Download Newton's Laws of Motion: Understanding Force and Momentum and more Exams Physics in PDF only on Docsity! Physics 3A: Newton's Laws Now we shift from quantitative descriptions of motion to quantitative descriptions of what causes motion: Newton's three laws of motion First, though we need to expand our "notion of motion" There is more to "motion" than just velocity and acceleration Would you rather be hit by a bowling ball moving 2 m/s or by a nerf football moving 2 m/s? So need more than velocity to describe full motion Define momentum as: where m is the mass of the object, v is the velocity (vector) and p is the momentum (vector) p m v (8.1) Equation (8.1) is a vector equation, so: What are the dimensions of momentum? What changes momentum (motion) of an object? FORCE Newton found that the change in momentum per unit time was equal to the force applied: (8.2) Fav p t but, p is a vector, so: Fav p t Physics 3A: Newton's Laws p ML T px m vx py m vy pz m vz So lets look at the concept of force What is a force? "a push or pull" Can classify forces as: Contact Forces: physical contact is made between the objects Examples? hitting a home run, performing CPR, when you hit the ground after falling out of bed, pulling the fire alarm Field Forces: no contact is made between the objects Examples? pushing one magnet with another magnet without them touching, gravity pulling on you as you fall out of bed Force is a vector (magnitude and direction) Strong person demo. Physics 3A: Newton's Laws (Do Example) We define the units of force to be the Newton (N) Back to Newton's three laws of Motion These laws govern the motion of objects of macroscopic size i.e. bigger than atoms ( > 10-10 m) (Objects smaller than this size are governed by Quantum Mechanics (see P3C) Newton formulated these laws by studying the relationship between forces and the changes in motion they cause (Newtonian Mechanics (Classical)) What did Newton observe? (actually first done by Galileo) "Every body persists in its state of rest or uniform motion in a straight line unless it is compelled to change that state by forces impressed on it." Physics 3A: Newton's Laws My translation: "A body at rest will remain at rest until acted upon by a net force." "A body in straight-line uniform motion will remain in that motion until acted upon by a net force." This is Newton's First Law What does this mean in terms of what we have learned about motion? That "no net forces applied" means "no acceleration" The 1st Law indicates a relationship between being at rest and moving with constant velocity They are related by a change in reference frame All reference frames in which a body has zero acceleration with zero applied net force are called inertial reference frames Physics 3A: Newton's Laws a F Lets go back to the bowling ball and nerf ball example Your face has to apply a larger force to the bowling ball to stop it than it takes to stop the nerf ball. This comes from: Since v is same for bowling ball or nerf ball, it must be the difference in mass which causes this You could also say the bowling ball has more "resistance" to changes in its motion, so mass measures this "resistance" We call this Inertial Mass Inertial mass is a measure of an object's resistance to changes in its motion Fav p t m v t Physics 3A: Newton's Laws How can you measure mass? Consider: so Can also measure "mass" using gravity, but it is conceptually a different quantity, although they have the same value NOTE: mass and weight are not the same quantity! m1 m2 a2 a1 (4.1) Physics 3A: Newton's Laws Newton's Second Law quantifies how forces change motion We know that a force causes an change in motion, i.e. an acceleration Consider: Newton found that if F 2 is double F 1 , then a 2 is double a 1 then a is directly proportional to F Newton also found that (in diagram slide 80) that if m 2 is double m 1 then a 2 is half a 1 so that a is inversely proportional to m ! Physics 3A: Newton's Laws
