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 Went through kinetics and motion test - see previous post for both question paper and mark scheme. Thinking Distance = distance covered between first seeing an object in the road and applying the brakes Braking Distance = distance covered between applying the brakes and stopping Stopping Distance = Thinking Distance + Braking Distance The thinking distance can be calculated using the simple equation: thinking distance = speed x reaction time (assuming constant speed) We generally assume that when the car is braking that the acceleration is constant. Hence, a velocity time graph would look like that shown below: Factors affecting thinking distance are: speed and reaction time (affected by: age, alcohol, drugs, tiredness, distractions)
Factors affecting braking distance: speed, mass, type of road surface, quality of tyres and brakes, weather conditions (i.e. wet or icy), gradient of road [for each of these, how do you think it affects the braking distance?] Assuming a constant deceleration, we find that braking distance is proportional to velocity (see future section on Work and Kinetic Energy) Answer question 3 from page 75 (answers in next lessons blog post): TEST on Kinematics and Motion tomorrow
Below shows how we can derive the equation for displacement (s = ut + 1/2 at^2) and can also be found here |
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