# Simple physics question **URL:** \ **Category:** Factual Questions\ **Created:** [April 22, 2020, 5:45pm UTC](https://boards.straightdope.com/t/simple-physics-question/852363 "2020-04-22T17:45:29Z")\ **Posts on this page:** 20\ **Page:** 3
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [April 23, 2020, 2:59pm UTC](https://boards.straightdope.com/t/simple-physics-question/852363/42 "2020-04-23T14:59:02Z")
> [@CookingWithGas](#): > > If the person is pushing down, the force vector is _de_clined 25° from the horizontal. > > The physics of this problem is very straightforward but the English is lacking. The contributors here still can’t even agree on what the problem is. OP has adequately specified the problem with post 1 together with post 4: -force vector is 250 N, angled 25 degrees from horizontal -block moves 3m horizontal. Everything else is noise; you can calculate work from the above info. ---
**Author:** ![Mijin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mijin/32/9369_2.png) [@Mijin](https://boards.straightdope.com/u/Mijin)\ **Post date:** [April 23, 2020, 3:26pm UTC](https://boards.straightdope.com/t/simple-physics-question/852363/43 "2020-04-23T15:26:28Z")
> [@Snarky\_Kong](#): > > It’s not necessary. Not even a little. Put your finger on the desk in front of you and push at an angle. This is blowing your mind? > > Assume we know the angle. This is completely incomprehensible unless I draw a handle on the box? Well you’re certainly earning your name… From your description and diagram it seems you’re assuming that we’re pushing the top surface of the block, and there’s either enough friction that our hand can stick on the top enough to push it forward, or it’s a very small block such that we can wrap our fingers around the edges. I doubt that this is the framing; it’s just not the obvious way to try to move a block. Also pushing the rear face is something that will relate to actual mechanics experiments done in high school; pushing a block forward with a shear force on the top face is harder to set up for no apparent value. Pushing the rear face you can’t push the block down into the ground without sticking your hands; it would make no difference what angle you push if the surface is smooth. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [April 23, 2020, 3:29pm UTC](https://boards.straightdope.com/t/simple-physics-question/852363/44 "2020-04-23T15:29:40Z")
> [@Machine\_Elf](#): > > OP has adequately specified the problem with post 1 together with post 4: > > -force vector is 250 N, angled 25 degrees from horizontal > -block moves 3m horizontal. > > Everything else is noise; you can calculate work from the above info. The OP didn’t clarify the 2nd point until post #12. And while OP has specified the problem _as the OP interpreted it_, we don’t know if that matches the intent of the original problem. ---
**Author:** ![Snarky\_Kong](https://avatars.discourse-cdn.com/v4/letter/s/a183cd/32.png) [@Snarky\_Kong](https://boards.straightdope.com/u/Snarky_Kong)\ **Post date:** [April 23, 2020, 4:22pm UTC](https://boards.straightdope.com/t/simple-physics-question/852363/45 "2020-04-23T16:22:42Z")
> [@Mijin](#): > > Well you’re certainly earning your name… > > From your description and diagram it seems you’re assuming that we’re pushing the top surface of the block, and there’s either enough friction that our hand can stick on the top enough to push it forward, or it’s a very small block such that we can wrap our fingers around the edges. > I doubt that this is the framing; it’s just not the obvious way to try to move a block. Also pushing the rear face is something that will relate to actual mechanics experiments done in high school; pushing a block forward with a shear force on the top face is harder to set up for no apparent value. > > Pushing the rear face you can’t push the block down into the ground without sticking your hands; it would make no difference what angle you push if the surface is smooth. None of that is relevant. If I show you a free body diagram with the block represented as a rectangle and the force as an arrow, are you honestly telling me that you have no idea what to make of it unless I also describe in detail the mechanism by which the force is transmitted? I’m an engineer, I’ve taken my share of physics/mechanics courses. This wouldn’t cause me, or anyone I know, to bat an eye. Heck, I did a google image search for “physics practice problem block” and this is the first result: [Learn AP Physics - AP Physics 1 & 2 - Newton's Laws of Motion](https://www.learnapphysics.com/apphysics1and2/newtons_laws.php). ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 23, 2020, 4:40pm UTC](https://boards.straightdope.com/t/simple-physics-question/852363/46 "2020-04-23T16:40:00Z")
For the problem as written, we don’t even need to know whether the force is at an upward or downward angle. We’d need to know that to determine, for example, the acceleration of the box, but they aren’t asking for that. We have a force. We have a distance moved. We have the angle between the force and the movement. That’s all we need to determine the work, which is what we’re asked for. We also have the coefficient of friction, but that’s completely irrelevant, because we don’t need it. We don’t know the mass of the box, but we don’t need that, either. We don’t know the nature of the force we’re using to push, be it normal, frictional, some combination thereof, or some other completely different sort of force, and we don’t need that, either. ---
**Author:** ![Mijin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mijin/32/9369_2.png) [@Mijin](https://boards.straightdope.com/u/Mijin)\ **Post date:** [April 23, 2020, 6:32pm UTC](https://boards.straightdope.com/t/simple-physics-question/852363/47 "2020-04-23T18:32:21Z")
> [@Snarky\_Kong](#): > > If I show you a free body diagram with the block represented as a rectangle and the force as an arrow, are you honestly telling me that you have no idea what to make of it unless I also describe in detail the mechanism by which the force is transmitted? I haven’t said I don’t know what to make of it, you keep saying that. I just said it was an odd question. You would expect a physics homework question, particularly for beginner level, to play fair and be easy to intuitively imagine in the real world. The most obvious conception of what’s happening should just work, unless the question is intended to highlight some misconception. > [@](#): > > I did a google image search for “physics practice problem block” and this is the first result: [Learn AP Physics - AP Physics 1 & 2 - Newton's Laws of Motion](https://www.learnapphysics.com/apphysics1and2/newtons_laws.php). That’s interesting. I googled phrases like “mechanics block incline” and got either a block on an incline with a force perpendicular to the incline, or a block on an incline with the force pushing normal to g, either of which my objection doesn’t apply. But ok, I guess the problem does get phrased this way too. With the diagram I guess it’s clear enough. BTW I am also an engineer. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 23, 2020, 6:52pm UTC](https://boards.straightdope.com/t/simple-physics-question/852363/48 "2020-04-23T18:52:37Z")
> [@](#): > > You would expect a physics homework question, particularly for beginner level, to play fair and be easy to intuitively imagine in the real world. And yet you object to intuitive, easy-to-imagine real-world features like handles. ---
**Author:** ![hibernicus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hibernicus/32/2802_2.png) [@hibernicus](https://boards.straightdope.com/u/hibernicus)\ **Post date:** [April 23, 2020, 9:08pm UTC](https://boards.straightdope.com/t/simple-physics-question/852363/49 "2020-04-23T21:08:36Z")
> [@blood63](#): > > This is a question from my son’s homework, already submitted and marked. > I was a physics teacher and am curious about one of the answers. > > A person pushes a block a distance of 3.0m with a force of 250N 25 degrees from the horizontal. The coefficient of kinetic friction is 0.1. How much work does the person do? > > I see three possible answers. > > 1. 250N x 3m > b) 250Ncos25 x 3m > c) (250Ncos25 - force of friction) x 3m > > The correct answer (supposedly) is a). > I wonder why it is not b) since the vertical component of the force isn’t doing any work. > > Can anyone explain? I think you are misinterpreting the question (unless you have access to information you have not shared, such as a diagram). The block is pushed 3 metres up a slope angled 25 degrees from the horizontal. The force is 250N aligned with the direction of movement. Hence the correct answer is a). ---
**Author:** ![Mijin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mijin/32/9369_2.png) [@Mijin](https://boards.straightdope.com/u/Mijin)\ **Post date:** [April 24, 2020, 12:40am UTC](https://boards.straightdope.com/t/simple-physics-question/852363/50 "2020-04-24T00:40:03Z")
> [@Chronos](#): > > And yet you object to intuitive, easy-to-imagine real-world features like handles. Yes, if they are not specified in the problem and are necessary for the maths to work. ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [April 24, 2020, 1:12am UTC](https://boards.straightdope.com/t/simple-physics-question/852363/51 "2020-04-24T01:12:53Z")
> [@DPRK](#): > > The OP repeatedly insists the box was not pushed in the direction of the force. Any normal force will be cancelled out by the constraint of the floor supporting the box, which will exert a reaction force in the opposite direction. Leaving less than 250 N to actually slide the box. > > This is why I asked if there was an accompanying diagram, which would make things instantly clear to the reader. Repeat after me… “\*It just doesn’t matter! It just doesn’t matter! It just doesn’t matter! It just doesn’t matter! …” \*(Thanks, Bill Murray) We’re talking strict physics here. F=Ma F= dp/dt (change of momentum over change of time) If the 250N was too much, the block would take off like a shot, zooming past all those spherical chickens in the vacuum. If the 250N was too little to match the friction, the block would not move. The block moves because of the force applied. Thus, the force - 250N - is applied to the block. the block moved 3m - that’s the distance. Presumably, the force of the move is the force required, and if the block does not accelerate after the initial push it is because the forward force is balanced by the friction force counter to the direction of motion. the 250N was to force necessary to move the block against the forces of friction and the 25-degree gravity. The question is poorly written in not being explicit enough - is the force 25 degrees from the horizontal, or is the block going upward a distance on an incline 25° (or downward for each case) …or both up a ramp and pushed at the angle of the ramp? People confuse effort with what physics considers force. If you have to push really really hard on the sides of the box, to guarantee a static friction sufficient that your hands don’t slide but the box moves instead - that has nothing to do with the amount of force that goes into moving the box. If you don’t also push forward, then the box doesn’t move, you just somersault over the box (creating angular momentum for yourself). The force times the distance is the work. If we accept the alternate truth, that the force is 25° from horizontal pushing down (or up) on the box but the box moves horizontally 3m then yes, the answer is “b)” The coefficient of friction is irrelevant since the question is work the person does, not the floor, and we are told he exerts 250N of Force. The net work on the block (I assume) would be “c)” ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 24, 2020, 2:12am UTC](https://boards.straightdope.com/t/simple-physics-question/852363/52 "2020-04-24T02:12:06Z")
If the handles were necessary to make the problem work, then yes, it’d be reasonable to object to their absence of mention. But all we need is for some point on the box where the dude pushing it is making contact. Whether those points are handles or not changes the math not one whit. **hibernicus** , the OP clarified back in post #4 that the box is moving parallel to the floor. **md2000** , nowhere in the problem does it state that the box is not accelerating, nor does it state that it started at rest. That, too, is irrelevant. ---
**Author:** ![blood63](https://avatars.discourse-cdn.com/v4/letter/b/97f17d/32.png) [@blood63](https://boards.straightdope.com/u/blood63)\ **Post date:** [April 24, 2020, 2:39am UTC](https://boards.straightdope.com/t/simple-physics-question/852363/53 "2020-04-24T02:39:07Z")
OP here Wow, I thought this thread was dead. The interest in this question must be a quarantine side effect. I know my original question was fuzzy but I thought I cleared it up in a later post. Here is the question as it appears on the assignment. A 160 kg object is pushed with a force of 250N (25 degrees from horizontal) a distance of 3m. (coefficient of kinetic friction = 0.11, coefficient of static friction = 0.13) a) Find the acceleration of the object once the object begins moving. b) Assuming a uniform acceleration, determine the time it takes for the object to travel the 3m. c) Determine the work done by the person pushing. d) Determine the work done by friction. e) Determine the power of the person pushing the object. I taught this course for many years and I think we can assume the surface is flat. My interest is in part c) only and only for curiosity. I loved these types of questions. I wish my son did as well but that is a different forum. ---
**Author:** ![excavating\_for\_a\_mind](https://avatars.discourse-cdn.com/v4/letter/e/e8c25b/32.png) [@excavating\_for\_a\_mind](https://boards.straightdope.com/u/excavating_for_a_mind)\ **Post date:** [April 24, 2020, 1:36pm UTC](https://boards.straightdope.com/t/simple-physics-question/852363/54 "2020-04-24T13:36:08Z")
> [@blood63](#): > > OP here > Wow, I thought this thread was dead. > The interest in this question must be a quarantine side effect. > > I know my original question was fuzzy but I thought I cleared it up in a later post. > > Here is the question as it appears on the assignment. > > A 160 kg object is pushed with a force of 250N (25 degrees from horizontal) a distance of 3m. (coefficient of kinetic friction = 0.11, coefficient of static friction = 0.13) > a) Find the acceleration of the object once the object begins moving. > b) Assuming a uniform acceleration, determine the time it takes for the object to travel the 3m. > c) Determine the work done by the person pushing. > d) Determine the work done by friction. > e) Determine the power of the person pushing the object. > > I taught this course for many years and I think we can assume the surface is flat. > My interest is in part c) only and only for curiosity. I loved these types of questions. > I wish my son did as well but that is a different forum. Thanks for a better re-statement of the problem. There are still two issues I have an issue with, and without clarification, the problem is really indeterminate. First, what does “25 degrees from horizontal” mean? Is that 25 degrees upward from horizontal or downward? Particularly in light of the other questions in the problem, that is very important. Second, when you state “…I think we can assume the surface if flat” without a diagram or that being explicitly stated in the original problem, I am not sure that is a safe assumption. One additional piece of information given is the answer to part c) is reported to be 250N x 3m (If I am reading the OP correctly and assume 1) and a) mean the same thing.) From the reported correct answer, it is obvious that the direction of the applied force and the direction of travel are equal. If they were not, that would not be the correct answer. Furthermore, if we knew the correct answers were to the other questions, we would be better able to understand the problem. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 24, 2020, 1:48pm UTC](https://boards.straightdope.com/t/simple-physics-question/852363/55 "2020-04-24T13:48:03Z")
Was there or was there not a diagram with the problem? If there was, did the diagram or did it not show the box on a level surface? And did it show the force as angled upwards or downwards? ---
**Author:** ![blood63](https://avatars.discourse-cdn.com/v4/letter/b/97f17d/32.png) [@blood63](https://boards.straightdope.com/u/blood63)\ **Post date:** [April 24, 2020, 2:20pm UTC](https://boards.straightdope.com/t/simple-physics-question/852363/56 "2020-04-24T14:20:42Z")
> [@Chronos](#): > > Was there or was there not a diagram with the problem? > > If there was, did the diagram or did it not show the box on a level surface? And did it show the force as angled upwards or downwards? No diagram. I think my interpretation of the question is correct because it is the simplest. Flat surface. Person pushing down on the object 25° above the horizontal. The vertical component of the force should not be counted when calculating the work done. My son has a good teacher. He just made a mistake. With distance education, it is more difficult for him to ask for clarification. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 24, 2020, 2:31pm UTC](https://boards.straightdope.com/t/simple-physics-question/852363/57 "2020-04-24T14:31:30Z")
OK, so we _don’t_ know that the block is moving horizontally, then. Nor do we know that it’s not. And if it’s not moving horizontally, we don’t know if the motion is up or down the ramp. Without a diagram, it’s a poorly-stated problem. And your description to us of the problem was also poor, because you made it sound (in post 4 and others) like the problem did specify a level floor, when it didn’t, and that was just your interpretation of what was simplest (while others interpreted the ramp as being the simplest). ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [April 24, 2020, 2:41pm UTC](https://boards.straightdope.com/t/simple-physics-question/852363/58 "2020-04-24T14:41:24Z")
Also, it’s impossible to answer parts (a), (b) and (d) of the question without knowing whether the force is 25 degrees upwards from horizontal, or down. The whole question is poorly written. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [April 24, 2020, 3:15pm UTC](https://boards.straightdope.com/t/simple-physics-question/852363/59 "2020-04-24T15:15:22Z")
I am forever amazed when I see such a lack of precision from teachers in a subject that demands precision (I am talking about the OP’s son’s teacher, not the OP). I am also amazed at how badly some textbooks are written. My son has been home from college for a few weeks and asked me a statistics question. I looked at his text (an online text). It uses terms without defining them first, and shows examples that are not clearly tied to the material they are supposed to support. No wonder the kid gets confused. I have pulled out my wife’s statistics text (I didn’t keep mine) to read a much better explanation of the topic to be able to explain it to him. ---
**Author:** ![excavating\_for\_a\_mind](https://avatars.discourse-cdn.com/v4/letter/e/e8c25b/32.png) [@excavating\_for\_a\_mind](https://boards.straightdope.com/u/excavating_for_a_mind)\ **Post date:** [April 24, 2020, 6:55pm UTC](https://boards.straightdope.com/t/simple-physics-question/852363/60 "2020-04-24T18:55:08Z")
OK, just for grins, I ran some calculations and if this object was on an inclined plane, once the angle of incline exceeded 7.41 degrees, it would slide backwards. That it, the friction would not be sufficient to prevent it from sliding. At an angle of 25 degrees, a 250N force would not be sufficient to prevent it from sliding backwards. So, you could say that the assumption that the surface is flat and level is implied by the problem itself. Can the OP tell us if this is a High School or college-level course? To expect someone to solve it without knowing calculus seems to me to be rather cruel. as the student is expected to memorize and use the various formulae without understanding the relationship between them. IMO, expecting people to solve these types of problems without knowing the calculus is what makes many people hate physics. Finally, the last two questions of the problem > [@](#): > > d) Determine the work done by friction. > e) Determine the power of the person pushing the object. are also poorly worded. For d), friction, of course, does not do work. I have to wonder if the expected answer to this is “0”, or is it asking how much work is performed to overcome the effects of friction (which, of course, would be all of it). For e), there is no way of knowing the power of the person pushing. You can calculate the power needed for the person to be able to push the object and one can only say that the person must be capable or producing more power than that in order to be able to push the object 3 meters in the 4.8 seconds it would take under the given circumstances. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [April 24, 2020, 7:58pm UTC](https://boards.straightdope.com/t/simple-physics-question/852363/61 "2020-04-24T19:58:32Z")
The expanded statement does specify a coefficient of kinetic friction which will oppose the box sliding in any direction and perform (perhaps negative, depending on how you want to call it) work that way. ETA if the box is sliding 3.0m backwards down a ramp, despite the person pushing on it, we can still say work is being done, some of it by gravity… [Previous page](https://boards.straightdope.com/t/simple-physics-question/852363.md?page=2) [Next page](https://boards.straightdope.com/t/simple-physics-question/852363.md?page=4)