Moments And Levers P
Übersicht
Try to shut a heavy door by pressing on it a centimetre from the hinges. You will push until your arm aches and the door will barely shift. Move your hand out to the handle, use a fraction of that effort, and the door swings closed. Nothing about the door has changed and nothing about your strength has changed. What changed is the distance, and distance is the second half of every turning problem in physics.
This lesson gives you the quantity that measures a turn: the moment of a force, worth force multiplied by perpendicular distance, measured in newton metres. You will use it to balance a beam by setting the clockwise turns against the anticlockwise ones, to work out an unknown force or an unknown distance from that balance, to predict the exact moment a crate or a bus stops leaning and starts falling, and to explain how a crowbar lets a pair of hands move a boulder that no pair of hands could ever lift.
Ziele
- The turning effect of a force is called the moment. The relationship between the moment, M, turning force, F, and perpendicular distance, d, from the point where the force is applied to the pivot is: M = F ×d
- If an object is not turning, the total clockwise moment must be exactly balanced by the total anticlockwise moment about any pivot. Students should be able to calculate the size of a force, or its distance from a pivot, acting on an object that is balanced.
- If the line of action of the weight of an object lies outside the base of the object there will be a resultant moment and the body will topple. Examples should include vehicles and simple balancing toys.
- Simple levers can be used as force multipliers.
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- What is the unit of the moment of a force? A. newton B. newton metre C. newton per metre D. metre Answer: B
- A force of 25 N acts at a perpendicular distance of 0.40 m from a pivot. What is the moment of the force about the pivot? A. 0.016 N m B. 10 N m C. 25.4 N m D. 62.5 N m Answer: B
- A beam is pivoted at its centre. A force of 60 N acts 0.50 m to the left of the pivot. A force F acts 0.25 m to the right of the pivot and the beam is balanced. What is F? A. 30 N B. 60 N C. 120 N D. 240 N Answer: C
- An object standing on a flat surface will topple when A. its weight is greater than the contact force from the surface B. the line of action of its weight lies outside its base C. its centre of mass is above the middle of its base D. the clockwise and anticlockwise moments about its edge are equal Answer: B
- A crowbar allows a small effort to move a much larger load. This is because a simple lever acts as a A. distance multiplier B. energy multiplier C. force multiplier D. speed multiplier Answer: C
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Simple Machines
Simple machines are devices that can be used to multiply or augment a force that we apply – often at the expense of a distance through which we apply the force. The word for “machine” comes from the Greek word meaning “to help make things easier.” Levers, gears, pulleys, wedges, and screws are some
"College Physics: Simple Machines" by OpenStax, OpenStax, Rice University, licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Source: https://openstax.org/books/college-physics/pages/9-5-simple-machines. Access for free at https://openstax.org/books/college-physics/pages/1-introduction
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