The problem
Consider a block of mass m sliding down an infinitely long inclined plane shown below with a frictional force that is given by
- [math]F_f = \mu mg[/math]
200 px
Find the blocks speed as a function of time.
Step 1: Identify the system
- The block is the system with the following external forces, A normal force, a gravitational force, and the force of friction.
Step 2: Choose a suitable coordinate system
- A coordinate system with one axis along the direction of motion may make solving the problem easier
Step 3: Draw the Free Body Diagram
200 px
Step 4: Define the Force vectors using the above coordinate system
- [math]\vec{N} = \left | \vec{N} \right | \hat{j}[/math]
- [math]\vec{F_g} = \left | \vec{F_g} \right | \left ( \sin \theta \hat{i} - \cos \theta \hat{j} \right )= mg \left ( \sin \theta \hat{i} - \cos \theta \hat{j} \right )[/math]
- [math]\vec{F_f} = - \mu mg \hat{i}[/math]
Step 5: Use Newton's second law
in the [math]\hat i[/math] direction
- [math]\sum F_{ext} = mg \sin \theta - \mu mg= ma_x = m \frac{dv_x}{dt}[/math]
- [math]\int_0^t g \left ( \sin \theta - \mu \right ) dt = \int_{v_i}^{v} dv [/math]
- [math]v= v_i - g \left ( \mu -\sin \theta \right ) t [/math]
The amount of time that lapses until the blocks final velocity is zero
[math]t= \frac{v_i}{\left ( \mu - \sin \theta \right ) }[/math]
After the above time the blocks speed is zero. The friction will change from being kinetic to static after the above time interval.
- [math]v(t) =\left \{ {v_i - g \left ( \mu -\sin \theta \right ) t \;\;\;\;\;\;\;\; t\lt \frac{v_i}{\left ( \mu - \sin \theta \right ) } \atop 0\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\; t\gt = \frac{v_i}{\left ( \mu - \sin \theta \right ) }} \right .[/math]
- [math]V(x) =\left \{ {0 \;\;\;\; x \lt 0 \atop V_o \;\;\;\; x\gt 0} \right .[/math]
Forest_UCM_NLM#Block_on_incline_with_friction
