Lecture03

# Lecture03 - PHYSICS 149: Lecture 3 Chapter 2 2.1 Forces 2.2...

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PHYSICS 149: Lecture 3 • Chapter 2 –2 .1 Fo rces – 2.2 Net Force – 2.4 Newton’s first law

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Forces • Forces are interactions between objects • Different type of forces: – Contact vs. non-contact forces – Contact forces are due to EM interactions • Forces always exist in pairs – When you push on the ground to walk, the ground pushes back on your foot • Measuring forces: Hooke’s law F = kx • Forces are vectors : magnitude plus direction Lecture 3 Purdue University, Physics 149 2
Force • Force: a push or pull that one object exerts on another – Force can also be defined as any action that alters a body’s state of rest or of constant speed motion in a straight line. • Forces always exist in pairs – Anything that exerts a force also has a force exerted on it. – For example, when you push on the ground to walk, the ground pushes back on your foot. • Two types of forces on macroscopic objects: – Long-range forces: Forces that do not require the two objects to be touching. Example: gravity and electromagnetic forces. – Contact forces: Forces that exist only as long as the objects are touching one another. Example: kicking a ball. Lecture 3 Purdue University, Physics 149 3

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How to Measure a Force? • The SI unit for force is the Newton (N). • One way to measure forces is using springs (F x). • Hooke’s low for an ideal spring: • F = kx where k is the spring constant Lecture 3 Purdue University, Physics 149 4
Net Force • More than one force can act on an object at once. • The “net” force means the “total force”. • Be careful since the force is a vector. • The sum is easy if all forces act in the same direction. • If F net = 0 we have “translational equilibrium”. Lecture 3 Purdue University, Physics 149 5

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• If two vectors are in the same direction, their sum is in the same direction and its magnitude is the sum of the magnitudes of the two. • If two vectors are in opposite directions, the magnitude of their sum is the difference between the magnitudes of the two vectors, and the direction of the vector sum is the direction of the larger of the two. Lecture 3
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## Lecture03 - PHYSICS 149: Lecture 3 Chapter 2 2.1 Forces 2.2...

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