# What is the average power of the horse?

I have been working on this problem for awhile now and I cant get anywhere. Can anyone help?

1) A horse pulls a cart with a force of 155.0 N at and angle of 32.0o with respect to the horizontal and moves along at a speed of 13.8 km/hr. How much work does the horse do in 8.6 min?

2) What is the average power of the horse?

tiny-tim
Homework Helper
Welcome to PF!

Hi qman316! Welcome to PF! (I assume it means that the road is horizontal, but the angle of the force from the horse is 32º)

1) Do you know how to calculate work? First off, draw a picture. Once you have that think of what work and power really are. how do you find work?
how do you find power?
what equations do you use?

i also think it may be better to figure out number 2 before number one. but that my be just me.

All the equations I have to figure out Work involve knowing the distance, which I do not know in this problem. Is there another way to find work without knowing a distance?

hmmm....well since the horse and cart are moving at a constant speed then the NET work is zero (the work the horse does and the work friction does cancel out resulting in no change in KE and thus no acceleration). However, the work the horse does is found by projecting the line of force caused by the horse onto the direction of motion. You would then multiply this by the speed to get the power (rate at which work is done). You can then multiply the power by the time (you need to convert minutes to hours though) to get work.

not all the equations to find work requires distance. do you know what the formula for kinetic energy is?

tiny-tim
Homework Helper
All the equations I have to figure out Work involve knowing the distance, which I do not know in this problem. Is there another way to find work without knowing a distance?
Hi qman316! But you do know the distance …

you know the time and the speed, so the distance is … ? how i would calculate it is to multiply the force from the horse by the distance travelled (which is just the velocity multiplied by the time)... F*d=Work

am i missing something?

the angle really has nothing to do with it unless your figuring out the work done by gravity right?

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tiny-tim
Homework Helper
Hi shamrock5585! how i would calculate it is to multiply the force from the horse by the distance travelled (which is just the velocity multiplied by the time)... F*d=Work

am i missing something?
Yes … you're missing how horses and carts work! (how old is this book? )

oh … and this isn't your thread, so you won't give the answer, will you? so.... d=v*t which is 13.8km/hr(.1433hr), d=1.978 then F*d=W which is 155N*1.978=306.59, what did I do wrong cuz it says 306.59 is the wrong answer?

HallsofIvy
Homework Helper
The horse moves at a speed of 13.8 km/hr. Since your are asked for average power, not work, you can start by assuming any time you like- say 1 hour. How far does the horse pull the cart in one hour?

the question in part a asks for the work and b the average power

tiny-tim
Homework Helper
Hi qman316! so.... d=v*t which is 13.8km/hr(.1433hr), d=1.978 then F*d=W which is 155N*1.978=306.59, what did I do wrong cuz it says 306.59 is the wrong answer?
You left out the angle! Kurdt
Staff Emeritus
Gold Member
If its still wrong after that check the units.

ur answer should probably be in newton meters

i was suggesting that the angle did not mean anything because i assumed the force was at an angle of 32 degrees instead of the force being horizontal and the box moving up a hill at angle 32 degrees...

Either if the horse is walking on a slope of 32 degrees, or if the 'connection' from the horse to the cart is at an angle of 32 degrees, you still need to use it!

I assume it is meant that the connection is not mounted horizontally between the horse and the cart. Then, the force that is given is the total force, at an angle of 32 degrees.

What is the equation for work? I saw you use "W = Fd" before but that is wrong! Can you see from this what you did wrong?

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What is the equation for work? I saw you use "W = Fd" before but that is wrong! Can you see from this what you did wrong?
Let me just correct myself there (since I cannot seem to edit my post anymore).
The equation 'W = Fd' is valid, but only under a certain condition, which has to do with the direction of the force.

well wouldnt W=Fd still be valid but force would be Fcos32 now?

to clarify what i meant before is that if the horse was walknig up the path the angle does not matter... the angle between force and displacement is 0 so Fcos0=F because cos0 =1

so the horse is walking horizontal with a rope tied to a box pulling it up a slope of angle 32 degrees?

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tiny-tim
Homework Helper
well wouldnt W=Fd still be valid but force would be Fcos32 now?

to clarify what i meant before is that if the horse was walknig up the path the angle does not matter... the angle between force and displacement is 0 so Fcos0=F because cos0 =1

so the horse is walking horizontal with a rope tied to a box pulling it up a slope of angle 32 degrees?
Hi shamrock5585! You're concentrating too much on the horse.

The horse is connected to the cart by a shaft or rope, and the force on the cart (and the reaction force on the horse) comes entirely from the tension in the shaft or rope, which will be along its length, at an angle of 32º. so what am i doing wrong? its been a little while since of done problems like this and i dont have equations in front of me im just going from my head... i was pretty sure it was the the force times the cos of the angle between force and dislacement times distance to get work... no?

oh yeah... and can ya blame me for concentrating on the horse too much haha the question asks what is the work done by the horse

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Here, I quickly drew up an image (don't mind the horse lol) of what I believe is the case: The 'rope' is attached to the horse under an angle of 32 degrees.

Only the horizontal component $\vec{F}_{hor}$ contributes to actually pulling the cart, so only this component does work.

The equation for work is:
$$W = \vec{F} \cdot \vec{d} = Fd \cos\theta$$

In this equation, F is the total force.

ok so based on that i was right...

i got an answer of ******

anybody confirm? or do i have a unit error?

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