Chapter 4

# Chapter 4 - Chapter 4 Newton's Laws of Motion Our first...

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1 Chapter 4: Newton’s Laws of Motion Our first encounter with a set of “laws” or rules We’ll need to define the forces acting on an object Newton’s laws tell us about forces and the motion they can produce Intuitively, we know that force is a “push” or “pull” Force F is a vector quantity F less ‘obvious’ forces Forces as Vectors We already learned the math for vectors components vector addition In 1686, Newton presented The Laws of Motion: Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant velocity, unless it experiences a net force. also known as “inertia”

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2 inertia “at work” friction is a force - we want a situation with no forces, hence no friction plenty of friction less friction almost no friction no forces, so an object moving keeps moving at the same velocity forever The first law told us what happens if the total force on an object is zero - what if it isn’t ? We know from experience that different objects resist a change in motion differently. • push a door • push a train Not the same response! There should be a law that tells us how an object responds to a force… Newton’s Second Law The acceleration of an object is directly proportional to the resultant force acting on it and inversely proportional to its mass. The direction of the acceleration is the direction of the resultant force. F = m a This is a lot of words - I can’t remember all that! A formula would be a lot more useful: actually the ‘resultant’, or total, force You are in a spacecraft moving at a constant velocity. The front thruster rocket fires incorrectly, causing the craft to slow down. You try to shut it off but fail. Instead, you fire the rear thruster, which exerts a force equal in magnitude but opposite in direction to the front thruster. How does the craft respond? 1. It stops moving.
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Chapter 4 - Chapter 4 Newton's Laws of Motion Our first...

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