Question

A 5kg object slides down a frictionless surface inclined at an angle of 30º from the horizontal. The total distance moved by the object along the plane is 10 meters. The work done on the object by the normal force of the surface is? The answer is zero. They they that the angle is 90degrees?? But it says 30 degrees! where do they get cos(90) from? me I get cos(30)

Answer 1

We know work by force = F.X

i.e.,( dot product of F and X)

= F*X*cos(Ɵ)

where (Ɵ) = angle b/w F and X

now normal makes 90 degree angle with displacement

so cos(90) = 0 learnt from tables

work by normal will be 0

  

Since they don't make 90 degrees with displacement, forces will do nonzero function

   (OR)

To the inclined surface the normal force is perpendicular

Hence the distance between displacement and normal force

i.e., (Ɵ)=90degree

we know normal force Fn = M*g*cos30

work done = F*s*(Ɵ = 0