A review of pulleys, mechanical advantage, an inclined surface and gears. This is not part of the physics syllabus for many A level boards, but may be included in some Applied Maths courses.
With my poore English i understand much more theory now than the time i was to the school!!!!!!!!!! The only good i remeber is this: "That is saves in power lose in distance". I like too mutch your lesson. Thanks.
Hello, DrPhysicsA. First of all thank you for your wonderful video. Would you be so kind as to help me understand the one issue I continue to have? I believe it is the least intuitive facet of pulleys. Essentially I cannot seem to reason displacement. For example around minute 19:00 you have a pulley system with a mechanical advantage of 3. Yet to me it seems as if for value x of rope you drag out, you would get value x/2 of displacement. Clearly this would violate conservation of energy, and therefore, clearly the displacement would be x/3, but I simply cannot reason it out. Please help.
What happens when your red cables / ropes have weights, (especially in a pulley system where there is more than one cable / rope)? Will the tension in them still cancel out?
at 18:03, shouldn't the tension from pulling the rope upwards be downwards instead of upwards? and also why did you consider the tension from the force applied upwards when you didn't do that in any other examples.
bec the force was pulling from downwards position, while the example ur're pointing at the force was pulling upward so it was bearing the weight also that's why he divided it by 3 instead of 2
Hello Sir I was wondering if you forgot to mention the catch in the last example, where even though you are pedalling the wheel 4x as fast you will still have to apply 4x the force needed to move the wheel. For instance cycling on gear 6 is much harder than cycling on gear 1
In the example at around 18:10, why is the tension in the rope that you would be pulling on directed the same way as the force you would be pulling with? Why isn't the vector for tension directed towards the pulley attached to the mass instead, opposing the direction of the force? Thanks in advance :)
There isn't an explanation for this, because he made a mistake (it can happen to anyone). I tested my theory in all other examples and it checked up in every example except the one at 19:00. The thing is, if that tension is directed in the same direction as the force is, then the system won't be at equilibrium.
its a little late but nonetheless, imagine you're holding a rope which has its one end attached to a weight and then you suddenly cuy, the rope jumps back towards you a little, suggesting that there is a force in the string towards you. On the other hand when you cut the rope, and the weight fell through, there was no such consequence in that part of the rope. Therefore, there is always a force towards you. P.S. This is only my insight to this.
sir can you put some examples of the six simple machines.. I mean questions.. please 😭 and thanks for the pulleys video really helped, but I need a variation of questions.
hello, as doubted several times by several comments now, the two wheel pulley system should have the same mechanical advantage of 2 whether being pulled up or down, because in both systems one would get the same rope displacement for the same force being applied. otherwise the conservation of energy would be violated.(video 19min). the following presentation of the three wheel pulley system with the mechanical advantage of 3 corroborates this view.
Equilibrium is essentially the situation where no net force is acting on the system. There isn't a generalized 'equation' to depict the equilibrium condition, but you can create one for every system. All you have to do is to balance ALL the forces acting on the system, resulting in no net force. Simply, forces with equal magnitude but having exactly opposite direction cancel out each other and so the resultant is zero.
Question: The one at 29:00 where the Force is a fifth Mg, that will cause a moment wont it? as the Net force acting upwards is not acting through the CoM... Hence that arrangement is not practical if you need to keep the mass level... or am i wrong?
Dylan Vignola Yes, I thought the same thing. If the two pulleys on the block were placed differently with one in the center, and the other symmetric to the point where the rope is fixed, then there would be no moment. However, this problem treats the block as a point mass, even though it isn't. If you consider it to be a point mass then there would be no moment. In the real world this block would tip, with the right side lifting higher than the left.
As I explain, the net tension in the rope must be zero. The tension in both sides of the rope must be upwards because that is where the force is exerted to overcome the gravitational force which would otherwise cause the load to fall to the ground.
I am pretty sure he made a mistake in the example at 19:00 the tension at the most right cannot me directed in the same direction as the force is, which means the required force has to be bigger than mg/2 in both cases, not mg/2 in one case and mg/3 in the other case. I tested my theory in all the other examples and my theory worked in all other examples except the one at 19:00, therefore I believe he made a mistake.
Marcelo Silva Imagine the rope at the right is not inclined and goes up vertically to the third floor then the rope goes over(through) another pulley(it’s established on the roof just for the sake of changing direction of the rope to the right as in the first example 9:54 ) ;now ,grab the rope; go to the right three meters; you’ll observe that it has shortened one meter at the below system
Hello DrPhysicsA on the system @23:19 why do you only include 3T are there not a total of 4T. Stated prior to the problem that you were improving the pulley system however, this was simply just a replication of the first. should the EQ be mg=4t thus =T=mg/4. Stated that to lift a mass it will require a quarter of the force however four times the distance in the downwards direction. to lift a pulley system 1meter
If you were to push the mass up by 1 m, then each of the three ropes surrounding the lower pulley would slacken by 1 m. You would therefore have to pull the rope a total of 3 m to take up all this slack.
Don't think this explain my question however I think my question is answered because tension and force will be in the same direction if the end rope is pulled upward
I am not finishing watching the whole video yet, but there is a question coming up in my mind. Should I conclude that if I want to use less force to lift up a heavy object, then the more pullies the better? And the force that exert upward to pull up the object is even better to save more force.
I am a bit confused around the 34th minute.. Why is the force acting down the slope due to the mass mgsin(alpha)? Isn't sin(alpha) = opposite/hypotenuse? We know mg is the opposite so why do you take it here to be the hypotenuse? Help!!
Hi DrPhysics... I'm afraid you've made a conceptual mistake when you were explaining the reasons why the net tension is zero.. The truth is that, what you called "net tension zero" only happen if the rope element's mass is negligible (it's easy to demonstrate by applying Newton's 2th Law of Movement)... if not, there will be a "net tension", necessary to accelerate that element. I suggest make the demonstration beginning from 2 different tensions, T and T', apply 2th Law of movement to the element of rope of negligible mass and so get T = T' . Elasticity or Rigidity has nothing to do with transmission of tension of one extreme of the body to another. Regards from Venezuela! I've learnt a lot with your work... Thanks!
Thanks to your kind remarks. I think I mentioned, or at least I hope I did, that for the purposes of this video we have to regard the rope as massless.
