Problem 2: Estimate the electric power requirement, in kW, of a 1,400 ft2floor area (three bedroomshome) with three occupants. U
1 answer:
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Consult the attached free body diagram.
By Newton's second law, the net force on the crate acting parallel to the surface is
∑ F[para] = (370 N) cos(-20°) - f = 0
(this is the x-component of the resultant force)
where
• (370 N) cos(-20°) = magnitude of the horizontal component of the pushing force
• f = magnitude of kinetic friction
The crate is moving at a constant speed and thus not accelerating, so the crate is in equilibrium.
Solve for f :
f = (370 N) cos(-20°) ≈ 347.686 N
The net force acting perpendicular to the surface is
∑ F[perp] = n - 1480 N - (370 N) sin(-20°) = 0
(this is the y-component of the resultant force)
where
• n = magnitude of normal force
• 1480 N = weight of the crate
• (370 N) sin(-20°) = magnitude of the vertical component of push
The crate doesn't move up or down, so it's also in equilibrium in this direction.
Solve for n :
n = 1480 N + (370 N) sin(-20°) ≈ 1606.55 N ≈ 1610 N
Then the coefficient of kinetic friction is µ such that
f = µn ⇒ µ = f/n ≈ 0.216
Answer: 88 m/s
Explanation:
If we are talking about an acceleration at a uniform rate, we are dealing with constant acceleration, hence we can use the following equation:
(1)
Where:
Is the final velocity of the plane (we know it is zero because we are told the pilot stops the plane at a specific distance)
Is the initial velocity of the plane
is the constant acceleration of the plane
is the distance at which the plane stops
Isolating from (1):
(2)
(3)
Finally:
This is the veocity the plane had before braking began