Order-of Magnitude Calculations Sometimes it is necessary to know a quantity only within a factor of 10 This is know as an order of magnitude For example that is the total mass of everyone in this class? mass of a person m- 75 kg Number of people n 475 la~7575kg=5625kg~6103kg Physics 121: Lecture 2, Pg 6
Physics 121: Lecture 2, Pg 6 Order-of Magnitude Calculations Sometimes it is necessary to know a quantity only within a factor of 10 This is know as an order of magnitude For example that is the total mass of everyone in this class? mass of a person m ~ 75 kg Number of people n ~ 75 mTotal ~ 75 ´ 75 kg = 5625 kg ~ 6 ´ 103 kg
Problem solution method Five steps: 1) Focus the Problem draw a picture- what are we asking for? 2)Describe the physics what physics ideas are applicable what are the relevant variables known and unknown 3) Plan the solution what are the relevant physics equations 4)Execute the plan solve in terms of variables solve in terms of numbers 5) Evaluate the answer are the dimensions and units correct? do the num bers make sense? Physics 121: Lecture 2, Pg 7
Physics 121: Lecture 2, Pg 7 Problem Solution Method: Five Steps: 1) Focus the Problem - draw a picture – what are we asking for? 2) Describe the physics - what physics ideas are applicable - what are the relevant variables known and unknown 3) Plan the solution - what are the relevant physics equations 4) Execute the plan - solve in terms of variables - solve in terms of numbers 5) Evaluate the answer - are the dimensions and units correct? - do the numbers make sense?
Chap 2: Forces and vectors In classical mechanics Need to study interactions between objects Described by forces We have an idea of what a force is from everyday life Physicist must be precise A force is that which causes a body to accelerate (See Newton's Second Law) A Force is a push or a pull A Force has magnitude& direction(vector) F Physics 121: Lecture 2, Pg 8
Physics 121: Lecture 2, Pg 8 Chap.2: Forces and vectors In classical mechanics Need to study interactions between objects Described by forces We have an idea of what a force is from everyday life. Physicist must be precise. A force is that which causes a body to accelerate. (See Newton’s Second Law) A Force is a push or a pull. A Force has magnitude & direction (vector). F
Fundamental Forces Example of Forces Hooke' s law for ideal spring: F=-kX Units of a force are 1 N 1 kg m/s 2 Fundamental Forces Gravity(more later) >) For motion of planets, etc Strong and weak nuclear forces(not here !) >)Explains behavior of nucleus in atoms Electromagnetic force(next semester in PHY122) >)Relevant for electric systems, chemical properties, etc Physics 121: Lecture 2, Pg 9
Physics 121: Lecture 2, Pg 9 Fundamental Forces Example of Forces Hooke’s law for ideal spring: F = -k x Units of a force are 1 N= 1 kg m/s2 Fundamental Forces Gravity (more later) »For motion of planets, etc. Strong and weak nuclear forces (not here !) »Explains behavior of nucleus in atoms Electromagnetic force (next semester in PHY122) »Relevant for electric systems, chemical properties, etc