Problem with f=ma and E=0.5mv^2

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In summary, the conversation discusses the calculation of the force, work, kinetic energy, and distance of an object with a mass of 1 kg being accelerated at a rate of 1 m/s^2 over 10 seconds. After various calculations, it is determined that the distance traveled is not 55m and the correct method for calculating distance is through integrals. The question has been answered and the person is thankful for the help.
  • #1
Nal
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EDIT: The question has been answered, and I don't need help anymore. Thanks a lot everybody!

Hi. This is something that has been puzzling me for a while, probably just due to my own stupidity. It seems to me that this should be really simple, but I don't know what I'm doing wrong though, so now I'm putting it out here.

Imagine an object with a mass of 1 kg being accelerated at a rate of 1 m/s^2 over 10 seconds.
We can then calculate the force being applied to the object
F=1*1=1 N

We can also calculate how far the object has moved over these 10 seconds
qovlur.jpg


According to Wikipedia, the work (W) is the product of a force times the distance through which is acts, measured in Joules, so
W=F*d=m*a*d=55 J

The velocity v at which the object moves is t*a=10 m/s
Then we can calculate the kinetic energy E

E=0.5*m*v^2=50 J

But shouldn't E and W be the same?
For example, if we take the object, now with the kinetic energy E and make it go up a hill on a planet where the gravitational acceleration is 1 1m/s^2 (which is =a from before, of course) we will convert the kinetic energy to potential energy, and we can calculate how far up the hill the object will go, as E(kin)+E(pot) is constant

0.5*m*v^2=m*a*h
0.5*1*10^2=1*1*h
h=50 m

But shouldn't h be 55 metres, as that was the distance the object moved when we accelerated it?
I hope you understood what I meant, I'm absolutely horrible at explaining my thoughts.
Thanks in advance! :)
 
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  • #2
The distance traveled is not 55m.
Check your calculation
 
  • #3
technician said:
The distance traveled is not 55m.
Check your calculation

Distance traveled over time

after 1s: 1 m (velocity 1 m/s)
2s: 3 m (velocity 2 m/s)
3s: 6 m (velocity 3 m/s)
4s: 10 m (velocity 4 m/s)
5s: 15 m (velocity 5 m/s)
6s: 21 m (velocity 6 m/s)
7s: 28 m (velocity 7 m/s)
8s: 36 m (velocity 8 m/s)
9s: 45 m (velocity 9 m/s)
10s: 55 m (velocity 10 m/s)
 
  • #4
Your calculations are incorrect. Given a constant acceleration a, distance d as a function of time is given by [itex]d=\frac 1 2 a t^2[/itex].
 
  • #5
Ah, well that obviously solves it. Thanks a lot.
 
  • #6
Nal said:
We can also calculate how far the object has moved over these 10 seconds
qovlur.jpg

[/QUOTE

Can you elaborate more on the method you use in finding the distance?
 
  • #7
azizlwl said:
Nal said:
We can also calculate how far the object has moved over these 10 seconds
qovlur.jpg

[/QUOTE

Can you elaborate more on the method you use in finding the distance?

I should update the main post. I realize now that I should have used integrals to calculate the distance travelled, however I've never learned about any of that. My question has already been answered. Thanks for taking your time to reply, though!
 

1. What is the problem with the equations f=ma and E=0.5mv^2?

The problem with these equations is that they only apply to objects moving at constant velocity or at rest. They do not account for objects that are accelerating or have varying velocities.

2. Why do f=ma and E=0.5mv^2 not work for accelerating objects?

These equations were derived for motion in a vacuum with no forces acting on the object other than the force being studied. In real world situations, there are often other forces at play that affect an object's acceleration and velocity, making these equations insufficient.

3. Can f=ma and E=0.5mv^2 be used for any type of motion?

No, these equations can only be used for linear motion. They do not account for rotational motion or motion in multiple dimensions.

4. Are there any cases where f=ma and E=0.5mv^2 can be used accurately?

Yes, these equations can be used accurately for objects moving at constant velocity or at rest, as long as there are no other forces acting on the object. They can also be used for small displacements and short time periods.

5. What are some alternative equations to use for more complex motion scenarios?

For more complex motion scenarios, other equations such as Newton's second law (F=ma) and the work-energy theorem (W=ΔE) can be used. These equations take into account the net force and work done on an object, respectively, and can be applied to a wider range of motion scenarios.

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