You and your friend Peter are putting new shingles on a roof pitched at 25 . You
ID: 1774647 • Letter: Y
Question
You and your friend Peter are putting new shingles on a roof pitched at 25 . You're sitting on the very top of the roof when Peter, who is at the edge of the roof directly below you, 5.5 m away, asks you for the box of nails. Rather than carry the 2.0 kg box of nails down to Peter, you decide to give the box a push and have it slide down to him.
If the coefficient of kinetic friction between the box and the roof is 0.50, with what speed should you push the box to have it gently come to rest right at the edge of the roof?
Explanation / Answer
initial speed vi = ?
final speed at the edge of roof vf = 0
distance travelled l = 5.5 m
work by gravitational force Wg = m*g*sintheta*l
work done by frictional force Wf = -uk*m*g*costhet*l
from work energy relation
Wtotal = change in kinetic energy
Wg + Wf = Kf - Ki
m*g*sintheta*l - uk*m*g*costheta = (1/2)*m*(vf^2-vi^2)
(2*9.8*sin25*5.5) - (0.5*2*9.8*cos25*5.5) = (1/2)*2*(0-vi^2)
initial push vi = 1.814 m/s <<<<<+++++++++ANSWER
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