As with all sports, skateboarding involves a lot of intriguing physics. I’ve marveled at the maneuvers of skilled skateboarder Alex Hewitt (my grandson). When traveling along a horizontal surface, Alex crouches and then springs upward with his skateboard to continue horizontal motion along a nearly half-meter-high elevated surface (above).
He could easily do the same while wearing roller skates, which would be no big deal because the roller skates would be attached to his feet. But in no way is his skateboard so attached. So how does the skateboard manage to follow him along his upward trajectory? Furthermore, by what means does the board gain gravitational potential energy with no applied upward force and no apparent loss in kinetic energy?
This amazing feat bothered me because it seemed to contradict the laws of physics. I then watched a slow-motion video of Alex to learn how he does it.
Exerting a torque about the axis of the rear wheels
The leap that Alex executes is called an ollie, a blend of the physics of linear and rotational motion. While heading for the elevated surface, he crouches and springs directly upward while exerting a downward force on the tail of the board that produces a torque about the rear wheels. (Torque = force × distance about a rotational axis.) This quick downward snap of the tail, with or without its making contact with the ground, causes the board to rotate upward into the air (Figure 1).
The same thing happens when you give a sharp tap to the rounded end of a spoon lying on a table. The spoon flips up into the air, just as Alex’s skateboard does. The center of masses of both the spoon and board are raised by this snap-and-flip action.
Exerting a second torque about the board’s center of mass
Controlling lift goes further. While the airborne board rotates upward, Alex slides his forward foot toward the nose of the board and produces a second torque, in the opposite direction (Figure 2).
This second torque raises the tail, puts the board in contact with the back foot, and levels the board before it meets the elevated surface (Figure 3, below). So we see the results of two torques, one that flips the board upward and one that levels it off. Skillfully executed, this sequence enables Alex and his skateboard to meet the elevated surface.
The kinetic energy of Alex and his skateboard before and after the ollie is practically the same. Yet he and the board have gained substantial gravitational potential energy as the board rides atop the elevated surface. Does the ollie maneuver violate the conservation of energy? No, it does not. Here’s why: First, the energy that propels Alex himself is straightforward. By crouching and leaping, he converts bodily chemical energy into mechanical energy just as if he had jumped up from rest.
But what about the board? From where does it get the energy to move upward and follow Alex? The answer involves the work-energy
principle of mechanics (work done = change in energy). For straight-line motion, work = force × distance moved. For rotational motion, work = torque × angle moved.
Alex does work on the board when he produces a torque that flips it into projectile motion. As with all projectiles, acquired kinetic energy converts to gravitational potential energy.
Another explanation, the simplest, bypasses rotational mechanics and energy conservation. The force that Alex exerts on the tail of the board as he jams downward on it is significantly greater than both his weight and that of the board. In action-reaction fashion, this downward push produces an upward normal force (the perpendicular support force) that is also greater than the combined weights of
Alex and the board (Figure 4). This increased normal force launches both Alex and the board into projectile motion.
One of the beauties of physics is that puzzles often have more than one explanation.
Hooray for the conservation of energy and for skateboarding in general. ■
Paul G. Hewitt (firstname.lastname@example.org) is the author of the popular textbook Conceptual Physics, 12th edition, and coauthor with his daughter Leslie and nephew John Suchocki of Conceptual Physical Science, 6th edition.
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