She pushed a mass of 45kg and 3 meters up - how much work against gravity did she do?


Matthew Clemens, Physics and Math Teacher, Parent, Tutor, and Professional Ski Instructor

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The work SHE did can be calculated two ways: 1) Use W=Fdcos(theta) where F is the force SHE exerted (impossible to know from this problem not knowing how she lifted the mass - constant velocity, with some acceleration, ...), d is the displacement of the mass, and theta is the angle formed by the force vector and the displacement vector. So it looks like we need to use option 2. So... 2) The works she did is equal to the gravitational potential energy she gave the mass. It is easiest to assume the mass started with zero Joules of gravitational potential energy (although any number could be assumed as it is the change in the potential energy that resulted from the work she did). The final gravitational potential energy can be found using mgh, where m is the mass of the object (45 kg), g is the acceleration due to the force of gravity at the location of the object (assuming Earth, 9.8 m/s/s), and h is the vertical displacement (3 m). So the work she did = GPE = (45)(9.8)(3).

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