1) A standard bathroom scale is placed on an elevator. A 26 kg boy enters the el
ID: 1330826 • Letter: 1
Question
1) A standard bathroom scale is placed on an elevator. A 26 kg boy enters the elevator on the first floor and steps on the scale. What will the scale read (in newtons) when the elevator begins to accelerate upward at 0.4 m/s2?
2) A person pushes two boxes with a horizontal force of 140 N going to the right. The boxes are on a horizontal frictionless floor as shown in the figure below.
If the mass of box A is 5 kg and the mass of box B is 3 kg, calculate the magnitude of the action–reaction pair between the two boxes.
Explanation / Answer
(1)
In this case, Force on cable acts upwards and weight acts downward. Thus, the net force equation can be written as follows:
Fnet = F - mg = ma
Solve for F
F = ma +m g
= m ( a + g )
= 26 kg ( 0.4 m/s2 + 9.8 m/s2 )
= 265.2 N
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(2)
Boxes A and B push on each other with equal forces less than 140 N
In this case, the acceleration is,
a = F / mA + mB = 140 N / ( 5 kg + 3 kg ) = 17.5 m/s2
From Newton's second law,
FB = mB a = 3 kg ( 17.5 m/s2 ) = 52.5 N
Boxes A and B push on each other with equal forces, 52.5 N which is less than 140 N
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