How Do You Calculate the Kinetic Energy of a Moving Body?

Click For Summary

Homework Help Overview

The discussion revolves around calculating the kinetic energy of a body with a given mass and initial horizontal speed after a certain time interval, considering both horizontal and vertical motion. The subject area is kinematics, specifically focusing on the kinetic energy formula and vector addition of velocities.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants discuss the need to consider the body’s velocities as vectors due to the horizontal and vertical components being at right angles. There are attempts to clarify the correct application of the kinetic energy formula and the impact of gravitational acceleration on vertical motion.

Discussion Status

The discussion has evolved with participants providing feedback on the original poster's calculations and assumptions. Some guidance has been offered regarding the correct treatment of velocity components, but there is no explicit consensus on the final kinetic energy value.

Contextual Notes

There are indications of confusion regarding the inclusion of gravitational effects and the proper calculation of velocity components. The original poster has acknowledged mistakes in their initial calculations, and participants are actively correcting and refining the approach.

kaspis245
Messages
189
Reaction score
1

Homework Statement


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 ?

Homework Equations


Ek = mv2/2
s = vot + gt2/2

The Attempt at a Solution


2qn5h6a.png

Is it correct?

P. S. There is a mistake: Ek = 3200 J
 
Last edited:
Physics news on Phys.org
It's wrong. First of all, you should add the velocities as vectors. And, in this case, those vectors are at 90º...
 
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
 
OK, from your answers I understood that:

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:
You have forgotten g...
 
What about now?
 
kaspis245 said:
What about now?

Now it's right...
 

Similar threads

Confused about a conservation of energy problem
  • · Replies 4 ·
Replies
4
Views
2K
Work done during a collision -- Change in Kinetic Energy & change in Momentum
  • · Replies 1 ·
Replies
1
Views
2K
Potential and Kinetic energy equations including drag coefficient
  • · Replies 1 ·
Replies
1
Views
2K
Resistance force changing the velocity of an object
  • · Replies 8 ·
Replies
8
Views
1K
How Is Potential Energy Converted to Kinetic Energy in Physical Systems?
  • · Replies 6 ·
Replies
6
Views
1K
Calculating Work and Kinetic Energy on an Inclined Plane
  • · Replies 2 ·
Replies
2
Views
3K
Calculating Kinetic & Potential Energy: Is it Correct?
  • · Replies 12 ·
Replies
12
Views
2K
Using Orbital Energy to Calculate Velocity
  • · Replies 3 ·
Replies
3
Views
2K
Change in the Kinetic Energy of a System
  • · Replies 2 ·
Replies
2
Views
3K
What's the source of increase in rotational energy of carousel?
  • · Replies 9 ·
Replies
9
Views
3K