Pulleys, wheels, and gears Part 1: Pulleys and efficiency, Slides of Physics

Download Pulleys, wheels, and gears Part 1: Pulleys and efficiency and more Slides Physics in PDF only on Docsity! 4/11/14 1 Pulleys, wheels, and gears Part 1: Pulleys and efficiency Objectives •  Calculate the mechanical advantage of pulley systems. •  Calculate the efficiency of a machine. Jasmine uses a block and tackle system, pulling with 10 N of force over a distance of 5 m. By doing so, she lifts a 40 N weight by 1 m. 1.   What is the mechanical advantage of her block and tackle? 2.   What is the efficiency of her block and tackle? Assessment Physics terms •  pulley •  block and tackle •  efficiency •  mechanical advantage Equations The mechanical advantage is the ratio of the output force to the input force. Ideal mechanical advantage is the mechanical advantage in an ideal, frictionless world. It equals the input distance divided by output distance. 4/11/14 2 Equations Efficiency is the ratio of work output to work input. Pulleys and ropes A pulley is a simple machine that can change the direction of a force. Pulleys and ropes Combinations of pulleys can be used to change the magnitude of a force as well as its direction. A combination of pulleys is called a block and tackle system. Investigation How do you use pulleys to lift a heavy object? Does a pulley system reduce the work you need to do? Investigation 12B: Pulleys appears on page 339 of your textbook. Investigation 1.  Use the spring scale to measure the weight of the mass provided. 2.  Run cotton or nylon string over one pulley. Attach the mass and the spring scale to the ends of the string. Part 1: Block and tackle pulley system Be sure to pull at a slow, constant speed. Measure the force F and distance d needed to lift the mass by 20 cm. Investigation Part 1: Block and tackle pulley system 3.   Construct a two-pulley and a three- pulley block and tackle system. Carefully thread each pulley system as shown in the diagram. Repeat the measurements of distance d and force F for each. 4/11/14 5 Efficiency Click this interactive calculator on page 341. Efficiency Calculate the efficiency of the three pulley system you used in the investigation. Do you think adding more pulleys to your system makes it more efficient or less efficient? Why? Jasmine uses a block and tackle system, pulling with 10 N of force over a distance of 5 m. By doing so, she lifts a 40 N weight by 1 m. 1.  What is the mechanical advantage of her block and tackle? 2.  What is the efficiency of her block and tackle? Assessment Jasmine uses a block and tackle system, pulling with 10 N of force over a distance of 5 m. By doing so, she lifts a 40 N weight by 1 m. 1.   What is the mechanical advantage of her block and tackle? 4 2.   What is the efficiency of her block and tackle? The output force is 40 N, and the input force is 10 N. MA = F o /F i = 4 Assessment Jasmine uses a block and tackle system, pulling with 10 N of force over a distance of 5 m. By doing so, she lifts a 40 N weight by 1 m. 1.  What is the mechanical advantage of her block and tackle? 4 2.  What is the efficiency of her block and tackle? 80% Assessment The output force is 40 N, and the input force is 10 N. MA = F o /F i = 4 Efficiency = work output/work input = (40 J) / (50 J) = 80% Part 2: Wheels and gears 4/11/14 6 Objectives •  Calculate the mechanical advantage of wheels and gears. 1.   The wheel on a well has a 40 cm radius and the shaft that lifts the bucket has a radius of 5.0 cm. What is the mechanical advantage? 2.   If the input gear turns once and the output gear turns 7 times, what is the mechanical advantage? Assessment Physics terms •  wheel and axle •  gear •  gear ratio Equations The mechanical advantage for a gear system is the ratio of the torque on the output gear to the torque on the input gear. The mechanical advantage for a wheel and axle is the radius of the wheel divided by the radius of the axle Wheel and axle A wheel and axle provide a mechanical advantage. Compare this wheel and axle to the lever system. Can you guess the formula for the mechanical advantage of the wheel and axle? Wheel and axle A wheel and axle provide a mechanical advantage. The mechanical advantage is the ratio of the wheel radius to the axle radius. 4/11/14 7 Wheel and axle Click the interactive calculator on page 342. Wheel and axle If the wheel radius is 60 cm and the axle radius is 5 cm, what is the mechanical advantage? 0.60 0.05 Wheel and axle 0.05 12 If the wheel radius is 60 cm and the axle radius is 5 cm, what is the mechanical advantage? MA wheel = 12 0.60 Gears Gears are a related type of simple machine. Gears differ from a wheel and axle in that the output gear rotates in the opposite direction of the input gear. The gear ratio is the ratio of output turns to input turns. Gears The mechanical advantage of a gear system is the ratio of the torques. Notice: the mechanical advantage is the inverse of the gear ratio. Gears Click the interactive calculators on page 343.