chapter06 - Work by a Constant Force Kinetic Energy...

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1 •Work by a Constant Force •Kinetic Energy •Potential Energy •Work by a Variable Force •Springs and Hooke’s Law •Conservation of Energy •Power
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2 The total energy of the Universe is unchanged by any physical process. At the most fundamental level, there are three kinds of energy: Energy due to motion ( Kinetic Energy), Stored Energy due to interactions (potential energy), and rest Energy. Energy may be converted from one form to another or transferred between bodies.
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3
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4 Work is an energy transfer by the application of a force . For work to be done there must be a nonzero displacement . It is only the force in the direction of the displacement which does work The unit of work and energy is the joule (J). 1 J = 1 Nm = 1 kg m 2 /s 2 .
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5 In general, the work done by a force F is defined as cos r F W where F is the magnitude of the force, r is the magnitude of the object’s displacement, and is the angle between F and r (drawn tail-to-tail).
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6 Example A sled is dragged along a horizontal path at a constant speed of 1.5 m/s by a rope that is inclined at an angle of 30.0° with respect to the horizontal. The total weight of the sled is 470 N. The tension in the rope is 240 N. How much work is done by the rope on the sled in a time interval of 10.0 s? F r x x y F w N An FBD for the box at left:
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7 It is only the force in the direction of the displacement that does work. x y F w N The work done by the force F is: x F r F W x x F cos r x The work done by the force N is: 0 N W The work done by gravity ( w ) is: 0 g W The normal force & Gravity are perpendicular to the displacement. F r x
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8 The net work done on the box is: x F x F W W W W g N F net cos 0 0 cos
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9 2 2 1 mv K is an object’s translational kinetic energy. This is the energy an object has because of its state of motion. The kinetic energy of an object moving with a speed v is equal to the work that must be done on the object to accelerate it to that speed starting from rest It can be shown that, in general . K W net
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10 Example: The extinction of the dinosaurs and the majority of species on Earth in the Cretaceous Period (65 Myr ago) is thought to have been caused by an asteroid striking the Earth near the Yucatan Peninsula. The resulting caused widespread global climate change. If the mass of the asteroid was 10 16 kg (diameter in the range of 4-9 miles) and had a speed of 30.0 km/sec, what was the asteroid’s kinetic energy?
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11 Objects have potential energy because of their location (or configuration ).
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