D. Roberts PHYS 121 University of Maryland Physic² 121: Fundament°ls of Phy²ics I September 18, 2006

D. Roberts PHYS 121 University of Maryland Outline Why do things move? Newton’s Laws of Motion –In Chapter 4 Skipping “Freely Falling Objects” and Chapter 3 for now. Will come back to them later Forces

D. Roberts PHYS 121 University of Maryland Why do things move? We’ve developed a language and some tools to describe motion –Position –Velocity –Acceleration –Graphs –Equations of motion Now we want to talk about what makes things move in the first place

D. Roberts PHYS 121 University of Maryland Demo C4-03

D. Roberts PHYS 121 University of Maryland Discussion What can you say about what you just observed? –Try to use some of the physics language we’ve developed –…and maybe even try to use it correctly What happens when I tap the ball: –Along the direction of motion –Opposed to direction of motion What happens when I’m not tapping the ball?

D. Roberts PHYS 121 University of Maryland Newton’s First Law An object at that is at rest will remain at rest, or an object this is moving will continue to move in a straight line with constant velocity, if and only if the net force acting on the object is zero New ideas we need to understand: –Force –Net Force This Law basically says no cause is needed for an object to move. –Uniform (constant) motion is an object’s natural state Translation from original Latin: “Every body perseveres in its state of rest, or of uniform motion in a right (straight) line, unless it is compelled to change that state by forces impressed thereon”

D. Roberts PHYS 121 University of Maryland Forces Usually think of a force as a push or pull Vector quantity May be a contact force or a field force –Contact forces result from physical contact between two objects –Field forces act between disconnected objects Also called “action at a distance” More generally: –A force is what causes an object to change its velocity (accelerate) –Converse: If an object is accelerating, there must be a non-zero net force acting on it

D. Roberts PHYS 121 University of Maryland Contact and Field Forces

? A soccer player first kicks a soccer ball and then a cannon ball using the same kick. Which ball will leave his foot going faster? The soccer ball because it’s lighter. 2.The cannon ball because it’s heavier. 3.Both the same since he used the same kick. 4.The soccer ball, but for some other reason. 5.The cannon ball, but for some other reason. 6.Both the same, but for some other reason.

D. Roberts PHYS 121 University of Maryland Question? A soccer player first kicks a soccer ball and then a cannon ball using the same kick. Which ball will leave his foot going faster? –1. The soccer ball because it’s lighter. –2. The cannon ball because it’s heavier. –3. Both the same since he used the same kick. –4. The soccer ball, but for some other reason. –5. The cannon ball, but for some other reason. –6. Both the same, but for some other reason.

D. Roberts PHYS 121 University of Maryland Newton’s Second Law The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. –F and a are both vectors Can also be applied three-dimensionally

D. Roberts PHYS 121 University of Maryland Units of Force SI unit of force is a Newton (N) US Customary unit of force is a pound (lb) –1 N = lb See table 4.1 in Textbook