Forces and Motion: A Common Misconception - Testing Physics
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- Опубликовано: 16 июл 2024
- We put a common misconception, about what the motion of an object tells us about the forces acting on that object, to the test. As a bonus, we find that the gravitational force acting on an object has nothing to do with whether that object is moving up or down.
Full Series Playlist: • Testing Physics
0:00 Introduction - The Scenario
2:00 Review of Newton's 2nd law
3:49 Direction of Acceleration
7:47 Making the Prediction
12:07 Experimental Setup
16:24 First Experimental Trial (Lifting Object)
20:10 More Trials
28:56 Results - Correcting the Misconception
31:45 Changing the Scenario (a New Prediction)
34:41 First Experimental Trial (Lowering Object)
36:52 More Lowering Trials
43:26 Overall Conclusions
45:07 Btw, Gravitational Force Doesn't Depend on Velocity
Opening Image Credit: NASA, ESA, CSA, Janice Lee (NSF's NOIRLab) webbtelescope.org/contents/me...
![What is a Force & Types of Forces in Physics? - Gravity, Normal Force, Contact Forces - [1-5-1]](http://i.ytimg.com/vi/HPfmSLDtP3c/mqdefault.jpg)
Great experiment Dr Koop, my observations are as follows.
1. The initial acceleration is a good demonstration of inertia.
2. The coasting phase with no acceleration proves Newton's first Law.
3. The deceleration data proves Newton's second law.
4. The entire experiment when all forces are analysed proves Newton's third law.
I'd clarify a couple points to not oversell what we can conclude from this experiment:
I demonstrated the part of Newton's 2nd law that claims the net force is in the same direction as acceleration, but since I didn't measure the specific accelerations that resulted from the net force, it wasn't a full demo. I have a previous video that goes through that part in more detail: ruclips.net/video/TMlMrpztGGI/видео.html
As for Newton's 3rd law, since I'm only measuring the forces acting on the hanging mass, I can't use that data alone to demonstrate the 3rd law. Newton's 3rd law always involves two forces _acting on different objects_ (the action and reaction forces). If I had a second force sensor rigidly connected to the hanging mass and use that to hook onto the other sensor (so the two sensors are pulling on each other), those two forces would always match (whether accelerating or not) and that would demonstrate the 3rd law. If you haven't seen it (and are interested) the video I made on the buoyancy force and (IMHO) an interesting example of Newton's 3rd law: ruclips.net/video/e0iGRmSwAf8/видео.html
Sorry for the self promotion of other videos.
The video was informative as I was learning about tension in rope and Newton's laws
Glad it's helpful! As I mention near the end, this style of question (comparing the strengths of forces if the object is moving in a certain way or vice versa) shows up in a lot of intro physics courses. Might be a different kind of case (I mention a jet where the engines apply a forward force and air resistance pushes backwards, or could be pushing a crate across the ground where you push forwards and friction pushes backwards, or a skydiver falling with gravity pulling down and air resistance pushing up, etc.) but we can approach all these cases with very similar methods.
@@PhysicistMichael many time when I use the formula to get the answer I get it but feel unsatisfied due to the lack of understanding of the concept and demonstration- the way science is taught these days is understable as not all the experiment can be performed or recreated in the classroom for the students. leading to science been taught from book and student not understanding it but just memorizing it- even if u show the experiment and not data for me that is more than enough but here you literally compared it like science practical book that is amazing
Thankyou Michael, if I could set up a weight on a string lifted at a constant speed say by an electric motor the string tension force recorded on the force scale would stay the same, that would be a nice demonstration.
That would likely be an improvement (and I encourage anyone interested to replicate these experiments, especially if they can improve on the methods). I would warn that you'd probably need to do some fine tuned experimenting with the motor since the needed output torque varies during the initial acceleration and coasting phases. The other difficulty I noticed with this (and in similar experiments), is that during the changes in motion (accelerating to coasting and back), I was getting some vibrations in the string which seemed to cause some of the fluctuations in the force data. Since I had long enough coasting phases to highlight averages over multiple fluctuations when measuring the tension, that wasn't a major problem, but those same fluctuations could be a problem getting the motor to pull at a constant speed during the coasting. Just some thoughts if you do end up trying this.
Thank you Michael,
Nice clear explanation and demonstration.
Would I be correct in saying that your experiment also demonstrated Newtons 1st Law.😊👍
In part, yes, since Newton's 1st Law is a special case of Newton's 2nd Law.
@@PhysicistMichael Thank you for the reply Michael, never heard it said in that way, so thank you for giving something to look into and better my understanding.
Very appreciated.👍😊
@@PhysicistMichael Not to split hairs to atomic levels, but doesn't Newton's 1st law talk about an object with "no forces" acting upon it rather than "zero net force"?
Just to clarify, that's not all that Newton's 1st law does. It also (indirectly) sets up the concept of an inertial reference frame, which is the condition that an observer needs to be in for Newton's laws to work.
@mindlessmarbles9290 to split hairs at subatomic level, "no force" is still a special case of "zero net force"
@@PhysicistMichael "no force" is a special case of "zero net force" but the pulley example was a situation where forces canceled out to zero net force.
Anyways, I was reminded of this article: Check out ScientificAmerican "Mistranslation of newtons first law discovered after nearly 300 years."
(Apologies if I'm being annoying.)