Work and Kinetic Energy Question.

[elementis0]

Hey guys, I am learning about the work-kinetic energy thereom and I am having trouble with a concept of it which arose from an example problem in my book.
Here it is:

Homework Statement

A man wishes to load a refrigerator onto a truck using a ramp at angle theta, he claims
that less work should be required to load the truck if the length L of the ramp were increased.
Is this claim valid?

Homework Equations

Basic Kinetic Energy and Work Eqn's

The Attempt at a Solution

Ok so I know how to solve the problem pretty much, my question on this problem is more a conceptual question.
In order to solve the problem I must notice that the total change in kinetic energy is zero, and this is because of what the book says which is "For the system of the fridge and the hand truck, the change in kinetic energy is zero. the normal force exerted by the ramp on the system is directed at 90 degrees to the displacement of its point of application and so does no work on the system"

But why is this? The fridge is moving isn't it? So if it has a velocity and is going a distance shouldn't there be kinetic energy from the work by the man?
I know when looking at net work I should look at the work being done by the entire system,
but I can't help but thing that the gravity is doing no work but that man is, therefore work is being done and kinetic energy is positive, but the book says the work done by the man added to the work done by gravity is zero?

Would love to have this question cleared up to help my understanding of this concept!

Where is my logic flawed?

 

[PhanthomJay]

Hey guys, I am learning about the work-kinetic energy thereom and I am having trouble with a concept of it which arose from an example problem in my book.
Here it is:

Homework Statement

A man wishes to load a refrigerator onto a truck using a ramp at angle theta, he claims
that less work should be required to load the truck if the length L of the ramp were increased.
Is this claim valid?

Homework Equations

Basic Kinetic Energy and Work Eqn's

The Attempt at a Solution

Ok so I know how to solve the problem pretty much, my question on this problem is more a conceptual question.
In order to solve the problem I must notice that the total change in kinetic energy is zero, and this is because of what the book says which is "For the system of the fridge and the hand truck, the change in kinetic energy is zero. the normal force exerted by the ramp on the system is directed at 90 degrees to the displacement of its point of application and so does no work on the system"

But why is this? The fridge is moving isn't it?

yes, but it is assumed to be moving at constant speed, so that implies the initial and final kinetic energies are the same, since v is the same at the beginning and at the end; thus, there is no change in the KE of the system

So if it has a velocity and is going a distance shouldn't there be kinetic energy from the work by the man?

the work done by the man changes the potential energy of the system

I know when looking at net work I should look at the work being done by the entire system,
but I can't help but thing that the gravity is doing no work but that man is, therefore work is being done and kinetic energy is positive, but the book says the work done by the man added to the work done by gravity is zero?

Would love to have this question cleared up to help my understanding of this concept!

Where is my logic flawed?

W_net = delta KE; since delta KE is 0, then W_net is zero. The work done by the man is the negative of the work done by gravity, that is, the work done by the man is mgh in any case, regardless of the angle of incline to reach the same height.

 

[elementis0]

So then also in that case,
would the change in potential energy be mg(del_h)?

 

[PhanthomJay]

So then also in that case,
would the change in potential energy be mg(del_h)?

Yes, where delta h is not given, but it would just be the vertical height from road grade to the top of the ramp (floor of the truck).