to it. lol. Anyway, your complete explanation was in need.When a total amount of work W is done on any object, the kinetic energy of that object changes by a quantity W - if negative work is done on the object, the kinetic energy changes by a negative amount and if positive work is done on the object it's energy changes by a positive amount.
Suppose you throw an object upwards, gravity is the only force that does any work on the object. That work is negative, so the kinetic energy of the object decreases until the object comes to a stop. When it falls down, gravity does positive work on the system thus increasing it's kinetic energy.
In this case, taking the first situation, the force is constant at magnitude F, over the entire displacement s. Thus, the work done by that force is Fs. By the
Work-Kinetic Energy theorem, this is equal to the increase in kinetic energy of the system. This value is given as 4 Joules (8-4 = change in KE = +4 Joules).
In the upcoming situation, the force is 2F, displaced through a distance 2s. Thus, the total work done is 4Fs. This is also the change in kinetic energy of the object.
From above, we know that Fs is 4 Joules, so 4Fs = 4*4 = 16 Joules increase. From 4 Joules, the increase of 16 Joules takes it to 20 Joules = B.
Well, I just noticed unique111's solution, and I have to say as far as I know it's absolutely correct, (so no need for the disclaimer, unique111! it's totally right!)
Good Luck for all your exams!
When a total amount of work W is done on any object, the kinetic energy of that object changes by a quantity W - if negative work is done on the object, the kinetic energy changes by a negative amount and if positive work is done on the object it's energy changes by a positive amount.
Suppose you throw an object upwards, gravity is the only force that does any work on the object. That work is negative, so the kinetic energy of the object decreases until the object comes to a stop. When it falls down, gravity does positive work on the system thus increasing it's kinetic energy.
In this case, taking the first situation, the force is constant at magnitude F, over the entire displacement s. Thus, the work done by that force is Fs. By the
Work-Kinetic Energy theorem, this is equal to the increase in kinetic energy of the system. This value is given as 4 Joules (8-4 = change in KE = +4 Joules).
In the upcoming situation, the force is 2F, displaced through a distance 2s. Thus, the total work done is 4Fs. This is also the change in kinetic energy of the object.
From above, we know that Fs is 4 Joules, so 4Fs = 4*4 = 16 Joules increase. From 4 Joules, the increase of 16 Joules takes it to 20 Joules = B.
Well, I just noticed unique111's solution, and I have to say as far as I know it's absolutely correct, (so no need for the disclaimer, unique111! it's totally right!)
Good Luck for all your exams!
When a total amount of work W is done on any object, the kinetic energy of that object changes by a quantity W - if negative work is done on the object, the kinetic energy changes by a negative amount and if positive work is done on the object it's energy changes by a positive amount.
Suppose you throw an object upwards, gravity is the only force that does any work on the object. That work is negative, so the kinetic energy of the object decreases until the object comes to a stop. When it falls down, gravity does positive work on the system thus increasing it's kinetic energy.
In this case, taking the first situation, the force is constant at magnitude F, over the entire displacement s. Thus, the work done by that force is Fs. By the
Work-Kinetic Energy theorem, this is equal to the increase in kinetic energy of the system. This value is given as 4 Joules (8-4 = change in KE = +4 Joules).
In the upcoming situation, the force is 2F, displaced through a distance 2s. Thus, the total work done is 4Fs. This is also the change in kinetic energy of the object.
From above, we know that Fs is 4 Joules, so 4Fs = 4*4 = 16 Joules increase. From 4 Joules, the increase of 16 Joules takes it to 20 Joules = B.
Well, I just noticed unique111's solution, and I have to say as far as I know it's absolutely correct, (so no need for the disclaimer, unique111! it's totally right!)
Good Luck for all your exams!
okay so u meanwhen work is done against gravity its negative and the kinetic energy decreases right just confirming (developing a concept actually )
There's some very tight wording there, so just want to confirm:
When the force of gravity itself does negative work, the kinetic energy decreases. This is correct.
However, when work is done against the force of gravity, it implies there is some force opposing gravity (suppose you are lifting an object with your hands, you are doing work against gravity. Suppose you throw the object up: as long as it is in contact with your palm and it is moving upwards with you hand, the normal force you exert on it does work against gravity. When you let go of the object, no force opposes gravity so gravity does negative work on it until it stops and falls back down).
It all depends on the displacement. When an object is moving upwards, gravity is doing negative work because the displacement is in the opposite direction to the force (displacement is upwards, force acts downwards). This is negative work, and shows itself as the object slows down. When the object is falling down, gravity does positive work because the displacement (downwards) and the force of gravity (downwards) are in the same direction.
So when work is done against gravity, it implies some force aside from gravity doing that work (as far as i know - the distinction is important, because CIE examiners like to use the most specific wording in their problems and options). In that case, depending on the displacements, the work can vary.
If the object is falling, but the force opposing gravity is stronger than the force of gravity, then the object slows down (because force opposing gravity is upwards and displacement is downwards, that force does negative work. Because both gravity and displacement are downwards, gravity does positive work. However, since the opposing force is stronger than gravity, it does more negative work than gravity does positive work, so the change in kinetic energy is overall negative. In other words, the kinetic energy decreases).
Suppose the object is going upwards and the force opposing gravity is weaker than gravity. In that case also the object slows down (because force opposing gravity is upwards and displacement is upwards, that force does positive work. Because gravity acts downwards and displacement is upwards, gravity does negative work. However, since gravity is stronger than the opposing force, it does more negative work than the opposing force does positive work, so the change in kinetic energy is overall negative. In other words, the kinetic energy decreases).
Hope this helped!
Good Luck for all your exams!
thanks alot
but....in the last example you gave how can gravity act upwards like doesnt gravity always acts downwards?
Ummm.... where did I write that?
Gravity always acts downwards for objects on Earth or any other spherical surface, so i'm not sure where I wrote that bit.
Questions 4 (A) , 13 (D) ,14 (A) ,16 (C) ,22 (B), 32 (D) Help pls. in these questions http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Physics (9702)/9702_w09_qp_11.pdf
Strain energy = 1/2 x F x e = ( 17 x 0.3) / 2 = 2.55 J
Strain energy = 1/2 x F x e = ( 17 x 0.3) / 2 = 2.55 J
as they r askng for the best estimate so we will round it to 2.0 J
but there is another option 2.6, why it is not the answer?
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