Newton's Second Law of Motion and Forces: Concepts and Equations - Prof. Wolfgang Losert, Study notes of Physics

Download Newton's Second Law of Motion and Forces: Concepts and Equations - Prof. Wolfgang Losert and more Study notes Physics in PDF only on Docsity! 1 Phys141 – Mon 9/19 • Today: Chapter 5.4-7 (won’t get to 5.8) Administrative: – Midterm 1: Oct 5 Mix of conceptual questions and quantitative questions Material up to Chapter 8 • For Wed: Read: Ch 5.8 (if you had not yet done so for today) Chapter 6.1 and 6.2 (skip rest of Ch 6) Newton’s Second Law In inertial frame: Force = Mass * acceleration or – Need force to accelerate – Larger Mass, less acceleration for same force – Law for an individual object – More than one force can be applied to one object: vector sum of all forces yields net force on object! ΣF = m a Force acceleration Mass = Units of Force? dimensional analysis of Newton’s 2nd law: F=ma Derived units: Newton (N) 1N =1 2 m kg s 2 m kg s Does the reading on the scale change when the string breaks (Demo C4-61) 33% 33% 33% 1 2 3 1. Yes 2. No 3. None of the above. 4039383736353433323130292827262524232221 2019181716151413121110987654321 Field forces act through empty space – can act over long distance Contact forces involve physical contact between two objects Gravity Electrical Magnetic 2 Fundamental Forces • Gravitational force – Between two objects • Electromagnetic forces – Between two charges • Nuclear force – Between subatomic particles • Weak forces – Arise in certain radioactive decay processes Other forces (including all contact forces) are result of fundamental forces Examples: Friction force: Caused by complicated sum of electromagnetic forces between many particles in the two surfaces that rub against each other. Spring force: Result of electromagnetic forces between atoms in the spring Force a wall exerts on a ball thrown against the wall: Result of electromagnetic forces between atoms in the ball and wall More About Newton’s Second Law Like any vector equation (see e.g. 2D motion) Newton’s Second Law can also be expressed in terms of components: ΣFx = m ax ΣFy = m ay ΣFz = m az Equilibrium: Zero Net Force Net force is equal to zero: Equilibrium – The acceleration is equal to zero – The velocity is constant • The object, if at rest, will remain at rest • If the object is moving, it will continue to move in same direction at same speed 0 0 and 0x y F F F = = = ∑ ∑ ∑