A `1 kg` mass is projected down a rough circular track `(radius = 2.0 m)` placed in vertical plane as shown. The speed of the mass at point `A` is `3 m//s` and at point `B`, it is `6.0 m//s`. How much is work is done on the mass between `A` and `B` by the force of friction?

Options

A. `- 7.5 J `
B. `-8.5J`
C. `-6.5J`
D. `-24j`

Answer

C

Explanation

Apply work energy theorem from A to B
` W _("air" ) = Delta "K.E." `
` 1 xx 10 xx 2+ W_("fr" ) =(1)/(2) (1) xx(6^(2)-3^(2))`
` 20 + W_("fr" ) = 13.5 implies W_("fr" ) =-6.5J`