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

#### gazeem

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.

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#### Dale

Mentor
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

Homework Helper
Gold Member
2018 Award
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.

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

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