Physics Help Forum Calculating Mechanical Energy
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 Dec 27th 2018, 01:46 AM #1 Junior Member   Join Date: Dec 2018 Posts: 1 Calculating Mechanical Energy Hi Everyone, I can't seem to figure this problem out. Please help! 1. The problem statement, all variables and given/known data A ball was launched upwards and vertically at a speed 3 m/second up to a height 4m. Calculate the mechanical energy of the ball if its weight is 5 Newtons and has a mass 0.5kg . 2. Relevant equations potential energy= mass x gravitational acceleration x height kinetic energy= 1/2 mass x velocity x velocity mechanical energy= potential energy + kinetic energy 3. The attempt at a solution mechanical energy = potential energy at maximum height maximum potential energy =mass x gravitational acceleration x maximum height = 0.5 x 10 x 4 = 20 joules So, mechanical energy equals 20 joules? Does this question seem right to everyone? I feel like there is a problem in the question itself. Does anybody else feel the same? Many thanks, Pamela Pepper
 Dec 29th 2018, 03:08 AM #2 Senior Member     Join Date: Jun 2016 Location: England Posts: 874 Your logic and your solution seem fine to me. Perhaps you were surprised by the numerical size of the solution, (perhaps expecting kilo-joules or similar magnitudes). If we consider the definition of a Joule: Then one Joule is equal to the energy transferred to (or work done on) an object when a force of one Newton acts on that object in the direction of its motion through a distance of one metre. If we give the object a mass of 1Kg then 1 Newton will accelerate it at 1 m/s/s. from S=ut+1/2at^2 we can get the time taken for the object to travel 1 metre (accelerating at 1m/s/s from zero). 2=t^2 thus t=1.4142... In other words 1 Joule will accelerate 1 Kg at 1m/s/s over a distance of 1 metre in just under one and a half seconds. Looking at this analysis of the magnitude of a joule (with respect to our more familiar units) will hopefully reassure you that 20 joules is not an unreasonable solution. __________________ ~\o/~
 Dec 29th 2018, 06:31 AM #3 Senior Member   Join Date: Apr 2017 Posts: 484 "A ball was launched upwards and vertically at a speed 3 m/second up to a height 4m." So the ball was launched upwards and has a velocity 3m /sec at a height of 4m. From search "Mechanical energy is the sum of kinetic and potential energy in an object that is used to do work. In other words, it is energy in an object due to its motion or position, or both." so the mechanical energy is 20 + (0.25 x 9) = 22.25 J
Jan 1st 2019, 07:19 AM   #4
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 Originally Posted by pamelapepper2019 Hi Everyone, I can't seem to figure this problem out. Please help! 1. The problem statement, all variables and given/known data A ball was launched upwards and vertically at a speed 3 m/second up to a height 4m. Calculate the mechanical energy of the ball if its weight is 5 Newtons and has a mass 0.5kg . 2. Relevant equations potential energy= mass x gravitational acceleration x height kinetic energy= 1/2 mass x velocity x velocity mechanical energy= potential energy + kinetic energy 3. The attempt at a solution mechanical energy = potential energy at maximum height maximum potential energy =mass x gravitational acceleration x maximum height = 0.5 x 10 x 4 = 20 joules So, mechanical energy equals 20 joules? Does this question seem right to everyone? I feel like there is a problem in the question itself. Does anybody else feel the same? Many thanks, Pamela Pepper
You can calculate the total mechanical energy at any height since its a constant.

So you seem to have it right.

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