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Understanding Force and Pressure in Physics

Explore the concept of force and pressure in physics, covering states of matter, momentum, and energy. Learn how molecules create pressure and exert force on surfaces. Discover the relationship between impulse, momentum, and external forces.

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Understanding Force and Pressure in Physics

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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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