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Lecture214Week3

# Lecture214Week3 - This Week Forces on an object Newtons...

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This Week Forces on an object Newtons laws Relating force to acceleration Riding in an elevator What we feel going up and down Cars and Trains Reaction /action What makes us walk or a car move Sailing Up Wind 12/09/11 Physics 214 Fall 2011 1

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12/09/11 2 Physics 214 Fall 2011 What causes motion In our everyday life we observe that objects change their state of motion. In fact everything that happens in the Universe results from a change in motion. That is a static inert object does not contribute to any of the things we consider to be useful. The functioning of our body depends on continual change throughout our bodies. These changes are produced by forces and in our everyday life there are just two forces. Gravity acts on mass Electric charge F = Gm 1 m 2 /r 2 F = kq 1 q 2 /r 2
12/09/11 3 Physics 214 Fall 2011 Newtons Second and First Law Second Law The acceleration of an object is directly proportional to the magnitude of the imposed force and inversely proportional to the mass. The acceleration is in the same direction as the force F = ma F and a are vectors unit is a Newton (or pound) 1lb = 4.448N First Law An object remains at rest or in uniform motion in a straight line unless it is acted on by an external force . F = 0 a = 0 so v = constant http://www.physics.purdue.edu/academic_programs/courses/phys214/movies.php ( anim0003.mov) (anim0004.mov)

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12/09/11 4 Physics 214 Fall 2011 Force at the earths surface F = GM E m/r E 2 But F = ma = mg so g = GM E /r E 2 Famous experiment by Cavendish Measured F = Gm 1 m 2 /r 2 using two known masses in the laboratory and so measured G Then using g = GM E /r E 2 he determined the mass of the earth r E
12/09/11 5 Physics 214 Fall 2011 Mass and weight Newtons second law enables us to measure relative mass. If we apply the same force to two objects and measure the accelerations then. F = m 1 a 1 and F = m 2 a 2 so m 1 /m 2 = a 2 /a 1 We then need to have one mass as a calibration and a kilogram is the mass of a piece of platinum held in Paris. Since gravity acts proportional to mass then the force near the earths surface is F = mg this is the weight of an object so if we compare F 1 = m 1 g and F 2 = m 2 g then weight 1/weight 2 = m 1 /m 2

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12/09/11 6 Physics 214 Fall 2011 Inertia Inertia = tendency of an object to resist changes in its velocity. Since F = ma and a = Δv/t then Ft = mΔv So if a force acts for a time t the change in velocity will be smaller for larger masses so it is mass that determines inertia. In particular if t is very small and m is large then F can also be large but Δv can still be very small .
12/09/11 7 Physics 214 Fall 2011 Friction In our everyday world any object which is moving feels a force opposing the motion --- this is friction. An object which is sliding The air resistance on your car These types of friction result in energy being lost and minimizing friction is very important.

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