Hi David/Khan Academy, The acceleration of the Centre of Mass (CM) of the system in NOT DOWNWARDS but at an angle to the vertical, the horizontal component being provided by the acceleration of 5 kg mass in the horizontal direction. If we take the system to be combined 5kg and 3kg, then the external forces acting on the system are not only the forces of gravity acting on the masses , the normal reaction on the 5kg mass (which is nullified by weight of 5kg) but also the NORMAL REACTION at pulley, point outwards (towards top right corner of screen) at an angle of 45 degrees with horizontal (for this specific problem), which is not at all mentioned. It is this force that provides the horizontal component of the system's acceleration. It is the vector sum of these TWO forces that drives the system (and not the single force mg) and the acceleration vector of the CM of the system, is the vector sum of this Reaction (at pulley) and mg (3kg mass) divided by the total mass of the system. In this specific problem, since the strings forms an angle of 90 degree at the pulley, the horizontal and vertical components of the Reaction force are equal in this case ( and makes an angle of 45 degree with horizontal) QUESTION: Why is the external Normal Reaction at pulley, that provides the horizontal component of the acceleration of the system, NOT at all considered ? QUESTION: Why is "system acceleration" and vector sum of External Forces assumed to be in vertical (downward) direction, when it is not so ?
Excellent....!!! I'd say the quick response to your questions is that "it gives you the right answer." However, YOU are correct on the physics of this problem that there is a contact force acting at the pulley on the string (part of the system) and that the acceleration of the system is down and to the right.
I don't get why the acceleration of the 3 kg mass must be negative. Does that mean that if the 3 kg mass and pulley were on the left, the acceleration would somehow be different? I meant about the hard way
It's saying that down/left is negative and up/right is positive. It's just a frame of reference. If the 3kg mass was moved to the left side, the 3kg mass would still be moving downward, so its acceleration would still be negative. But the 5kg mass would now be moving to the left, so its acceleration would now be negative. Just think of the signs as directions.
Think about it like this: It doesn't matter if the acceleration is negative or positive (it does, but for a min ignore it), we just know that the acceleration that 3kg experiences is the same that the 5kg experiences. However, the DIRECTION they experience the acceleration is different. The reason he says the Acceleration of the 5kg is equal the negative acceleration of the 3kg is because if it was the other way around (no negative) then when you solved your equation you would find that the 3kg mass is experiencing a positive acceleration. As Humans we know which direction its experiencing the acceleration (down) so we it doesn't matter if its positive or negative. but based on the math it would seem that the 3kg math is moving up. (since it has a positive acceleration) Its a way of keeping consistency. In your example where the 3kg is on the left side, then the equation would indeed be: -5kg = 3kg.
Friction is always acting on the block. The question will tell you whether you should ignore friction or take it into consideration. This question just ignores it to make the question simpler.
In real life, it depends on the surface it's on. But if you're given a question, the question would usually give you the friction force. If not, it's most likely frictionless
Sorry to say this, but you can do a better job in explaining things in a simple way. Great teachers makes difficult things simple. Teachers who have to demonstrate their potential makes easy things hard. Dislike this presentation completely.
best explanation on the internet. Thank you
This just shifted my whole perspective, thank you!
Hi David/Khan Academy,
The acceleration of the Centre of Mass (CM) of the system in NOT DOWNWARDS but at an angle to the vertical, the horizontal component being provided by the acceleration of 5 kg mass in the horizontal direction.
If we take the system to be combined 5kg and 3kg, then the external forces acting on the system are not only the forces of gravity acting on the masses , the normal reaction on the 5kg mass (which is nullified by weight of 5kg) but also the NORMAL REACTION at pulley, point outwards (towards top right corner of screen) at an angle of 45 degrees with horizontal (for this specific problem), which is not at all mentioned. It is this force that provides the horizontal component of the system's
acceleration.
It is the vector sum of these TWO forces that drives the system (and not the single force mg) and the acceleration vector of the CM of the system, is the vector sum of this Reaction (at pulley) and mg (3kg mass) divided by the total mass of the system.
In this specific problem, since the strings forms an angle of 90 degree at the pulley, the horizontal and vertical components of the Reaction force are equal in this case ( and makes an angle of 45 degree with horizontal)
QUESTION: Why is the external Normal Reaction at pulley, that provides the horizontal component of the acceleration of the system, NOT at all considered ?
QUESTION: Why is "system acceleration" and vector sum of External Forces assumed to be in vertical (downward) direction, when it is not so ?
Excellent....!!! I'd say the quick response to your questions is that "it gives you the right answer." However, YOU are correct on the physics of this problem that there is a contact force acting at the pulley on the string (part of the system) and that the acceleration of the system is down and to the right.
God bless
Physics is becoming too easy with you guys))
Thanks.
“The Organic Chemistry Tutor” can stop now, Khan will take over from here.
Ahh I finally understand. Thank
I don't get why the acceleration of the 3 kg mass must be negative. Does that mean that if the 3 kg mass and pulley were on the left, the acceleration would somehow be different? I meant about the hard way
it shouldn't be negative since the 5 kg mass holds it so it won't go downwards.
It's saying that down/left is negative and up/right is positive. It's just a frame of reference. If the 3kg mass was moved to the left side, the 3kg mass would still be moving downward, so its acceleration would still be negative. But the 5kg mass would now be moving to the left, so its acceleration would now be negative.
Just think of the signs as directions.
Think about it like this: It doesn't matter if the acceleration is negative or positive (it does, but for a min ignore it), we just know that the acceleration that 3kg experiences is the same that the 5kg experiences. However, the DIRECTION they experience the acceleration is different. The reason he says the Acceleration of the 5kg is equal the negative acceleration of the 3kg is because if it was the other way around (no negative) then when you solved your equation you would find that the 3kg mass is experiencing a positive acceleration. As Humans we know which direction its experiencing the acceleration (down) so we it doesn't matter if its positive or negative. but based on the math it would seem that the 3kg math is moving up. (since it has a positive acceleration) Its a way of keeping consistency. In your example where the 3kg is on the left side, then the equation would indeed be: -5kg = 3kg.
Omg! Thank you!!! 🙏🙏🙏🙏🙏
Thanks for this!
is it tension or the weight of the object accelerating the system.please explain
The weight is the answer the tension is just opposite to the acceleration
So how do you determinate whether the block is frictionless or not? I mean when does friction acts on a block?
Friction is always acting on the block. The question will tell you whether you should ignore friction or take it into consideration. This question just ignores it to make the question simpler.
In real life, it depends on the surface it's on. But if you're given a question, the question would usually give you the friction force. If not, it's most likely frictionless
respect
amazing concept!
What if there are multiple pulleys?
if you dont translate to spanish. How to understand the upget of Khan Academy.
Why is u taken 0.3 ?
Rinisha Pillai random
Sorry to say this, but you can do a better job in explaining things in a simple way. Great teachers makes difficult things simple. Teachers who have to demonstrate their potential makes easy things hard. Dislike this presentation completely.
Who cares about ur opinion we loved it