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Lecture 04

Lecture 04. WEB. Force on the “Wall”. Time. What’s Happinin?. One Ball:. What is the “ Average ” Force??. F. One Ball:. Force on the “Wall”. Time Interval. 1 2 3 4 5 6. Force on the “Wall”. F. Time Interval. Averaging.

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Lecture 04

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  1. Lecture 04 WEB

  2. Force on the “Wall” Time What’s Happinin? One Ball:

  3. What is the “Average” Force?? F One Ball: Force on the “Wall” Time Interval 1 2 3 4 5 6

  4. Force on the “Wall” F Time Interval Averaging

  5. Last time we defined pressure • The force is created by molecules “bouncing” off of the wall or surface. • We need to quickly look at why there is a force.

  6. Movement PRESSURE: HIGH LOW

  7. Remove the Wall High Low Pressure FLOW

  8. Weather Map Now you know! H

  9. Molecules Move Around! WALL

  10. An Aside …States of Matter As the temperature of a material increases, the TEMPERATURE increases as well. (Definition of Temperature shortly)

  11. STATES OF MATTER • Solids • Liquid • Gas

  12. SOLIDS • Hold their shape. • Relatively heavy • Examples • rocks • houses • mountains • computers • salt, sugar

  13. LIQUIDS • Conforms to the shape of the container • Can be light or heavy • Water is the most important example

  14. GAS • Will conform to any container. • If we increase the size of the container, the gas will EXAPAND to fill the new container. • Liquids or solids will not do this. diluted LIQUID GAS

  15. Which is Heavier? • A cubic foot of a solid. • A cubic foot of a liquid. • A cubic foot of a gas. • You can’t tell without more information. Cross-Tab Label 0/0

  16. WHY DO WE WE CARE ABOUT GASSES?? • We breath air which is a gas. • Sound travels from the source to our ears through the air. • It also can travel through solids and liquids. • Whales can “talk to each other” over a distance of hundreds or perhaps thousands of miles of water! • Note: Sound does NOT travel over wires from one phone to another. VERY DIFFERENT

  17. States of Matter – ExampleWATER • Below 32 degrees F … SOLID • ice • Between 32 and 212 degrees F … LIQUID • water • Greater than 212 degrees F … GAS • steam

  18. We need two more concepts • Momentum • Energy

  19. Consider a mass m with a “velocity” v v m IMPORTANT DEFINITION momentum = (mass) x (velocity) or p=mv

  20. Remember??

  21. DEFINITIONS • IMPULSE = (Force x time it acts)=F x t • IMPULSE = CHANGE IN MOMENTUM

  22. Force on the “Wall” Time How Big is this force?? Average Force

  23. One Crash v pinitial=mv v pfinal= - mv pfinal – pinitial = 2 mv F x t = 2mv

  24. To Calculate the Pressure • Use the previous equation to estimate F • Estimate the time of the “bounce” … short! • Look up the mass. • Check out the velocity • Then, find out how many collisions take place per second. • Multiply all these things together to find pressure. • We ain’t gonna do dat!

  25. An Aside on an “inelastic” crash INTERNAL Forces

  26. Conservation of Momentum External Force = 0 Conservation of Momentum

  27. InitialMomentum = M vinitial M M M M Final Momentum = (2M) x vfinal

  28. So • Initial Momentum = M vinitial • Final Momentum = 2M vfinal

  29. Summary • Pressure is caused by the presence of molecules. • When the molecules crash into the walls of a container, they push on it and supply an “impulse” which is the force multiplied by the time that the force (crash) exists. • The wall applies a FORCE to the molecule as well as an impulse. • The molecule changes its momentum. • The force exerted on the wall by the billions of molecules that hit the wall each second creates the pressure.

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