Physics Topic 5 Notes 1 1 of 9
- Slides: 12
Physics Topic 5 Notes 1 1 of 9 © Boardworks Ltd 2009
What is work? What happens when a rocket is launched into space? When the rocket’s engines are fired, chemical energy in the fuel is transferred to kinetic energy in the rocket. This transfer of energy is called work. 2 2 ofof 9 9 © Boardworks Ltd 2009
Chart Measurement work 3 3 ofof 9 9 name W definition transfer of energy through motion units Joule (J) SI Tool J __ © Boardworks Ltd 2009
Work and energy What is the link between work and energy? work done = energy transferred This means the units for work are the same as the units for energy – Joules. For example, if a person does 500 J of work, then 500 J of energy is transferred. In the same way, if a person transfers 250 J of energy, then 250 J of work is done. 4 4 ofof 9 9 © Boardworks Ltd 2009
What factors affect work done? 5 of 9 © Boardworks Ltd 2009
Work or no work? • lift books? – yes, force and distance parallel • hold books? – no, no distance (really displacement) • walk with books? – no, only the component of force parallel to the distance counts • lower books? – yes, force and distance parallel 6 6 ofof 9 9 © Boardworks Ltd 2009
How is work calculated? The work done on an object can be calculated using this equation: W = Fnetd(cosq) l Force is measured in newtons (N). l Distance moved is measured in meters (m). l Work done is measured in Joules (J). 7 7 ofof 9 9 © Boardworks Ltd 2009
Factors affecting work done 8 of 9 © Boardworks Ltd 2009
Calculating work done A 20. 0 kg suitcase is raised 3. 0 m off the ground by a conveyor belt. How much work has been done on the suitcase? 2 m = 20 kg 1 d d= 3 m Fg 4 3 W = Fd = mgd 5 = (20 kg)(9. 81 m/s 2)(3 m) 6 = 590 J 9 9 ofof 9 9 © Boardworks Ltd 2009
Calculating work done Starting from rest, a 5. 0 kg block slides 2. 5 m down a 30. 0° incline. What is the work done by gravity? m = 5 kg 2 d = 2. 5 m 1 Fn q q = 30. 0° Fg 4 3 W = Fdsinq = mgdsinq 5 = (5. 0 kg)(-9. 81 m/s 2)(-2. 5 m)(sin 30°) 6 = 61 J 10 10 ofof 99 © Boardworks Ltd 2009
Sign of Work is important even though it is scalar! Negative Work Positive Work Fa Fa d d Fa d 11 11 ofof 99 Fa d © Boardworks Ltd 2009
Quiz Time 12 of 9 © Boardworks Ltd 2009