1) A block weighinp 300N is moved at Constant Speed over a horizontal surface by a force of 50.0N applied parallel to the surface. a.What is the coefficient of kinetic friction b.What will be acceleration of the block if uR=0?
SOLUTION at constant speed the acceleration is zero.
Normal reaction R=300N
let f=frictional force
50-f=0 f=50N but f=uR u=50/300=1/6
b. a=F/m=50/300=1/6N

2)The half life of a certain radioactive element is T. How long will it take for 75% of the sample of the element to decay.
SOLUTION The general formula is N(t)=N*2^(-t/T) N(t) = amount left after decay N=original amount N(t)=0.25, N=1 ==>0.25=2^(-t/T) log20.25=-t/T t=-Tlog20.25=2T