Work = Force X Distance (Measured in Joules)

Physics 10

Work = Force x distance (measured in Joules) Force = Mass x Acceleration (measured in Newtons) Power= Work/time (measured in Watts) (gravitational potential energy) PE = mgh (weight x height) KE = ½ mv2 (velocity is speed with direction) Work = ∆KE (change in KE)

1. Calculate a) Calculate the work done when a force of 3-N moves a book 4m. b) Calculate the work done in lifting a 300-N barbell 1.5 meters above the floor. (What is the potential energy of the barbell) c) Calculate the power expended when a 15-N force pushes a cart 6m in a time of .5 seconds. d) Calculate the power expended in lifting a 300-N barbell 1.5 meters in 2 seconds. e) How many Joules of potential energy does a 1-kg book gain when it’s elevated 3m? f) Calculate the increase in potential energy when a 20-kg block is lifted a vertical distance of 3m. g) Calculate the kinetic energy of a 500kg car has moving at 5 m/s. What is the KE if the car’s speed doubles? h) How much work is required to increase the kinetic energy of a car by 5000 J? 2. Answer in complete sentences a) Why is it easier to stop a lightly loaded truck than a heavier one that has the same speed? b) Why does one get tired when pushing against a stationary wall when no work was done on the wall? c) Which requires more work – stretching a strong spring a certain distace, or stretching a weak spring the same distance? Defend your answer. d) Two people who weigh the same climb a flight of stairs. The 1st person climbs the stairs in 30s, the 2nd in 40s. Which person does more work? Which uses more power? e) When a driver applies the breaks to keep a car going downhill at a constant speed and constant KE, the PE of the car decreases. Where does the energy go?

Gravity

m m F = G 1 2 , G = 6.67 x 10-11 N m2/kg2 d 2 3. a) By how much does the gravitational force between 2 objects decrease when the distance is doubled? Tripled? Increased tenfold? b) Calculate the force of gravity on a 1-kg mass at the earth’s surface. The mass of the earth is 6x1024 kg, and it’s radius is 6.4 x106 m. c) Calculate the force of gravity on the same 1-kg mass if it were is 6.4 x106 m above the earth’s surface (that is, if it were 2 Earth radii from the Earth’s center) d) Calculate the force of gravity between the Earth and the moon, mass is 74 x 1022 kg. The average Earth-Moon distance is 3.8 x108 m.