Homework Help: Calculation of Kinetic Energy

1. Dec 7, 2014

kaspis245

1. The problem statement, all variables and given/known data
A body, which has mass of m = 1 kg , is dropped with a horizontal speed of vo = 20 m/s . What is its kinetic energy after 4 s ?

2. Relevant equations
Ek = mv2/2
s = vot + gt2/2

3. The attempt at a solution

Is it correct?

P. S. There is a mistake: Ek = 3200 J

Last edited: Dec 7, 2014
2. Dec 7, 2014

NTW

It's wrong. First of all, you should add the velocities as vectors. And, in this case, those vectors are at 90º...

3. Dec 7, 2014

Maged Saeed

after 4 seconds ,,
It would have a vertical component of velocity equal to (v0-gt)"which is what you have calculated in your derivative , which is incomplete too , " and a horizontal component equal to (v0 cos [theta=0])(which you haven't included)

Just find the magnitude of them and substitute the value of velocity in the kinetic energy formula

4. Dec 7, 2014

kaspis245

vy = voy - gt
So, after 4 s :
vy = 0 - 10 m/s2 * 4s = -40 m/s

vx = 20 m/s (since it does not change)

v2 = (vy)2+(vx)2 = 2000 m2/s2

v = 45 m/s

Ek = mv2/2 = 1000 J

Corrected

Last edited: Dec 7, 2014
5. Dec 7, 2014

NTW

You have forgotten g...

6. Dec 7, 2014