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Find the potential gravitational and kinetic energy of a ball after 0.4 seconds

  • #1

Homework Statement



A 3.0 kg metal ball is held 3.5m above the ground. Find the potential gravitational and kinetic energy of the ball after it is let go and has fallen for 0.4s


Homework Equations



EK= 1/2mv^2

EP=mgh



The Attempt at a Solution



Okay I know how to find both the kinetic and potential energy, but how does the time effect it? Is there a certain equation I have to sub it in other than the ones given. Please help :)

Potential= EP=mgh
Ep=(3.0Kg)(9.8m/s^2)(3.5m)
EP= 102.9 J

Kinetic = EK=1/2mv^2
EK=1/2(3.0kg)(9.8m/s^2)
EK=14.7 J
 

Answers and Replies

  • #2
PeterO
Homework Helper
2,425
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Homework Statement



A 3.0 kg metal ball is held 3.5m above the ground. Find the potential gravitational and kinetic energy of the ball after it is let go and has fallen for 0.4s


Homework Equations



EK= 1/2mv^2

EP=mgh



The Attempt at a Solution



Okay I know how to find both the kinetic and potential energy, but how does the time effect it? Is there a certain equation I have to sub it in other than the ones given. Please help :)

Potential= EP=mgh
Ep=(3.0Kg)(9.8m/s^2)(3.5m)
EP= 102.9 J

Kinetic = EK=1/2mv^2
EK=1/2(3.0kg)(9.8m/s^2)
EK=14.7 J
It would seem you actually don't know how to find KE.

You have substituted an acceleration value in for the velocity.

The 0.4 seconds is there as in that time the object will have fallen some distance [so you won't use 3.5 in the PE formula], and will have gained some speed - the value of which you will use in the KE formula.

Normal kinematics for falling objects is used to find the distance and speed values.
 
  • #3


It would seem you actually don't know how to find KE.

You have substituted an acceleration value in for the velocity.

The 0.4 seconds is there as in that time the object will have fallen some distance [so you won't use 3.5 in the PE formula], and will have gained some speed - the value of which you will use in the KE formula.

Normal kinematics for falling objects is used to find the distance and speed values.


Thank you for helping me realize that, I appreciate it. :D
 

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