The introduction of cosine and its values influencing the amount of work really blew my mind for some reason. Being able to quantify things like that really makes me appreciate this crazy universe. These videos are immensely valuable and you are one of the reasons I have chosen to pursue an education in physics
You know you never really use too many examples or if you do they are simple, but honestly that should be the way they initial teach. It really helps me understand where things come from before practicing the actual examples later.
thanks very much for the explanations i am having a test tomorrow about work and energy and i feel much clever after i watched this video and i like the way you explain work and energy i cant wait to see more of your videos about KE and PE thanks for the video now even my sister carly can understand
I felt bamboozled knowing that this is the first video that I’ve seen that did not include comprehension in the end. But nonetheless you’re explanation are always superb thank you very much!
im so happy i found a youtuber that can help me with bioligy,chemistry and physicis this means so much to me.. u have no idea how ur helping me with my finals :)
Okay. So here is a very fundamental doubt. We know that work = energy. (It definitely is. Because, work=fd= 1/2mv(v)= energy And energy is the ability to do work..(1) That implies Work=ability to do work (using (1))?😮😮 How can two things which are exactly equal can be different from each other?
Thank you so much Sir!!! I learn a lot from you!!! You're a blessing to students just like me who can't easily catch up with what their professors where teaching! Again, Thank you so much!! ❤
Theeta is a greek letter, just like we use 'x' and 'y' to mark angles. Cos theeta means the effect of that force in the horizontal direction. For example if you hit a football in north-east direction, actually it had travelled in two perpendicular directions, north and east...the force which contributed for the force only to travel to east means Fcos(theeta)..its something like that..... And work energy theorem means that, the more energy u have, the more u can transfer it and do work...just like if u have more money, you can spend to buy more things and vice versa.. in physics, that spending to buy part should be replaced by 'making an object move'(do work)
In physics, "work" is when a force applied to an object moves the object in the same direction as the force. If someone pushes against a wall, no work is done on the wall because it does not move. However, depressing a letter on a computer keyboard requires work.
Thank you Mr. Dave, this is such a great summary; simply packed (?) and brief. I would like to share this on physics class. Please do continue doing this sort of video, you really did help me with many of my assignments at school, I often come back watching your videos if I need help on what kind of phrasing should I use to be more understandable. Also, the jokes. It's great.
This makes me think of the old expression every young man is scolded/initiated with into the world of manual labor: *”Let the tool do the work!”* - so.. the reason why we quickly came to realize just how wise this initially irritating/inane advice really was, is because according to the laws of physics work-energy theorem: We only do the work of changing the kinetic energy of the tool, but only the tool’s energy of motion can now do “the work”.. so ya, let the tool do the work!
The second law of thermodynamics (part of it) is that all physical processes create entropy and entropy the amount of energy not available to do work. It, along with the first law is the reason you can't create a perpetual motion machine, all the energy will "turn useless" which is the second part, that all the energy will become entropic in a closed system. Still trying to figure out entropy so that's why I bring this up.
So, if take the perpetual motion machine example, say a machine that (well, it doesn't succeed obviously) that is a closed system and tries to recycle the energy into itself, stops working, how? It's able to do it, initially, but then eventually it can't, what is going on? I'm trying to think of an example of something we could try to make into a perpetual motion machine that would obviously fail to do so, so I can try to figure it out. I'd say water that falls turning a wheel, in which the wheel pumps the water back up to fall on itself to turn it, but that's not exactly closed, because what starts the wheel turning in the first place?
A perpetual motion machine could exist in theory, if you were able to eliminate all losses, such as friction, viscosity, drag, and ohmic resistance. But the minute you try to extract any energy out of it for any application at all, it will no longer be in perpetual motion. You will slow down the motion, and deplete its energy. What people really mean by "a perpetual motion machine", is really a more like a perpetual WORK machine. A machine that can continuously deliver a work output, without any work input. It doesn't exist in reality. A planet in orbit around a star is one example of what would technically be a perpetual motion machine by the direct meaning of the words. But if you were to extract work out of the orbital speed of the planet to power your own machine, the planet will not keep orbiting at the same position. Its orbital energy will decrease, and its orbital radius (or more generally, semimajor axis) will also decrease. Continue extracting energy, and it will end up on a crash trajectory with the star.
You will notice that when theta equals zero, cosine of theta reduces to 1. Thus you will get the same answer as if you had calculated W=F*d*cos(theta), or W=F*d itself. The full generalized form of the work definition equation is: W = integral F dot dx where F is a vector at every point along the way, and dx the various infinitesimal displacement vectors that add up to the full path length. This reduces to a single dot product of force dot displacement (W = F dot d) when the force is constant at every point along the way, and the displacement is a simple straight line. The dot product is a special kind of multiplication that multiplies the aligned components of two vectors, hence the cosine term that develops. Since d already has another full time job in Calculus, it is common to use another letter to indicate displacement, such as x or s.
The definition of work in Physics as what happens when an applied force causes a displacement of an object is completely erroneous or wrong and work ,hence, is not really force times displacement(F•d). Work is a vector quantity(i.e. it's not a scalar) and ,in general, is defined as a "physical-displacement(ms)" by or due to a force, an impulse, or a kinetic force. The work-energy theorem stating that the net work done on an object equals the change in the kinetic energy of the object is completely erroneous or wrong too. It would be the net force on an object that is equal to the change in the kinetic energy of the object.
This lesson confused me. Work is equal to (magnitude of force) * (D) is it distance or displacement (ik their differences but you interchange them at 1:08 ) and you said work is scalar if the "D" in W=(F)(D) is displacement (thats what you and my professor said) then why is work a scalar quantity and not a vector quantity Especially when you apply work on an object by an angle in which you must use a cos(x)
Work is the product between magnitude of force and magnitude of displacement. More technically, it's the dot product between force and displacement. So obviously work is a scalar quantity! Hope it helps
It is displacement. That is why the cosine(theta) develops in the equation. You are taking a dot product of force and displacement, as opposed to just a scalar multiplication of distance and force. When the vectors are aligned, you get away with ignoring the fact that it is really displacement, and just multiplying magnitudes. But when vectors are not aligned, that's when you need to consider the cosine term to account for direction. Or you can use the "multiply corresponding components and add up the products" strategy of taking a dot product.
Not necessarily. That is true if the NET work done on the object is negative, but not just the work in general. Consider the example with you and your baby brother, both pushing a box. You push forward on it, and your baby brother being a little brat, pushes backwards on the box. Your baby brother is doing negative work on the box, but you are doing positive work on the box. It is possible that the both of you increase the speed of the box as a net result, because you do a lot more work than your baby brother. The net work is positive, even if your baby brother's contribution is negative and counterproductive to your effort.
If we push an object in space and that object keeps accelerating assume it gets infinite velocity at infinite displacement..we get work as F*infinity....which will be very large offcourse (infinite ) here kinetic energy Is also increasing and work is also being done....how is it possible that work is done and energy is gained !
Mr.Dave,in a previous video, you explained that scalers don't have negative magnitude, could you clarify how work has a positive and negative magnitude although it is considered as a scaler, plz?
Some scalars can be negative. Some make no sense for them to be negative. Some examples of scalars that can be negative: Work Potential energy Voltage Current Power Divergence Money (e.g. debt) Electrical charge Outputs of trigonometry functions Some examples of scalars that make no sense to have negative values: Speed Distance Length Volume Mass Kinetic Energy Coefficient of restitution Coefficient of friction Temperature in the absolute scales of Kelvin and Rankine
If you lift the box, but not strong enough to lift it anywhere, the work will be zero. You apply a force, but that force is unsuccessful in doing anything. If you lift the box and it moves upward, there will be a distance that is directly upward. The angle isn't 90 degrees, but rather it is zero, and the cosine is 1. This isn't the angle from the horizontal, but rather the angle between the force and the direction it moves.
Since the letter d already has another full time job in Calculus, it is common to replace d with s, as the symbol to represent distance and displacement. In an introductory physics class before they expect you to even think about Calculus, it is common to represent it with d. In a Calculus-based physics class, or in a class that prepares you for Calculus-based physics, they assign s as the variable of displacement. You might even see x and y as the variables for displacement to specify direction, or possibly even r, reflecting the fact that it is a radius vector from an origin. Again, it is all an arbitrary convention of what letters we assign, so none is necessarily any more correct than any of the others. What ultimately matters is the substance of the equation, rather than the choice of letters as variables.
I have a question professor Dave if you push the box at 90° and the box moves downward then the applied force is now parallel to the displacement of the object, I think there is work done given the definition of work
In this example, the box doesn't move downward, because it is constrained by the normal force from the floor. Assume we are talking about a rigid box and a rigid floor, rather than a compressible cardboard box.
Actually theta is the angle between force and displacement.In your situation the angle between force and displacement is still cos 0 as they are parallel to each other.remember that theta is not only the angle of force it is the angle between force and displacement
Do you have videos for conservation of momentum, radial acceleration, potential and kinetic energy equation examples? Please it would help a lot with physics
Scalars can be negative. Some scalars make no sense as negative numbers, others are used as negative numbers all the time. Some scalars make no sense as negative numbers, like speed, mass and volume. There are also scalars that make sense when using in relative scales, like temperature, pressure, and elevation, but ultimately are positive in absolute scales. Additionally, some scalars are defined in with an arbitrary zero point is, and thus can be either positive or negative. Potential energy, and related concepts like voltage, are examples. Beyond that, there are scalars that have a nature to make sense in both signs, such as electrical charge. Work is one such example that makes sense as a negative number, because it simply means that the agent of the force receives the energy, instead of providing the energy.
When we apply force to an object it'll cover infite distance (1st law of motion) but it actually nor happens due to friction so technically friction opposed the force we applied so how does it related to energy ? You doing the same work on rough surface and plan surface so how does you get kore tired there since you apllied the same energy
