The energy of a ball shot upwards

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AI Thread Summary
The discussion focuses on calculating the mechanical energy of a ball shot upwards, using the conservation of energy principle. The mechanical energy is expressed as the sum of kinetic and gravitational potential energy, with a specific example yielding 19.6 J. Participants debate the use of kinematics to find the instantaneous velocity at a given time, emphasizing that conservation of energy does not account for time. The challenge arises from missing values needed to calculate velocity, leading to questions about assuming constant velocity. Ultimately, the forces acting on the ball, primarily gravity, are acknowledged, prompting further exploration of the relationship between velocity and time.
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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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