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

In summary, when determining the kinetic energy of an object at the bottom of a declined ramp, it is more accurate to use the equation KE = initial PE - any energy lost due to friction, rather than including the work done by the vector component of gravity parallel to the direction vector of the object. This is because including this work would result in double accounting for the work done by gravity, which is already accounted for in the initial and final potential energies. It would be beneficial to solve the problem using both methods to gain a better understanding of the physical and mathematical reasoning behind their equivalence.
  • #1
gazeem
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TL;DR 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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  • #2
gazeem said:
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.
 
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  • #3
gazeem said:
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

gazeem said:
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
 
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Ah okay I see. Thank you both so much for your responses, makes more sense now.
 
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gazeem said:
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.
 
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1. Why doesn't the gravity vector contribute to KE sliding down a ramp?

The gravity vector does contribute to the kinetic energy (KE) of an object sliding down a ramp. However, the contribution is not as significant as the other factors that affect KE, such as the object's mass, velocity, and the angle of the ramp.

2. Does the gravity vector play a role in the velocity of an object sliding down a ramp?

Yes, the gravity vector does play a role in the velocity of an object sliding down a ramp. The force of gravity accelerates the object down the ramp, increasing its velocity as it moves.

3. How does the angle of the ramp affect the contribution of the gravity vector to KE?

The angle of the ramp does affect the contribution of the gravity vector to KE. The steeper the ramp, the more significant the contribution of the gravity vector will be to the object's KE. This is because the force of gravity is acting more directly on the object, increasing its acceleration and velocity.

4. Can the mass of an object affect the role of the gravity vector in KE on a ramp?

Yes, the mass of an object does affect the role of the gravity vector in KE on a ramp. The greater the mass of the object, the more force of gravity is required to accelerate it down the ramp, resulting in a larger contribution to KE.

5. Is the contribution of the gravity vector to KE on a ramp affected by the surface of the ramp?

The surface of the ramp does not directly affect the contribution of the gravity vector to KE. However, the surface can indirectly affect the object's velocity and KE by providing more or less friction, which can either aid or hinder the object's movement down the ramp.

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