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7answers
80 views

Initial speed is zero and so is power?

If I want to accelerate something from standstill to max speed, with a constant force (acceleration and mass don't change), the equation P = F * v would say that in the beginning we use 0 W power. How ...
2
votes
3answers
113 views

What happens to $P = F × v$ for a car moving on a frictionless surface and in vacuum?

I'm considering a scenario in which a car with an engine capable of producing maximum power of 200 hp is moving on a frictionless surface and in vacuum. Since no work is lost due to friction or air ...
2
votes
7answers
117 views

Why does incline on treadmill burn calories faster?

So I was at the gym today, running on the treadmill, when the question hit me: If I run with the treadmill on incline, I burn more calories. Since I run at the same speed, kinetic energy is constant, ...
4
votes
2answers
175 views

Did Usain Bolt really sustain 11 horsepower of exerted power during the 100 m sprint?

I've just come across a WP article which says: "The Jamaican sprinter Usain Bolt produced a maximum of 3.5 hp (2.6 kW) 0.89 seconds into his 9.58 second 100-metre (109.4 yd) dash" Out of curiosity, I ...
-1
votes
3answers
86 views

Why am I getting two answers from the same question? [closed]

In this problem, if I use the formula for power as $\frac{F\cdot s}{t}$, then the answer is 37.5 W. But if I use the formula $F\cdot v$, then the answer is 75 W. How is it possible that the man has ...
0
votes
1answer
38 views

Is the force from an engine constant as a vehicle reaches top speed?

I've come across a textbook question and I've got myself confused by it although it seems like it should be simple. The van accelerates with a (supposedly) constant resistance force up to its top ...
0
votes
1answer
74 views

Force exerted by a motor, velocity, and friction

Some motor make an object move on a surface very slowly. One can write at steady state : $F_{motor}-F_{friction}=0$. The friction is supposed to come from the surface so : $F_{motor}-\mu_s m g=0$ with ...
0
votes
1answer
26 views

Power - why do we use the driving force over the resultant force?

I'm an A-Level student, studying Further Maths. We've chosen the Further Mechanics as our optional module of the course. Further Mechanics, unfortunately, is not making a lot of sense to me, because ...
-1
votes
1answer
83 views

Calculating power with force as a function of time [closed]

While doing an AP question in Physics C today I answered the question differently from the professor but I'm not sure what part of my reasoning is incorrect. A $100\ \rm{kg}$ block is being pulled ...
0
votes
2answers
56 views

Power by normal force

Would the power created by the normal force always be zero? Since normal force is always perpendicular to the object?
1
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2answers
108 views

Does gravity matter when calculating work done horizontally?

My teacher gave the following question: A 212lb running back can complete the 40yd dash in 4.53s. What time would a 198lb running back have to post in order to be more powerful? My teacher says that ...
-1
votes
1answer
51 views

Can a tunnel through a planet be used as a power generator?

Assuming that it were possible to do so, if a tunnel from one side of a planet to the other were lined with copper wire, could dropping magnets down the hole be used to generate power?
0
votes
1answer
85 views

Tidal Power Gen Multiplier? [closed]

Regarding tidal power generation, the following scheme uses an outer tank and an inner tank that floats on the water of the inner tank. You fill the inner tank first (while it's at high-tide level) ...
-1
votes
1answer
79 views

When and how do we use the formula Power = force X velocity?

I'm unable to understand the solution given in this book mine... Why are we adding the resistive force? In fact, i would really appreciate if someone explains the question to me, I'm really struggling ...
-1
votes
1answer
54 views

How to calculate acceleration from power and weight? [closed]

Imagine a bike that is not moving initially at $t=t_0$. The bike and its cyclist have a combined mass of $m=60\,\mathrm{kg}$. If the cyclist starts to apply $p=100\,\mathrm{W}$ of power, then what ...
-1
votes
2answers
51 views

An engines power is 650kW and the shuttle's speed increases from 120 m/s to 160 m/s in t seconds [closed]

note sure if I'm missing something obvious, but i cant seem to find a good way to tackle this question. A space shuttel of mass 400kg is moving in a straight line where there is no resistance. the ...
0
votes
0answers
40 views

How to convert average horse power?

My question is " A locomotive engine draws a load $m$ pounds up an inclined plane making angle $a$ with horizontal, the coefficient of friction being $k$ . If starting from rest and moving ...
0
votes
4answers
132 views

For a car engine, why does velocity increase as force decreases?

Take the equation P = FV For a fixed power of an engine, applying a greater force gives a smaller velocity while a smaller force gives a greater velocity. To me this sounds counter-intuitive. I ...
0
votes
1answer
41 views

mechanical power in moving coordinate system

Suppose I have an object resting on top of a table. There are two forces acting on the object: gravitational pull and the supporting force from the table. It is pretty obvious, that the table is doing ...
0
votes
1answer
677 views

What does the Force in the equation P=FV represent?

So this is the question in context. A car of mass 1200 kg tows a trailer of mass 300 kg along a straight horizontal road. The resistance to motion is modeled as being a constant force of magnitude ...
-1
votes
1answer
119 views

Power and work of tension force of a rotating body [closed]

Suppose that we have a body rotating on a massless string with a uniform velocity. What can we say about the work done by this tension. Moreover, what is the power related to this work. I do not have ...
2
votes
1answer
101 views

What assumptions would I have to make to rigorously prove power equals force dotted with velocity, $P=\mathbf{F}\cdot\mathbf{v}$?

$\newcommand{\F}{\mathbf{F}}$$\newcommand{\r}{\mathbf{r}}$$\newcommand{\v}{\mathbf{v}}$$\newcommand{\d}[2]{ \frac{ d #1 }{ d #2 }}$ There’s a well-known known formula for the power $P$ exerted on a ...
-1
votes
1answer
285 views

The physics of a dog sled — how much power or energy over time?

I'm looking for some equations that govern the physics of a dog sled. Let's say we have 8 dogs pulling a sled. Each dog provides a force of F, the tractive effort. The sled weighs W and there's a ...
1
vote
1answer
50 views

Watts and Newtons revisited

At the junkyard, an electromagnet holds a 1 ton car freely in the air (no acceleration exc. gravity). Assuming 100% power efficiency, how many Watt electrical power are needed? I know that the magnet ...
-2
votes
2answers
163 views

Why isn't work a logical quantity? [duplicate]

Please read till the end, before you answer. I'll start off with the examples to get to the point. Drop a block from 1 meter and it will have speed v when it reaches the ground, with total work ...
0
votes
0answers
89 views

Wind Power in a Wind Tunnel

Two questions in one day! What? Okay, so the power equation for air flow is as follows: $$ P = 1/2 \cdot \rho \cdot A \cdot V^3 $$ In a wind tunnel due to the Bernoulli principle as the area ($A$) ...
0
votes
2answers
50 views

Power of an engine

Suppose a car of mass $m$ starts out with an acceleration $a$ in the positive direction (say +ve x-axis), against a constant Resistive force $R$. At the instant it's velocity is $v$, what is power of ...
0
votes
2answers
848 views

How do you find instantaneous power?

I was wondering is there a way to find instantaneous power when you are not given an equation relating work with time but only the numerical values of quantities like force, mass of body, acceleration ...
0
votes
1answer
280 views

Infinitesimal work

I am a newbie in Physics (Senior on highschool) and our teacher wrote in a proof $$\dfrac{dK}{dt}=\dfrac{dW}{dt},$$ where $K$ is the Kinetic energy of a body and $W$ is the Work. So now that I am ...
0
votes
2answers
223 views

what causes a decrease in velocity?

Let's assume that a car is travelling at a constant velocity (V) on a horizontal flat plane , hence the force of friction is balanced by the force applied by the engine. The power which the engine ...
1
vote
3answers
191 views

What does the equation $P=\vec{F}\cdot \vec{v}$ really mean?

Let's say that we have a car travelling at a constant velocity (V) on a flat plane , let's assume that it's applied force or the force applied from its engine ( Fapp ) balances the frictional force ( ...
-1
votes
1answer
26 views

Why is the power for an object of mass m moving at a higher velocity greater than that for the same object moving at a lower velocity?

I understand the equation Fv but I will like a more intuitive answer concerning why the rate at which energy is transferred must be greater
0
votes
1answer
87 views

What is the relation between a fixed force and work?

How would you translate a constant force into work or energy? If I have a way to output 100KN continuously, how can I translate that into watts? Is there a formula for that translation?
0
votes
3answers
512 views

Question on work, energy and power

The engine of a train is working at a constant rate. The maximum speed of the train up a certain incline is $V_{1}$ and the maximum speed down the same incline is $V_{2}$. Then if the train moves on a ...
1
vote
1answer
448 views

Crate pulled on an incline with constant speed. What is the total work done?

A rope is attached to a 50.0-kg crate to pull it up a frictionless incline at constant speed to a height of 3-meters. Note that the force of gravity has two components (parallel and perpendicular ...
0
votes
1answer
107 views

How many watts of power does the man use?

I was in the middle doing my homework perfectly fine until I encountered this question. How many watts of power does a man with a weight of 600.0 N have when he climbs a flight of stairs 3.0 m ...
2
votes
2answers
415 views

Does a rocket engine apply more power as the rocket's speed increases? [duplicate]

To clarify my level of knowledge, I'm a high school student all the way through AP Physics C: Mechanics. So, let's imagine that there is a rocket travelling through the vacuum of space (ignoring ...
-3
votes
1answer
443 views

Work, energy and power [closed]

A load of mass $m$ slides down a smooth plane with inclination title from point $A$ to $B$. The same mass is then dropped from $A$ to $C$. Could anyone decipher this $\downarrow$. I've edited the ...
0
votes
3answers
6k views

In the equation “Power=Force . Velocity”, if velocity is considered constant, how can force exist?

Power(P)=Force(F) . Velocity(V) "In the straightforward cases where a constant force moves an object at constant velocity, the power is just P = Fv. In a more general case where the velocity is not ...
3
votes
1answer
41 views

work of body remaining still in gravitational field

How to quantify power output of a statite? It's a kind of (non-)satellite, that remains immobile in relation to the central body (e.g. hovering over one of the poles of a planet) through continuous ...
2
votes
4answers
1k views

Can I use the work done to calculate power of a motor?

I am trying to calculate the energy/ power required to move a plate up and down in a liquid. My approach is to calculate the forces that must be applied by the motor to push/pull the plate based on ...
1
vote
1answer
35 views

Power expression for a dynamic force

Given that: $$W=\int_{x_0}^x \vec{F}\cdot d\vec{x}$$ and assuming $F$ is time dependent, we get: $$\frac{dW}{dt} = \int_{x_0}^x \frac{d\vec{F}}{dt}\cdot d\vec{x}+\vec{F}(x,t)\cdot \vec{v}$$ ...
0
votes
3answers
322 views

Power in a uniform circular motion

I've a circular motion and I want to find the power that the motion is generating. The formula's I got: $$\text{P}\left(t\right)=\text{W}'\left(t\right)=\text{E}'\left(t\right)$$ Where $\text{P}$ is ...
-1
votes
1answer
102 views

Impact force of a pillow?

My friend and I just had a discussion about how dangerous it would be to get hit in the face with a pillow thrown out of a train. If we say the train is traveling at 100km/h and the pillows weight is ...
2
votes
0answers
127 views

How to apply conservation of energy to active noise-cancelling headphones?

Active noise-cancelling headphones add the opposite waveforms of the ambient sound in order to cancel out this ambient sound by destructive interference. I do not understand why adding more power in ...
1
vote
2answers
234 views

Velocity over time applying constant power and considering air drag

I wanted to understand how velocity changes over time when applying a constant power to a body of mass $m$. I figured out that in this case, since $$P=d/dt(KE)=d/dt(\frac{1}{2}mv^2),$$ integrating I ...
1
vote
1answer
297 views

Power needed to raise a bucket at a constant speed by a pulley

This is the last section of this pulley exercise: The bucket in is filled with water so its total mass is now $45$ kg. The bucket is raised at a constant speed of $2.0$ $ms^{-1}$ using an ...
-1
votes
1answer
841 views

What does $P = Fv$ mean? [closed]

The engine of a car is developing a power of 90kW when it is moving on a horizontal road at a constant speed of 100 km h−1. Estimate the total horizontal force opposing the motion of the car. I ...
4
votes
2answers
383 views

How is the dot product a generalization of multiplication?

I've seen an interesting explanation for lots of what I previously thought were unmotivated definitions in Newtonian mechanics, namely that power is always defined as effort times flow. But when ...
2
votes
2answers
305 views

Why dont you take derivative of force in definition of power ? P=F.v

The derivative of work is $\bf F\cdot v .$ $$P(t)= \frac{\mathrm dW}{\mathrm dt}= \mathbf{F\cdot v}=-\frac{\mathrm dU}{\mathrm dt}\;.$$ But why not $$(\mathrm d(\mathbf F)/\mathrm dx)\cdot \mathbf x ...