Physical Science 8th grade (1).pdf

Newtons second law of motion relates to the

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Newton’s second law of motion relates to the acceleration of an object. Acceleration depends upon the mass of the object and the force(s) acting on the object. Newton’s third law is probably the best known of his laws of motion. It states that every action has an equal and opposite reaction. How things have changed When skateboarding was invented, a skateboard was simply a two-by-four on roller skate wheels. The goal was to start at the top of a hill and ride the skateboard to the bottom without crashing. It wasn’t long before skateboarders wanted the sport to be more challenging. Today, skateboarders use boards with wheels that reduce friction. They make incredible moves. Even the most basic skateboarding moves rely on the laws of physics. A good example is skateboarders in half-pipes. The half-pipe is a U- shaped ramp that usually has a flat section in the middle. In the half-pipe, controlled acceleration is absolutely essential. On flat ground, gaining speed is easy. Push with one foot, and you accelerate. Half-pipes are a little trickier. They require “pumping.” Pumping makes use of Newton’s second law. When skateboarders pump, they drop down into a crouch as they roll through the flat bottom of the U-shaped half-pipe. As they enter the sloped part of the ramp, called the transition, they straighten their legs. By standing up in this way, skateboarders raise their center of mass. The sudden shift in their center of mass gives them more energy. More energy means acceleration. The shift in the center of mass is in the direction the skateboarder is moving. The skateboard accelerates in that direction. Chapter 14 Connection
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305 U NIT 5 M OTION AND F ORCE This is like pumping on a swing. The swing goes higher if you lift your legs while passing through the bottom of the swing’s arc. At the top of the arc, you drop your legs. The more times you repeat this movement, the more energy you gain. Gained energy translates into swinging higher. This same concept is what helps skateboarders gain speed in the half-pipe until they have the enough height and speed to perform all kinds of stunts. Motion in action How does Newton’s third law affect skateboards? In essence, it is what allows them to move. For each of the skateboarder’s actions, there is an equal and opposite reaction. A skateboarder pushes against the concrete at the top of a half-pipe. The concrete pushes back. The skateboarder is in motion. Friction between the wheels and the concrete is a force that acts on the motion. Gravity is another force that acts on the skateboarder’s motion. Have you ever watched skateboarders in a half-pipe? If so, you know that they overcome the force of friction in some pretty dramatic ways. In one impressive stunt, skateboarders soar above the half- pipe and perform a move called the “frontside 180.” It’s a trick that puts a little more spin on physics. Skateboarders appear to hang in the air for a moment after flying out of the half-pipe. Then they skate back down the ramp.
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