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Ch9sum

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  • 1. m s –2 v is initial velocity Collision Course s= u+v xt u is final velocity 2 a is acceleration v = u +at acceleration = change in velocity s is displacement time t is time 1000 km 1000 km = final velocity—initial velocity per hour per hour time The Kinematic Equations average velocity = initial velocity + final velocity distance apart 10 km 2 s = ut + 1/2 at2 v2 = u2 + 2as Velocity of approach 2000 km These equations may not be m s –1 useful when motion is irregular (kinematic equations can only be used when acceleration is constant) so this is when graphs are useful. time to collision = distance apart_ Acceleration due to gravity: 9.8m s –2 What ever the shape of the graph: velocity of approach time to collision = 10 km____ (On earth!) • the slope (Gradient) at any point on a distance—time graph gives the speed 2000 km per hour at that point in time. time to collision = 1/200 hour = 18 sec • the area under a speed—time graph gives the distance travelled. Chapter 9 Computing the 225m s –1 Planes velocity Planes velocity, reversed -225m s –1 Next Move Aircraft appears Potential Energy Falling and going 212m s –1 to approach along this track Rules: As an object moves Relative velocity 1) Add velocity opposite to that upwards its potential Planes velocity of one plane to the velocities energy increases! of both. Planes velocity, reversed -225m s –1 2) Find the resultant relative velocity, Force to lift object = mg adding vectors tip to tail. Work done = force x 3) See if the direction of the relative displacement mgh mass (kg) velocity hits the plane. If so, take Energy going to gravitational Force (N) avoiding action. potential energy = mgh Vertical and horizontal components of F = ma Kinetic energy = Energy • Energy comes from the velocity are independent. transferred = work done = force x gravitational field: decrease Vertical component increases uniformly with time. acceleration (ms-2) distance in potential energy = mgh Start by calculating: force x time • Energy now carried by Since F = ma & at is the increase in v motion of object; increase Power = force x velocity Acceleration is often replaced with g (gravity) Force x time = mv in kinetic energy = 1/2mv2 because the acceleration of a falling object is the THIS IS MOMENTUM! • Decrease in potential Energy from train to surroundings = drag force x gravitational pull on it. force x displacement = force x time energy = increase in kinetic displacement (E = Fs) x average velocity Power used by train = rate of dissipation of energy If acceleration is uniform, the average P=E/t Momentum: how big a force is needed to stop in velocity is v/2 so: P=Fs/t with s = vt a given time. force x displacement = 1/2mv2 P = Fv Power used by train = drag force x velocity

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