Presentation on theme: "Scalar quantities have magnitude only. Vector quantities have magnitude and direction. Examples are: Scalar quantities: time, mass, energy, distance, speed."— Presentation transcript:
Scalar quantities have magnitude only. Vector quantities have magnitude and direction. Examples are: Scalar quantities: time, mass, energy, distance, speed. Vector quantities: Force, velocity, acceleration, displacement
Displacement / time graphs Displacement is the distance travelled in a given direction. The gradient of a displacement/time graph gives the velocity Describe the motion shown by each graph.
Velocity / time graphs Calculate the accelerations for times 0-4s, 4-7s and 7-10s. Calculate the distance travelled in the 10s. (Use the red line for this question)
Uniformly accelerated motion Uniformly accelerated motion means that the acceleration remains constant. The following symbols are used in the equations of motion. symbolquantityunit uinitial velocityms -1 vfinal velocityms -1 aaccelerationms -2 ttimes sdisplacementm
Equations of motion The following equations apply to linear motion with a uniform acceleration. v = u + at s = ut + ½ at 2 v 2 = u 2 + 2as The average velocity equation is also very useful: s/t = ½ (u + v)
Example A car accelerates from rest to a velocity of 20 ms -1 in 10s. Find the acceleration and the distance travelled in the 10s. First - write down what you know (making sure that the values are in the correct units) Then choose the equation to use. Then substitute in the values. Then calculate the answer. Clearly state the answer, with the correct units.
u = 0 ms -1 v = 20 ms -1 t = 10 s To find a: use v = u + at 20 = 0 + (a x 10) a = 2ms -2 To find s: use s = ut + ½ at 2 s = 0 + ½ x 2 x 10 2 s = 100m
Questions 1. How long will a car accelerating at 4.5ms -2 from rest at traffic lights, take to reach the speed limit of 18ms -1 ? 2. A train moving at 18ms -1 goes through a red light which causes its brakes to be applied automatically. The train stops after 4.5s. Find its acceleration and the distance it travelled beyond the red light. 3. A body with an initial speed of 3.6ms -1 accelerates for 4.5s at 1.4ms -2. How far does it travel while accelerating?
4. A rocket-powered sledge accelerates at 35ms -2 and takes 1.6s to pass through a 60m section of a test track. Find the speed at which it entered the section. What is its speed when it left? 5. An electron moving at a speed of 1.0 x 10 5 ms -1 travels 20cm through an electric field. It leaves in the same direction with a speed of 9.0 x 10 5 ms -1. Find (a) the acceleration of the electron while it is in the electric field, (b) the time it spends in the electric field.