Can someone please explain this to me. A hoist used in car repair applies a constant force to a 200kg engine to lift it 1.5m. How much work does the hoist do on the engine? the answer is 2.9*10^3 Nm. So my questions are 1. Isn't the unit supposed to me Joules? not Nm? 2. if the hoist is doing work on the engine, isn't the force tension? and not m*g? I understand it's positive work because the kinetic energy is becoming potential energy. But for some reason I can't seem to wrap my head around a clear picture of what's going on. Thank you soo very much.
1 Joule is 1 Nm the two units are identical and may be used interchangeably. 2. The hoist is under the car. The metal of the hoist is being squeezed together by the forces of the car, not being pulled apart so the force in the hoist is compression, not tension. And it doesn't matter whether the work is being done on the car ( lifting) or on the hoist ( descending). On top of the hoist a car is pushing down ( gravity) and at the bottom a hydraulic system is pushing up ( hoist) in between the rod is being pressed together and sqeezed. 3. As the force in the hoist is CAUSED by the force of gravity on the car, the forces applied , by the car on the hoist, and by the hoist on the car, are truly F= mg ( every action has an equal and opposite reaction). And remember that there are many different points at which you can analyze the forces. You can't mix and match. Pick any one point that is convenient and analyze it there Second as every action has an equal and opposite reaction, the direction of the forces depends on which body you choose to consider. The forces ON the car are the opposite direction to the forces BY the car. Unless you are consistent you will always become confused.
