Chapter 6 Friction and Projectiles

Chapter 6 Friction and Projectiles - Chapter 6: Friction...

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Chapter 6: Friction and Projectile Motion Equilibrium We will get into Newton’s laws in great detail in chapter 7, but will apply his first law at this time. To paraphrase the first law: If an object is not accelerating, the vector sum of the forces acting on the object is zero. Written as an equation it looks like this: F 0 Σ = . The letter Σ is called “sigma” and it means the sum of. Another phrase used instead of vector sum is to call it the net force . So the equation can also be written as: net F =0 . If the net force is zero (or the sum of the forces is zero), the forces on the object are said to be balanced. I f the net force is not zero (the sum of the forces is not zero), the forces are said to be unbalanced . So, if the object is not accelerating the net force is zero, and the forces are balanced. Force diagrams A force diagram (sometimes called a free body diagram) is a device we use to show the forces acting on an object, the direction they are acting and in some cases the relative size of the force. The steps we will use are shown below for the block in the diagram above as it slides across the counter at a constant velocity: 1. Draw a rough outline of the object in question. 2. Place an arrow on your outline for each force acting on the object. Point the arrow in the direction of the force. Label the arrow with the type of force, the object causing the force and the object the force is acting on. On the block, the label on the upward arrow represents the normal force of the counter on the block. The arrow to the right represents the tension force of the string on the block. The arrow to the left represents the kinetic friction force of the counter on the block. The arrow down represents the force of gravity of the earth on the block. Every label should end with the same letter. 3. If the object is in equilibrium, the arrows should be sized so they add up to zero. If the object is accelerating, the object will have a net force in the direction of the acceleration. We were told the block was sliding at a constant velocity, so the left and right arrows are of the same size. The block is not moving up or down so those two arrows are also of the same size. 1 cb F K f F N cb F g eb F T sb Force diagram for the block above as is moves across the counter at a constant velocity
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Objects on an incline Objects on an incline present a more difficult situation because of the orientation of the forces. Diagram a shows a force diagram for a block at rest on an incline. There are three forces exerted on the block, the earth is pulling it down (weight), the incline is pushing it perpendicular to the surface (normal) and pushing it parallel to the surface (friction). We know the all of the forces are balanced because the block is not accelerating. Where are the forces balancing the friction and the normal force? The weight force can be split into two components. One component is the part of the weight that is pulling the block down the plane. This component will be called F  , for force parallel to the surface of the plane.
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This note was uploaded on 10/15/2009 for the course PH 1511 taught by Professor Pro during the Spring '09 term at Cy-Fair College.

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Chapter 6 Friction and Projectiles - Chapter 6: Friction...

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