Tagged Questions
5
votes
2answers
64 views
Conceptually, what is negative work?
I'm having some trouble understanding the concept of negative work. For example, my book says that if I lower a box to the ground, the box does positive work on my hands and my hands do negative work ...
5
votes
2answers
164 views
First law of thermodynamics?
The first law says that the change in internal energy is equal to the work done on the system (W) minus the work done by the system (Q). However, can $Q$ be any kind of work, such as mechanical work? ...
0
votes
0answers
50 views
Work And Energy Question [closed]
$H = 3\text{ m}$,$m=2\text{ kg}$
The right side is rough.
I want to figure:
what is the coefficient of friction $\mu$?
How high and exceed the maximum return on the plane right body?
I know ...
0
votes
0answers
36 views
What is the total work done in this problem? [closed]
A 1800 kg trick airplane is 450 m in the air. At this point the plane takes a dive
with an initial speed of
42 m/s and accelerates to 64 m/s, dropping
a total distance of 120 m.
(a) Using the ground ...
0
votes
1answer
91 views
Energy needed to lift and bring down an object
A mass of 0.5 Kg needs to be moved from point A to another point (B) which is 1 meters above point A. The time for this movement should be 0.2 seconds, then the mass is kept at position B for another ...
0
votes
1answer
60 views
How to find work done due to friction [closed]
The force F=40N is applied on a 10kg block at an angle of 36 with the horizontal. The block moves a distance of 15m. If the surface is frictionless. If the coefficient of kinetic friction is 0.25, ...
1
vote
1answer
83 views
calculating work done by friction
I want to calculate the work done by friction if the length $L$ of uniform rope on the table slides off. There is friction between the cord and the table with coefficient of kinetic friction $\mu_k$.
...
2
votes
1answer
56 views
Work done by gravity on Water
Now according to me we would see change in potential energy of system and equate it to the work done by gravity.
But when we see this the first column lowers by $H/2$ and right one rises by $H/2$ ...
2
votes
1answer
147 views
How to understand the work-energy theorem?
How to understand the work-energy theorem?
I took a short lecture on physics for engineering last week. The lecturer emphasized that the work done on an object will cause the kinetic energy change as ...
0
votes
1answer
69 views
Why work to change velocity from 0 to 20 km/h is less then from 20 to 40? [duplicate]
Imagine spaceship in vacuum with mass = 1. At beginning, it has velocity 0, and kinetic energy 0.
$$W_1 = 0$$
Then, it turns on its engine, and changes velocity from 0 to 20 (delta v = 20). It's ...
1
vote
3answers
160 views
Integration by parts to derive relativistic kinetic energy
I have come across a weird integration during derivation of relativistic kinetic energy. Our professor states that i can get RHS out of LHS using integration by parts:
$$
\int\limits_0^x \! ...
0
votes
0answers
46 views
Work, energy and friction [closed]
A car loaded with bricks has a total mass of 18kg and is pulled at a constant speed by a rope. the rope is inclined 30 degrees with above the horizontal, and the cart moves 20.0 m on a horizontal ...
0
votes
0answers
27 views
How did scientists come up with Work? What was it fundamentally defined as? [duplicate]
Now before I get into the questions I want to make a couple of things clear, I know that there were similar questions like this and I've been through (what i think is) all of them and none of the ...
1
vote
2answers
143 views
Finding maximum speed in a work-energy problem
I have the following problem:
The Royal Gorge bridge over the Arkansas River is $310\text{ m}$ above
the river. A $57\text{ kg}$ bungee jumper has an elastic cord with an
unstressed length of ...
3
votes
3answers
119 views
When can one write $a=v \cdot dv/dx$?
Referring to unidimensional motion, it is obvious that it doesn't always make sense to write the speed as a function of position. Seems to me that this is a necessary condition to derive formulas ...
4
votes
2answers
122 views
Conservative Force and $1/r^2$
Does the inverse square law have anything to do with conservative behavior of the central forces?
7
votes
3answers
552 views
Is the normal force a conservative force?
Most of the time the normal force doesn't do any work because it's perpendicular to the direction of motion but if it does do work, would it be conservative or non-conservative?
For example, consider ...
1
vote
3answers
128 views
Can endergonic reactions occur outside of living organisms?
If the Gibbs free energy equation is defined as:
∆G = ∆H - T∆S
And the amount of energy/work released from a reaction is:
...
1
vote
2answers
182 views
Mechanics Question: Energy, Work and Power
I'm a pure mathematician by trade, and have been trying to teach myself A-level mechanics. (This is not homework, it is purely self-study.)
I've been working through the exercises and have come up ...
0
votes
1answer
69 views
What is the difference of work $W$ and thermal energy $Q$ in thermodynamic Stirling-process for ideal gas?
What is the difference of work $W$ and thermal energy $Q$ in thermodynamic Stirling-process (in simple form) for ideal gas?
I think that you need work to preserve this process and you bring thermal ...
5
votes
2answers
244 views
Intuitively Understanding Work and Energy
It is easy to understand the concepts of momentum and impulse. The formula $mv$ is simple, and easy to reason about. It has an obvious symmetry to it.
The same cannot be said for kinetic energy, ...
1
vote
1answer
389 views
Needed Energy For Lifting 200 kg weight
I'd like to learn how much energy I need to lift a 200 kilograms weight on normal earth conditions?
For example how much electric power do we need?
I'm not a physicist and not a student and this ...
0
votes
1answer
215 views
Energy used to stop / slow an object
I'm trying to workout how much energy (if any), I use (imagining me as an efficient machine rather than a complex bio-mechanical human) when I lower or catch a weight. I understand that when I push it ...
4
votes
5answers
2k views
Why there is a 1/2 in kinetic energy formula? [duplicate]
Possible Duplicate:
Why is there a $\frac 1 2$ in $\frac 1 2 mv^2$?
Hèllo, I have a question about kinetic energy formula.
As you know, in kinetic energy formula, we have:
...
1
vote
3answers
108 views
Carrying water on person, or on the frame when bicycling
So, the question is as follows:
What is the difference in work exerted by the rider in the two following scenarios?
a) Rider + bike. Water carried in a holder on the frame
b) Rider + bike. Water ...
3
votes
1answer
341 views
I have a slight problem understanding the concept of “work”?
What I understand is that work is not the same as a car using gas or a crane lifting a car high up into the air. Let's use the crane as an example. And let me write out a few lines from the book.
...
3
votes
3answers
171 views
Why does it require such little energy to create the fastest thing in the universe?
I have noticed when I turn on the light switch in my house light comes from the bulb.
How is this light created?(process occurring in the bulb) and why is this small amount of electricity enough to ...
1
vote
2answers
559 views
Why is there a $\frac 1 2$ in $\frac 1 2 mv^2$?
For elastic collisions of n particles, we know that momentum in the three orthogonal directions are independently conserved:$$ \frac{d}{dt}\sum\limits_i^n m_iv_{ij} =0,\quad j=1,2,3$$
From this, it ...
0
votes
1answer
70 views
System moves away from equilibrium $\rightarrow$ it has energy added?
Suppose there is an isolated system $A$ at time $(-\infty, t_1)$, whose entropy is $S=S_{max}$, i.e. it is at thermodynamical equilibrium.
Between moments $[t_1, t_2)$ the isolation is violated and ...
1
vote
0answers
32 views
Is energy applied by the muscle when it holds a body in the air? [duplicate]
Possible Duplicate:
Why does holding something up cost energy while no work is being done?
We all know the following Newtonian formulas:
PotentialGravitationEnergy=m*g*h
Kinetic Energy = ...
1
vote
1answer
306 views
Work and Area under a Curve relating to Hooke's Law
If it takes work W to stretch a Hooke’s-law spring (F = kx) a distance d from its unstressed length, determine the extra work required to stretch it an additional distance d (Hint: draw a graph and ...
21
votes
7answers
3k views
Why does holding something up cost energy while no work is being done?
I read the definition of work as
$$W ~=~ \vec{F} \cdot \vec{d}$$
$$\text{ Work = (Force) $\cdot$ (Distance)}.$$
If a book is there on the table, no work is done as no distance is covered. If I ...
