# CH-4 - Dr Sabah AL-Naimi Qatar University Physics Program...

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Dr. Sabah AL-Naimi Qatar University Physics Program FALL 2009 PHYS 191 1 The Work done on an object by a constant force (F) is defined as: ) (cos S F W (4.1) Work Done by a constant force Where : F is the magnitude of the force. S is the magnitude of the displacement. θ is the angle between the force and the displacement . x F s y The SI unit of Work is the Joule ( J ), where: 1 Joule = 1 N.m When θ = 0 , then cos 0° = 1 and the work is : S F W When θ = 180 , then cos 180° = -1 and the work is : S F W

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Dr. Sabah AL-Naimi Qatar University Physics Program FALL 2009 PHYS 191 2 The kinetic energy K.E. of an object with mass ( m ) and moving with a speed ( v ) is given by: (4.2) Kinetic Energy and the Work-Energy Theorem 2 2 1 . . v m E K The SI unit of K.E. is the Joule ( J ). The Work-Energy Theorem states that: When a net external force does Work ( W ) on an object, the kinetic energy of the object changes from its initial value of (K.E) i to a final value (K.E) f ,the difference between the two values being equal to the work: ) ( 2 1 2 1 2 1 ) . ( ) . ( . . 2 2 2 2 i f i f i f v v m v m v m E K W or E K E K E K W Where v i is the initial speed. and v f is the final speed. Conservation of Energy Energy is conserved – This means that energy cannot be created or destroyed. – If the total amount of energy in a system changes, it can only be due to the fact that energy has crossed the boundary of the system by some method of energy transfer.
Dr. Sabah AL-Naimi Qatar University Physics Program FALL 2009 PHYS 191 3 Work done by Friction: The work W f done by fraction force f when an object slides over a rough surface is given by this form: (4.3) Work by Friction & by a Spring s f W f Where f is the friction force.

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## This note was uploaded on 10/12/2011 for the course PHYSICS 101 taught by Professor Khidr during the Spring '11 term at Qatar University.

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CH-4 - Dr Sabah AL-Naimi Qatar University Physics Program...

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