The energy of a ball shot upwards

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Homework Help Overview

The discussion revolves around the energy of a ball shot upwards, focusing on the application of mechanical energy concepts and the relationship between kinetic and gravitational potential energy. Participants explore the implications of missing values in the context of energy conservation and kinematics.

Discussion Character

  • Mixed

Approaches and Questions Raised

  • Participants discuss the conservation of mechanical energy and its application, questioning how to calculate velocity at a specific time without given height or other parameters. There is a consideration of using kinematics to find velocity, alongside discussions about the assumptions regarding forces acting on the ball.

Discussion Status

The conversation is ongoing, with participants exploring different methods to approach the problem. Some guidance has been offered regarding the use of kinematics to find velocity, while others express uncertainty about the assumptions needed to proceed with calculations.

Contextual Notes

Participants note the absence of specific values such as acceleration, final velocity, and displacement, which complicates the use of kinematics. There is also a mention of the assumption that the ball is moving close to the Earth's surface, which may influence the analysis of forces acting on it.

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Homework Statement
The ball with a mass of 200g is shot up with the instantaneous velocity of 14 m/s.
a) Determine the mechanical energy at the point of release
b) Find the speed of the ball after it has travelled for 0.2s
c) What is the kinetic energy of the ball after 0.2s
d) Use the conservation of energy principle to determine the max height the ball reaches
Relevant Equations
E(gravity)=mgh
E(kinetic)=1/2mv^2
E(mech1)=E(g)+E(k)
E(mech1)=E(mech2)
a) E(mech)=E(k)+E(g)
E(mech)=1/2mv^2+(0.2)(9.8)(0)
E(mech)=19.6 J

b) E(mech1)=E(mech2)
E(k)+E(g)=E(k)+E(g)
E(g)=E(k)+E(g)
0=1/2mv^2+(mgh)
*No height is given so I can't solve using this method. It says instantaneous velocity meaning the velocity at 0.2s is different.

c) E(k)=1/2mv^2
E(k)=1/2(0.2)(i would use the velocity from q.b)^2

d) E(mech1)=E(mech2)
E(k)+E(g)=E(k)+E(g)
E(g)=E(k)
mgh=1/2mv^2
h=v^2/2g
h=(how would i calculate v?)^2/2(9.8)

So, I am stuck on how to calculate a few values. I can't use kinematics as this is the energy unit.
 
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physicslady123 said:
So, I am stuck on how to calculate a few values. I can't use kinematics as this is the energy unit.
I'm pretty sure that you are allowed to use kinematics to find the velocity a given time. Conservation of energy method does not consider time in its application, so clearly you need another method for that (hence: kinematics).
 
gneill said:
I'm pretty sure that you are allowed to use kinematics to find the velocity a given time. Conservation of energy method does not consider time in its application, so clearly you need another method for that (hence: kinematics).

I still wouldn't be able to calculate velocity for part b by using kinematics as I have 3 missing values (no acceleration, no final velocity, and no displacement). Would I have to assume that the velocity is constant?
 
physicslady123 said:
I still wouldn't be able to calculate velocity for part b by using kinematics as I have 3 missing values (no acceleration, no final velocity, and no displacement). Would I have to assume that the velocity is constant?
Presumably the ball is assumed to be moving close to the Earth's surface. So what forces are acting on the ball?
 
gneill said:
Presumably the ball is assumed to be moving close to the Earth's surface. So what forces are acting on the ball?
gravity.
 
physicslady123 said:
gravity.
Right. So given that, can you write an expression for the velocity vs time?
 

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