Answer:
b) the second at the doorknob.
Explanation:
The torque applied by a force is given by the equation
where
is the torque
is the component of the force perpendicular to the direction between the axis of rotation and the point of application of the force
d is the arm (the distance between the axis of rotation and the point of application of the force)
In this problem, we have two equal forces F both applied perpendicular to a door, so
The first force is applied at the midpoint of the door; if we call L the width of the door, then the arm of this force is 
, so the torque applied by this first force is
The 2nd force instead is applied at the doorknob, so the arm in this case is L:
d = L
So, the torque exerted by the second force is
Therefore, we see that the 2nd force exerts a greater torque.
Answer:
moving the boxes and the furniture up to the fifth floor.
Explanation:
The work done when moving an object up or down is equal to its change of gravitational potential energy:
where
m is the mass of the object
g is the gravitational acceleration
is the change in height of the object
We have two opposite situations:
1) when an object is moved upward, is positive, so the work done W is positive as well: this means that we need to do work in order to move the object, because we have to "win" the effect of gravity, which pulls the object downward
2) when an object is moved downward, is negative, so the work done W is negative as well: this means that we do not need to do work on the object, because it is already done by gravity, which pulls the object downward.
Therefore, more work is done when we are moving the boxes and the furniture up to the fifth floor.
Answer:
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