Question 856062
These are not algebra problems but rather physics problems. 
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Q47: Gravity acts downwards. Resolve the force into components along the plane and normal to the plane. Along the plane, 
{{{a=g*cos(60)=9.8*cos(60)=9.8*0.5=4.9}}}{{{m/s^2}}}
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Q42:Look at the work done by the friction force. It equals the initial kinetic energy of the object.
{{{mu*N*d=(1/2)mv^2}}}
{{{mu*mg*d=(1/2)mv^2}}}
{{{mu=v^2/(2gd)}}}
{{{mu=8^2/(2(9.8)(10.2))}}}
{{{mu=0.32}}}
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Q49: Since there is a friction force, set up a force balance down the plane.
{{{mg*cos(60)-mu*N=ma}}}
{{{2(9.8)(0.5)-3.8=2a}}}
{{{2a=9.8-3.8}}}
{{{2a=6}}}
{{{a=3}}}{{{m/s^2}}}