Dynamics: Lesson 22 - Intro to Work in Dynamic Systems

[@zakkustowski6971]

I honestly cant believe that you started posting Dynamics again right when I needed it the most. Thanks for your hard work!

[@suhibghanem252]

I'm really grateful for this series of dynamics hoping u continue posting the rest of the series

[@georgesadler7830]

Professor Hanson, this is an excellent introduction to Work in Dynamics/ Dynamic Systems. This dynamics problem was easy to follow from start to finish.

[@mattrodriguez2750]

Happy to see you're back!

[@maryb7166]

So glad your back!! Love these videos so much

[@gosiajagusiak6414]

I'm a little confused about the signs of work and friction. I thought that if the force was in the object was moving in the direction of the force , the weight should be positive. Here the box is sliding down and the weight is acting down, why is it negative then? Same when it comes to friction, here the friction opposes the direction of the movement. Why isn't negative then? Can someone explain, please?

[@montesaraphaelt.23]

interesting one! it all goes to cartesian plane. weight has effect on crate that it goes down meaning (-y) direction. friction pushed the crate on the +x direction. just look at the fbd of the crate

[@gosiajagusiak6414]

@@montesaraphaelt.23 Yes, but isn't work just Force x displacement? I get why the force would be negative. I'm confused why work is negative if it's pointing at the same direction as the displacement?

[@alihajri7207]

You could do it different way than he did as he mentioned that y is tilted also x is tilted +x up hell where the movement occurs so Newton Second Law is applicable then 0.35N-50(9.81)sin35=50(-a) when you do the calculation you will have a=2.81 so F=ma=50(2.81) multiplied by distance 2.3 it will be positive answer check it out regards,

[@JiyouXu]

oh god this topic is so hard, thank you for making these videos!!

[@justinbeck560]

We want more videos. MORE!!!

[@christopherchiaro2328]

im also confused about work, i always thought work was positive when the force vector is in same direction as the displacement....so confused!!! should i forget that though?? should i just ask myself what the change in potential energy is when determining if force is negative or positive?

[@waseemsajjad2036]

So did you get the point? I think professor did mistake because work against direction of displacement is always negative

[@keepmahaney7412]

@@waseemsajjad2036 yeah that’s what I thought as well

[@SSTOKES]

I'm confused too...

[@carsonleischner5884]

yah I believe friction should be negative and weight in the x direction is positive. That will give the same answer but positive joules.

[@NexiOHome]

i think its because the work is being done by the box, aka losing potenital energy. Then you have the work aka friction force being done on the box, anything doing work on something, that "something" in this case, the box will have a positive work being done on the box. Also, the friction forcw wants to go in the postive direction, if there was no gravity making it go down (which goes against laws of physics the friction force would throw it up the hill. causing a rise in potenital energy...idk

[@gosiajagusiak6414]

Thank you so much for making those videos.

[@nolabean]

Can somebody tell me which of the videos in this dynamics playlist are kinetics and which are kinematics?

[@tayfunerdogan8644]

Sir finally i graduaded my university thanks to you

[@1234jhanson]

Woohoo! Congrats!

[@osmansari8088]

@@1234jhanson It's our turn

[@Mackyanguson]

Thanks so much for these videos....❤✔

[@alexorr4270]

I haven’t taken this class but can someone explain to me why he didn’t annotate gravity in his free body diagram?

[@alexorr4270]

I’m an idiot, never mind

[@montesaraphaelt.23]

because thats part of the weight=mass x gravity. dont call ur self idot, alex.

[@wekajakdeng4096]

The first to like

[@lukecaetano8405]

3rd

[@erfanmohagheghian707]

Oh man I can't believe you did this wrong. The work done by gravity is positive and the work done by friction is negative, since the angle between gravity and displacement is less than 90 (55 degrees) with + cos and the angle between friction and displacement is 180. The crate gains kinetic energy as it moves down, so the total work which is the change in kinetic energy is positive.

[@TheTastyHeisenburger]

I thought so too. You are correct that the work is positive in the direction of the force and gravity is the force causing the box to move. However he chose a negative sign to account for the height change I believe. He set up his coordinates so that Z is positive pointing down, but logically the box is moving in the negative z direction so z = -1.32m. A more accurate equation would have been W = (50kg)(9.81m/s^2)(-1.32m). I wish he had been more explicit about it but regardless he is still correct. (edit: I realized he has y for the vertical direction but the point remains)

[@erfanmohagheghian707]

@@TheTastyHeisenburger Jesus! Work is scalar. It does not depend which direction you pick as positive. Total or net work or the work from all the forces is equal to the change in kinetic energy. If the object is speeding up, the net work is positive. Period. He did it wrong.

[@alihajri7207]

He forgot the Weight component on X axis [0.35 N-W sin35=50 (-a)] ; (-a) since it is sliding downwards opposite to +x. Check it yourself regards,

[@alihajri7207]

Then F=ma