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

[gazeem]

TL;DR

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.

 

[Dale]

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.

 

[A.T.]

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

 

[gazeem]

Ah okay I see. Thank you both so much for your responses, makes more sense now.

 

[PeroK]

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.