Physics Prelim

Displacement is a measure of the change in position of an object. Displacement is a vector quantity. • In order to fully describe any vector quantity, a direction must be specifi ed as well as a magnitude. eModelling: Numerical model for acceleration doc-0050 Watch this eLesson: Motion with constant acceleration eles-0030 Watch this eLesson: Ball toss eles-0031 Explore more with these weblinks: The moving man Speeding cars Check your formula RESOURCES 2.5 Exercise 2 (ms–1) t (s) 5 10 15 20 30 25–10–5–15 0 5 10 15 35 40 FIGURE 2.14 The following velocity –time graph describes the motion of a car for a 40 s period of time. Using the graph, determine (a) the car’s acceleration during its fi rst 5 s of motion, (b) the times when the car was stationary and (c) the car’s displacement at the end of the 40 s. • Speed is a measure of the rate at which an object moves over distance and is a scalar quantity. Velocity is the time rate of displacement and is a vector quantity. • The velocity of an object measured by a moving observer is referred to as the relative velocity. The relative velocity is the difference between the velocity of the object relative to the ground and the velocity of the observer relative to the ground. • Average speed = distance travelled time interval • The average velocity of an object, vav during a time interval, t, can be expressed as vav = s/t. • Instantaneous speed is the speed at a particular instant of time. Instantaneous velocity is the velocity at a particular instant of time. • Acceleration is the rate at which an object changes its velocity. Acceleration is a vector quantity. The average acceleration of an object, aav can be expressed as aav = Δv Δt , where Δv = the change in velocity during the time interval Δt. • When the acceleration of an object is constant, the following formulae can be used to describe its motion: v = u+at s = 1 2 (u + v)t s = ut + 1 2at2 • The instantaneous velocity of an object can be found from a graph of its displacement versus time by calculating the gradient of the graph. Similarly, the instantaneous speed can be found from a graph of its distance versus time by calculating the gradient of the graph. • The displacement of an object during a time interval can be found by determining the area under its velocity-versus-time graph. Similarly, the distance travelled by an object can be found by deter mining the area under its speed-versus-time graph. • The instantaneous acceleration of an object can be found from a graph of its velocity versus time by calculating the gradient of the graph