9. The acceleration of an object down on an incline that makes an angle with the

Question

9. The acceleration of an object down on an incline that makes an angle with the horizontal and has a friction coefficient is
A. g sin
B. g cos
C. g (sin + cos)
D. g (sin - cos)
E. g tan

10. The acceleration of an object sliding up on an incline that makes an angle with the horizontal and has a friction coefficient is (positive x direction is up on the incline!)
A. -g sin
B. g (sin + cos)
C. - g (sin + cos)
D. g (sin - cos)
E. -g (sin - cos

9. The acceleration of an object down on an incline that makes an angle with the horizontal and has a friction coefficient is B. cose C. g (sin + cose) D. g (sin- cose) sine u cos 10. The acceleration of an object sliding up on an incline that makes an angle with the horizontal and has a friction coefficient is (positive x direction is up on the incline!) A. -ug sine B. g (sine+ cose) C. . g (sine+ cos) D. g (sine- cose) E. -g (sine-u cose)

Explanation / Answer

when the object is moving down

gravitational force Fg = - m*g*sintheta acts down the plane

frictional force f = +u*m*g*costheta acts up the plane

net force Fnet = -m*g*sintheta + u*m*g*costheta

from newtons law Fnet = -m*a

-m*a = -m*g*sintheta + u*m*g*costheta

a = g*(sintheta - u*costheta)

option D

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when the object is moving up

gravitational force Fg = -m*g*sintheta acts down the plane

frictional force f = -u*m*g*costheta acts down the plane

net force Fnet = -m*g*sintheta - u*m*g*costheta

from newtons law Fnet = +m*a acts up the plane

m*a = -m*g*sintheta - u*m*g*costheta

a = g*(-sintheta - u*costheta)

a = -g*(sintheta + u*costheta)

option C

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