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Transcript
Focus Question(s) If you shoot a dart at a low angle (0), then continue to shoot at a greater and greater angle (90), how will that effect… the distance the dart travels? the time the dart spends in the air? Chapter 6 Motion in Two Dimensions 6.1 - Projectile Motion A projectile is an object that is shot through the air. Three types of projectiles: Projectiles shot vertically (90 degrees) Projectiles shot horizontally (0 degrees) Projectiles shot at some angle between 0-90 degrees Projectiles The path that a projectile takes while in flight is called the trajectory. The shape of this path is called a parabolic curve (or parabola). The horizontal distance that the projectile travels is called its final displacement. The highest point in the trajectory is called the max height. Hangtime The time a projectile spends in the air is called its hangtime. If shot horizontally, the time is only the time down. If shot at any angle greater than 0 degrees, the projectile will travel up for a while, then down. The time up is equal to the time down. The closer the launch angle is to 90 degrees, the more time the projectile will spend in the air (assuming that the initial velocity is constant). Diagram of Horizontal Projectile vx vx= horizontal velocity dy dy= vertical height t = time to fall t dx= final displacement dx Diagram of an Angled Projectile dy-max Time up equals time down dx Horizontal Projectile formulas The horizontal velocity of a projectile does not change while in flight. Same formula from Chapter 2: dx vx t Horizontal Projectile formulas To find the time a horizontal projectile stays in the air, you only need to know how high it was dropped from (the only acceleration for a projectile is that of gravity): 2dy t g Horizontal Projectile Formulas To find the vertical velocity of a projectile that was initially shot horizontally, you will need to know the time that it was in the air: vy f vyi at Example 1 A ball is thrown from the top of a 125m tall structure with an initial horizontal velocity of 15m/s. How long will it take for it to hit the ground? How far will it land from the base of the structure? What was the vertical component of the velocity immediately before the ball hit the ground? Example 2 An arrow is shot horizontally from 2.0m above the ground and lands 75m away from the shooter. How long was it in the air? What was the initial speed of the arrow? Homework P. 150, #1-3 Journal #32 10/23/08 A bullet is dropped from 2.0m at the exact same time that a bullet is shot horizontally from 2.0m. Which bullet will hit the ground first? Why? Journal #32 Answer Velocity is not part of the time formula, so it has no effect on time. 2dy t g Answers to HW Answers to HW 6.2 - Circular Motion There are many objects that do not travel in straight lines or move along a trajectory. Some objects travel in a circular motion: Blades of a fan Cars going around a curve Satellites orbiting the earth Uniform Circular Motion The movement of an object or particle trajectory at a constant speed around a circle with a fixed radius (r). As an object moves around the circle, the length of the radius does not change. The acceleration of the object is toward the center of the circle causing the velocity to stay at a tangent. A tangent line is a line that passes through a single point of a circle and is perpendicular to the radius of that circle. Diagram of Circular Motion Centripetal Acceleration Centripetal acceleration always points to the center of the circle. Its magnitude is equal to the square of the velocity, divided by the radius of circular motion. 2 v ac r How can you measure the speed of an object moving in a circle? Speed is still calculated as distance traveled divided by the time it takes to travel that distance. In the case of circular motion, the distance is the circumference of a circle or (2πr). The time it takes to go around a circle one time is called the period (T). (2r) v T The “New” Net Force? We know that an object must have a force acting on it for it to accelerate… so what is this mysterious force that is pulling or pushing towards the center of the circle? A net force that in a direction towards the center of a circle is called the centripetal force. Centripetal Force The centripetal force is really a costume artist… it can be whichever force is causing the acceleration to the center of the circular motion. Examples: A car going around a curve - Friction Force A satellite in orbit - Force of Gravity A ball on a string - Tension Force The first step for any circular motion problem is to identify the force that is acting as the centripetal force. Using Newton’s Second Law Again Newton’s 2nd Law stated that net force on an object is equal to the mass times the acceleration. This holds true, but we substitute in centripetal acceleration: Fnet mac Does Centrifugal Force Exist??? If you have ever been in a car that suddenly turned to the left, your body may have been thrown to the right… does that mean that there is some outward force? No… the car around you has moved and you have tried to maintain your original path (according to Newton’s 1st Law). Example 1 An athlete whirls a 7.00kg hammer 1.8m from the axis of rotation in a horizontal circle. The hammer makes one revolution in 1.0s. What is the centripetal acceleration of the hammer? What is the tension in the chain? Example 2 If a 0.40kg stone is whirled horizontally on the end of a 0.60m string at a speed of 2.2m/s, what is the tension in the string? HW: P. 156, #12-15