Newton’s Second and Third Laws. Acceleration and Force. Acceleration is directly Proportional to the Force Acceleration α Net Force If the net force is increased, then the acceleration must increase by the same factor as long as mass is held constant. Force and Acceleration.
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Acceleration and Force • Acceleration is directly Proportional to the Force • Acceleration α Net Force • If the net force is increased, then the acceleration must increase by the same factor as long as mass is held constant.
Force and Acceleration • Acceleration is always in the direction of the net force.
Acceleration and Mass • Acceleration is inversely proportional to the mass of the object. • Accelerationα • If the mass increases, then the acceleration decreases as long as the force remains constant. • If the mass is doubled, then the acceleration is cut in half.
Force, Mass and Acceleration • The acceleration is directly proportional to the net force divided by the mass • Accelerationα
Newton’s 2nd Law of Motion • Newton’s Second Law – The acceleration of an object is directly proportional to the net external force acting on the object and is inversely proportional to the mass of the object. • Fnet= ma
Equation Variables and Units • Newton’s Second Law variables • Fnet: net force (Newton – N) • m: mass (Kilograms – kg) • a: acceleration (meters per second² - m/s²)
Example Problem #1 • What net force is needed to move a 3.2kg book across a table with an acceleration of 2.1 m/s² to the right? • Answer: 6.7 N to the right
Example Problems #2 • The net force on the propeller of a 0.75kg model airplane is 17 N forward. What is the acceleration of the airplane?
Example Problem #3 • A ball pushed with a net force of 13.5 N to the right accelerates at 6.5 m/s² to the right. What is the mass of the ball?
Example Problem #4 • Two people push on a box resting on a frictionless floor. One person pushes to the left with a force of 17 N and the other person pushed with a force of 37 N to the right. If the mass of the box is 10.0 kg, what is the acceleration of the box?
Newton’s 3rd Law • Newton’s Third Law – If two bodies interact, the magnitude of the force exerted on object 1 by object 2 is equal to the magnitude of the force simultaneously exerted on object 2 by object 1, and these two forces are opposite in direction. • For every action, there is an equal and opposite reaction.
Forces Always Exist in Pairs • Forces always exist in pairs. • If you push on a wall with 100 N, the wall presses back on you with 100 N. • Equal and opposite, as long as there is no acceleration. • If Earth is pulling you down with a force equal to your weight, what is the second force?
Action-Reaction Pair • Action-Reaction Pair – A pair of simultaneous equal but opposite forces resulting from the interaction of two objects. • The action and reaction occur at the same exact time.
Field Forces • Field Forces also exist in pairs as well. • Gravity is an example of a field force. • If you drop a ball the Earth pulls down on the ball, but the ball pulls up on the Earth by the same amount. • But why doesn’t the earth move and the ball does?