Forces 13 • Boxes on top of Boxes in Lifts • Mech1 Ex10E • 🚀

[@jessicatan4830]

Thank you soooo much! I was really confused about this kind of question and this video really saved me!

[@lizavoloboeva3500]

Thank you! So helpful!

[@poissonerie6975]

07:19 why is the weight of B of 0.6g not acting upon A? I thought A would experience an equal and opposite force from B as well, hence the resultant force would be R-0.4g-0.6g?

[@BicenMaths]

But an equal and opposite force isn't from the weight of B, it is from the reaction forces instead! It's a really tricky concept, but we need to think of reactions rather than weights.

[@piecectrld3495]

how do you know where the tension is acting? i understand everything else but i cant seem to get my head around this one!!

[@BicenMaths]

It's best to try and imagine that you actually are the particle/thing you are considering the forces on - think about what each of those different parts would experience. You would only experience the tension if it is directly connected to you.

[@exquisitepuma7318]

For part a of the question, why did you only consider the weight of the boxes and not the reaction force of B pushing down of the lift?

[@BicenMaths]

It’s almost as if we pretend that the boxes and the lift have merged to become one single particle - so just consider the overall weight and the tension. If we considered just the lift, we would need to take B’s reaction pushing down into account!

[@exquisitepuma7318]

@@BicenMaths ok that makes sense, thank you

[@shamirjilani1496]

Why do we not include the force exerted by the cable on the scale pan in part A or would this just cancel out with tension

[@BicenMaths]

The force exerted by the cable on the scale pan is actually the tension!

[@shamirjilani1496]

1 more question, if we decided to just look at the scale pan and not the whole system why is it the weight of block A and block B are not included but the pushing force of B on the scale Pan is?

[@rev0cs]

for part b of the question, wouldnt the force exerted onto B by A just be the weight? why do we need to change it around

[@BicenMaths]

No it wouldn't - this is all to do with Newton's law about equal and opposite reactions. Your instinct tells you its the weight, but it's actually the reaction of them in contact with each other. This is why this topic is so tricky!

[@rev0cs]

@@BicenMaths ah i see

[@heyitsadelinaaa]

how likely is it that we get a question like this in our paper 3 tomorrow? i've done all a level past papers and never seen questions to do with lifts/boxes

[@liokalebic]

I'm praying it doesn't come up

[@BicenMaths]

Unlikely if we use the past papers as a guide - but we know that there are always unusual questions that appear, so this is a potential... it depends if you want to go purely from past experience or not!

[@heyitsadelinaaa]

@@liokalebic same literally what is this😭 my teacher never covered lifts

[@heyitsadelinaaa]

@@hhh11.you mean my grade in the exam?

[@hashimabbas6731]

🎉​@@BicenMaths

[@zoey3605]

hi sir, how likely do you think it is that lifts will come up on the stats and mechanics a level paper this thursday? i understand pretty much everything in mechanics, but this is the one thing i can never wrap my head around😭

[@BicenMaths]

Definitely a chance of it coming up, but I don't believe it has come up before - can't comment much more on this, as we'll only truly know when we see the paper! Good luck!

[@ariii1898]

Hi sir, I don’t get it why the force 4.12 is going downward in part c.

[@BicenMaths]

Because B feels A pushing down on it - we might think that it feels A's weight, but it actually feels the equal and opposite of the reaction force on A, which is why it is the 4.12 from earlier in the question! Really weird to think about, this is a challenging area of mechanics!

[@bharadwajeddanapudi8507]

Sir at 10:56, for part c, why did you use the mass of only B on the RHS of the equation rather than the combined mass of B and A since they both act on the scale pan?

[@BicenMaths]

Because the force diagram I’ve drawn is actually only considering box B, hence we only consider B’s mass in the calculation!

[@bharadwajeddanapudi8507]

@@BicenMaths oh I see. Thanks sir

[@olindaboulding7136]

If the scale pan has a weight, in part c, should we add it? I know that it says that the scalepan’ weight is light, meaning its weight is ineligible

[@edgymomof8313]

Hey Mr Bicen At 8:00, shouldn't the force we've calculated be -4.12N since it's opposite in direction?

[@BicenMaths]

But for forces we give the magnitude - I think at 8:00 though R is in the positive direction still though?

[@loldalegend]

for part c i included both the weights combined - 1g, use f=ma and still got the same answer. why cant we use this method as you said it wouldnt be putting down 0.6kg and 0.4kg

[@BicenMaths]

You can do that for c - but it's a bit more complicated for a and b!

[@JJ-fn4jj]

Is there a reason you didn't round the answers of the final question to 2sf? thankyou

[@BicenMaths]

No, but I should have done!

[@silvano6374]

Can there be a question where the lift has mass?

[@BicenMaths]

Yes - did one in my livestream on Mechanics yesterday evening!

[@jammydodger1277]

can you also use newtons 3rd law to say "the force exerted on mass B by the scale pan" is the same as "the force exerted on the scale pan by mass B"? (I know u dont need to use this to solve any of part A/B/C but just wondering anyways)

[@BicenMaths]

Yes absolutely! If the Q asks for something like you wrote, you are always allowed to switch them round and it will be the same magnitude of force, just in the opposite direction.

[@malanhemal6574]

sir, why can't 0.4g act in the direction of acceleration on A and also being the weight? (Newton's 3rd Law)

[@BicenMaths]

I’m not sure about what you mean, but with these diagrams we are only considering one particle at a time. B only experiences the reaction of A, not just the weight of A. The reaction is to do with the weight but it’s not the same.

[@malanhemal6574]

@@BicenMaths Oh ok sir. So you could reaction acting download will be a force with weight + extra force. Correct sir?

[@BicenMaths]

Yes but it could also be the weight minus a force. When you’re in a lift and accelerating upwards you feel heaving. This isn’t because your weight has increased, but because the normal reaction from the floor of the lift has increased to have you accelerate. It’s this idea that the video is talking about!

[@malanhemal6574]

@@BicenMaths oh thank you sir

[@qamarhareth9986]

why haven't we taken B only instead of A and calculate force directly , why did we use A first

[@BicenMaths]

I’m not sure I quite understand your question - can you rephrase it?

[@samarthbhatnagar7982]

Sir for part (b) you used newtons 3rd law for calculating force exerted on B due to A by flipping the statement around. Then how would you solve it without flipping the statement around. when I tried to do it I got 10.1 N

[@BicenMaths]

You need to flip the statement round for this to work, actually!

[@BradleyPeacock-or8lr]

Challenge question 2 from review exercise 2 is also good practice

[@fatmabatmaz2]

Had to revise this for y2 connected particles

[@FarhanRahman-i7j]

Hi sir does weight always have to be kg

[@BicenMaths]

Yes the standard unit for mass is kg. (Careful, the units for weight is Newtons, not kg! Annoying as in every day life we say weight when we really mean mass...!)