energy and Impulse Example 3 R E = 6.371 10 6 m G = 6.67384 10 -11 - PowerPoint PPT Presentation

Class 27: Examples on conservation of energy and Impulse

Example 3 R E = 6.371  10 6 m G = 6.67384 × 10 -11 m 3 kg -1 s -2 Escape velocity = ?

Power Power is the rate of work done by a force: dW  P dt     But dW F d r     dW d r         P P F P F v dt dt Unit for power: Watt (W)  Js -1 Same as the one we use in electrical appliances

Normal force Like friction, normal force is NOT a conservative force. For example: 2kg 4kg 3m/s 4m/s 1 1     2 2 ( 4 )( 3 ) ( 2 )( 4 ) 18 16 34J Total mechanical energy before collision = 2 2  Total mechanical energy after collision 34J (energy is lost) Mechanical energy is not conserved in a collision.

Impulse and Momentun F    d v     Impulse I F dt Newton' s Law F m dt  d p   F dt     Momentum p m v      d v          I F dt m dt m d v m v dt      I p

An important case In a collision, normal force F  and  t  0 #1 #2 I 2  1 I 1  2     I I   1 2 2 1        p p 2 1        p p 0 2 1 No change in total momentum in a collision.