12.23

To transport a series of bundles of shingles A to a roof, a

contractor uses a motor-driven lift consisting of a horizontal

platform BC which rides on rails attached to the sides of a

ladder. The lift starts from rest and initially moves with a

constant acceleration a₁ as shown. The lift then decelerates

at a constant rate a2 and comes to rest at D, near the top of

the ladder. Knowing that the coefficient of static friction

between a bundle of shingles and the horizontal platform is

0.30, determine the largest allowable acceleration a₁ and the

largest allowable deceleration as if the bundle is not to slide

on the platform.

0.8 m

B

65°