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Transcript
Newton 2nd Law • Today we understand Newton’s Second law and how it can be used to measure mass is a different way. Second Law of Motion: “The Law of Acceleration” The acceleration produced by a net force on an object is directly proportional to the magnitude of the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object v v F ma “sigma” = sum F and a are vectors What does this law tell us? • Objects that are not in equilibrium will accelerate. • Net force (sum of all forces) on an object causes acceleration. • Note the difference between a force and a net force. A good analogy is to compare deposits/withdrawals into a bank account with the account balance. click for applet Inertial and Gravitational Mass Inertial mass Relates to how a mass responds to an external force (also called a contact force). If you push a stalled car into motion you are testing its inertial mass. Gravitational mass Relates to how a mass responds to the force of gravity (also called a field force). If you lift up a stalled car you are testing its gravitational mass. r F ma inertial mass Fg mg gravitational mass In the equation for weight, g is no longer considered the acceleration due to gravity, but rather the gravitational field strength, with units of newtons/kilogram. Inertial and gravitational masses have been tested and are believed to always be equal in amount. This is why all objects freefall at the same rate of acceleration. Constant acceleration from gravity • Galileo found that balls of different masses fell at the same rate. • The force of gravity is stronger for more massive objects. • But it requires more force to accelerate a more massive object (inertia). • These two effects cancel each other out. How does this relate to gravity? • We have already talked object that are in free fall always fall with a acceleration of 9.8 or 10 m/s2. – Remember the acceleration in the y direction for all of those fun freefall problems? • Well this is why they do. F ma i Fg mg g F F g mi a mg g ag Mini Lab: Mass Two Ways • The first way is that you will measure the mass of the car using a triple beam balance. • What type of mass are you measuring? • The second way is to plot the Net Force vs Acceleration on a graph. • Y= x • ΣF = a • Slope of a F vs a graph is__________. • What type of mass are you measuring? Conclusion • Are the masses the same? • What is their percent difference?