Homework Assignment 3: Calculating Energy Transfers
1. A woman bungee jumper jumps from the top of a bridge with a height of 75m. The length of the bungee elastic is 25m and the woman has a mass of 60kg.
(a) Calculate the potential energy of the woman as she stands on top of the bridge.
(b) Calculatethemaximumspeedthatthewomanreachesandstateat what height this will happen.
2. An electric kettle is used to boil some water. The kettle holds 1.5 litres of water when full and operates from a 230V, 13 amp supply. If all the electrical energy supplied is converted to heat energy, how long will it take the kettle to boil from an initial temperature of 15°C? (The specific heat capacity of water is 4190J/kgK.)
3. A goalkeeper kicks the ball out of his hands straight up into the air. The ball reaches a maximum height of 30m.
(a) Calculate how much kinetic energy the keeper gives the ball if it has a mass of 2kg.
(b) Calculateatwhatspeedtheballleavesthegoalkeeper’sfoot.
4. A motor and gearbox system is used to raise a load from the ground to the roof of a building site. The load weighs 100kg and the motor runs from a 110 volt supply and draws a current of 15 amps. If the roof is 25m above the ground, calculate:
(a) The potential energy of the load when it has been raised
(b) Theamountofelectricalenergyusedinthattime
(c) The time taken to raise the load
(d) Theaveragespeedatwhichtheloadisbeingraised
(e) Thekineticenergyoftheloadwhenitisbeingraised.
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