Why doesn't the gravity vector contribute to KE sliding down a ramp?

  • B
  • Thread starter gazeem
  • Start date
  • #1
9
2

Summary:

For physics questions where, say, a block is at the top of a declined ramp, how come the KE once the block reaches the bottom is determined by:

KE = initial PE - any energy lost due to friction

rather than:

KE = initial PE - any energy lost due to friction + any work done by the vector component of gravity that is parallel to the direction vector of the object.
244340
 

Answers and Replies

  • #2
30,325
6,806
KE = initial PE - any energy lost due to friction + any work done by the vector component of gravity that is parallel to the direction vector of the object
The PE is the same as the work done by the vector component of gravity that is parallel to the direction vector of the object.
 
  • Like
Likes russ_watters, gazeem and Chestermiller
  • #3
A.T.
Science Advisor
10,730
2,311
Summary: For physics questions where, say, a block is at the top of a declined ramp, how come the KE once the block reaches the bottom is determined by:

KE = initial PE - any energy lost due to friction
Or more generally:
KE = (initial PE - final PE) - any energy lost due to friction

rather than:

KE = initial PE - any energy lost due to friction + any work done by the vector component of gravity that is parallel to the direction vector of the object.
Beacuse that would be double accounting for the work done by gravity, which is already included as:
initial PE - final PE
 
  • Like
Likes gazeem
  • #4
9
2
Ah okay I see. Thank you both so much for your responses, makes more sense now.
 
  • Like
Likes Dale
  • #5
PeroK
Science Advisor
Homework Helper
Insights Author
Gold Member
15,044
7,224
Ah okay I see. Thank you both so much for your responses, makes more sense now.
It would be a good exercise to solve the problem both ways: 1) using PE and 2) using W = Fd for the gravitational force down the slope.

This may give you an insight into why physically and mathematically they give the same result.
 
  • Like
Likes gazeem and russ_watters

Related Threads on Why doesn't the gravity vector contribute to KE sliding down a ramp?

  • Last Post
Replies
5
Views
3K
Replies
22
Views
5K
Replies
36
Views
19K
  • Last Post
Replies
18
Views
2K
Replies
9
Views
743
Replies
2
Views
2K
  • Last Post
Replies
19
Views
32K
Replies
2
Views
1K
Top