A book weighing 20. newtons slides at constant velocity down a ramp inclined 30.° to the horizontal as shown in the accompanying diagram. What is the force of friction between the book and the ramp?

1 answer

To find the force of friction acting on the book as it slides down the ramp at constant velocity, we first need to understand the forces acting on the book.

  1. Weight of the book (W): The weight of the book is given as 20 N.

  2. Components of the weight: The weight can be broken down into two components:

    • Parallel to the ramp (W_parallel): This is the component of the weight that acts down the ramp.
    • Perpendicular to the ramp (W_perpendicular): This is the component of the weight that acts into the ramp.

    The formulas for these components are: \[ W_{\text{parallel}} = W \sin(\theta) \] \[ W_{\text{perpendicular}} = W \cos(\theta) \] where \(\theta\) is the angle of the ramp (30° in this case).

  3. Calculating the components: \[ W_{\text{parallel}} = 20 , \text{N} \cdot \sin(30°) = 20 , \text{N} \cdot 0.5 = 10 , \text{N} \] \[ W_{\text{perpendicular}} = 20 , \text{N} \cdot \cos(30°) = 20 , \text{N} \cdot \left(\frac{\sqrt{3}}{2}\right) \approx 17.32 , \text{N} \]

  4. Force of friction: Since the book is sliding down the ramp at constant velocity, the net force acting along the ramp is zero. This means that the force of friction (f_friction) must balance out the component of the weight acting down the ramp (W_parallel). Therefore: \[ f_{\text{friction}} = W_{\text{parallel}} = 10 , \text{N} \]

So, the force of friction between the book and the ramp is 10 Newtons.