Try it on paper first. A hint, the key idea and the worked solution are here when you need them.
A uniform solid ball of mass and radius is launched along a rough horizontal floor. At the start its centre moves forwards at , and it spins backwards at (backspin), as shown in the figure. The coefficient of sliding friction between the ball and the floor is . Take .
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. , , , , backspin , .
Step 1. Friction at the start. The bottom of the ball moves forwards at because of the centre's motion, and forwards again at because of the backspin. So it slides forwards at , and friction acts backwards, with . It slows the centre at and reduces the backspin at .
Step 2. Angular momentum about the floor. Take a fixed point on the floor. Friction acts along the floor, so it has no torque about that point; the weight and the normal force cancel along one vertical line. So the angular momentum about that point is conserved. Taking forward rolling as positive:
When the ball rolls, and . So , forwards.
Step 3. Time. The centre slows at from to : . In that time the spin goes from to , which is , as rolling needs.
Step 4. Heat. At the start: . When rolling: . So has turned into heat.
Step 5. Coming back. The final velocity points backwards if , that is if : .
Answer: (a) Backwards; it slides for . (b) forwards. (c) . (d) More than .
Check. The heat is also friction times the distance the contact point slides over the floor. The sliding speed falls from to zero at , so it slides , and .
Watch out. Friction does not first stop the spin and then the motion: it changes both at once, from the very start, until the contact point stops sliding.
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