Kinetic Energy and Frictional Work of Child Jumping on Sled

Click For Summary

Homework Help Overview

The discussion revolves around a problem involving kinetic energy and frictional work related to a child jumping onto a sled. The scenario includes a child with a specified mass and velocity, a sled at rest, and the effects of friction on the sled after the jump.

Discussion Character

  • Mixed

Approaches and Questions Raised

  • The original poster attempts to apply conservation of momentum to find the velocity of the sled after the child jumps on it, followed by calculating the kinetic energy of the combined system. They also explore the work done by friction to stop the sled. Some participants question the calculations and the interpretation of the problem, particularly regarding the relationship between friction and the distance required to stop.

Discussion Status

Participants are engaged in verifying calculations and clarifying the problem's requirements. There is some agreement on the initial calculations, but questions remain about the interpretation of the frictional work and its relation to kinetic energy. Guidance has been offered regarding the need to recalculate kinetic energy and the implications of the friction force.

Contextual Notes

Participants note potential mathematical errors and the need for careful reading of the problem statement, particularly concerning the relationship between friction and the distance to stop the sled.

Jfxue
Messages
9
Reaction score
0
A 50 kg child running at 6.0 meter per second jumps onto a stationary 10kg sled. The sled is on a level frictionless surface.
1.FIND VELOCITY AFTER CHILD JUMPS ONTO SLED
M1V1+M2V2=(M1+M2)VF
300=60VF
VF=5 M/S

2.FIND KINETIC ENERGY of the sled with the child after she jumps onto the sled.
KE=1/2MV^2
1/2(60)(5^2)=750J

3.After a short time, the moving sled with the child aboard reaches a rough level surface that exerts a constant frictional force of 54 Newtons on the sled. How much work must be done by friction to bring the sled with the child to a stop?

WFf=(Ff)(D) =54d

find d
vf^2=vi^2+2ad
0=50^2 + 2(-54/60)d
1.8d=25
25/1.8 = 13.9M

now i got
wff=54x13.9 =750Jam i correct?
 
Last edited:
Physics news on Phys.org
you have made a mathematical mistake... 300/60 = 5. you have written 50
 
ooo but other than that I am correct?
 
Yes...looks OK.
The friction part of the question is a bit strange. If you are given the force of friction you are usually asked to calculate the distance (d) to bring the thing to rest.
Are you sure you read the question correctly? because the work needed to bring the sled to rest is always going to be = to the KE of the sled.
A large force of friction means it will come to rest after a short distance (and time)
A small force of friction means a longer distance and time.
Make certain to re-calculate the KE
 

Similar threads

Pulling an object at an angle with friction -- find the Kinetic Energy
  • · Replies 6 ·
Replies
6
Views
4K
Work and Kinetic Energy of a Sled
  • · Replies 4 ·
Replies
4
Views
5K
Kinetic Energy and Newton Forces - distance/speed
  • · Replies 3 ·
Replies
3
Views
2K
What's the coefficient of kinetic friction?
  • · Replies 8 ·
Replies
8
Views
4K
Calculating Work and Change in Thermal Energy
  • · Replies 2 ·
Replies
2
Views
2K
Finding work and the Change in Thermal Energy
  • · Replies 5 ·
Replies
5
Views
2K
Kinetic energy of a sled question
  • · Replies 4 ·
Replies
4
Views
8K
How Does Angling the Force Affect Kinetic Energy Increase in a Sled?
  • · Replies 9 ·
Replies
9
Views
3K
Potential energy -- child sleds down a slope and comes to a stop....
  • · Replies 21 ·
Replies
21
Views
3K
A 30 kg child moving at 4 m/s jumps onto a 50 kg sled that is initiall
  • · Replies 2 ·
Replies
2
Views
3K