Number of significant digits in 0.0028900 is ______
a) 5
b) 6
c) 7
d) 8

Explanation

Suppose the speed of the car at the topmost point of the loop si v. Taking the gravitatioN/Al potential energy to be ero at the platform and assuming that the car starts with a negligible speed the conservation of energy shows
`0=-mgh+1/2mv^2`
or, `mv^2=2mgh`
where m is the mass of teh car. The car moving in a circle must have radial aceleration `v^2/R` at this instant. The force on the car are mg due to gravirty and N due to the contact with the tgrack. Both these forces are n radial direction at the top o the loop. Thus, from Newton's law
`mg+N=(mv^2)/R`
or, `Mg+N=2mgh/R`
For h to be minimum N should assume the minimum value which can be zero. Thus,
`2mg (h_(min))/R=mg or h_(min)=R/2`