Worked answers to the homework questions (Q3 pg75 – OCR AS/A Level Physics A (Mike O’Neill)) can be found here - please let me know if you find any errors as it was quite late when I was writing it! Work Work is defined as force multiplied by displacement in the direction of the force If the force is at an angle to the displacement (as shown in the diagram below) then we multiply the component of the force (in the same direction as the displacement) by the distance moved. More generally the equation for work done is given by: W=F x cosθ x d So, W = Fdcos θ Defining the joule The unit of work is the joule. In fact, the joule is defined from the definition for work. Hence the joule is defined as the work done when a force of 1N moves a distance of 1m (hence 1 J = 1 Nm) [An aside - You should be able to convert units from one to another. In particular, you should be able to convert all SI units back to the so called 'base units' (kg, m, s, A). More information on SI units, base units and prefixes can be found in my notes here] Calculations of work 1. When using W = Fdcos θ, we can see that if the force is parallel to the direction of motion then θ = 0 and so cosθ = 1 and the equation simply becomes W = Fd 2. When the force is perpendicular (90°) to the direction of motion then θ=90° and so cosθ = 0 and so W = 0. Therefore, there is no work done when the force is perpendicular to the direction of motion. This is because there is not component of the force in that direction (e.g. when an object moves in a horizontal circle at constant speed) Further examples of calculations of work found in this PowerPoint Leave a Reply. |
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