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Transcript
Physics AS Unit 1 Mechanics Displacement Time Graphs • • • • • • • A: Positive acceleration B: Constant positive velocity C: Positive Deceleration D: Stationary E: Negative acceleration F: Constant negative velocity G: Negative deceleration (with negative displacement) Velocity Time Graphs • A: Increase in rate of positive acceleration (Starting to speed up) • • B: Constant positive acceleration • • D: Constant positive velocity • • • F: Constant negative acceleration C: Decrease in rate of positive acceleration (This is not slowing down but the rate of acceleration has slowed) E: Increase in rate of negative acceleration (starting to slow down) G: Stationary H: Increase in rate of negative acceleration (starting to speed up in opposite direction) Equations of Motion Equation Excluded element v = u + at s s = ut + ½at² v s = vt - ½at² u v² = u² + 2as t s = ½ut + ½vt a Using Vectors • By resolving vectors into horizontal and vertical components it allows for a resultant triangle (ABC) to be calculated. • A = X + α = Z Cosψ + γ Cosϴ B = Y + β = Z Sinψ + Z Sinϴ Using Vectors and Work • Two Triangles 3 Methods • 1: Find the component of the force which is acting in the direction of travel (B) and multiply by the distance travelled (C) • 2: Find the component of the distance travelled which is in line with the force (D) and multiply by the magnitude of the force (A) • 3: Calculate the energy transformed as this is also equal to the work done Galileo and Aristotle • Aristotle: An object has an inherent tendency to reach as low a point as possible. A moving object will continue to move until the force that started it runs out. • Galileo: Carried out experiments on slopes to ‘dilute’ the effects of gravity. Showed that a projectile had a constant horizontal velocity and a changing vertical velocity. Projectile Movement • Any thrown object will have a constant horizontal velocity (if air resistance is ignored) and a vertical acceleration of 9.81ms-2 • To solves these problems you therefore have to establish how long the projectile is in the air for. Force • A force is a push or a pull that results in either an acceleration or a change in shape. This is measured in Newtons (N) • • Force = mass x acceleration = change in momentum/time • Reactive forces are the equal and opposite forces experienced by two different objects • Mass is a scalar quantity which is defined as the property of an object that resists motion and is measured in kg • F = ma can be rewritten in the case of weight to read W = mg g is the gravitational field strength or acceleration of free fall g therefore has units of N kg-1 or m s-2 as these are the same thing A resultant force is the sum of two or more forces acting on the same object Torque, moments and turning effect • moment = magnitude of force x perpendicular distance (quick version) • • Two triangles same solution: • 2: Multiply (Y) x (B) (B) = The magnitude of the force applied (Y) = The perpendicular distance between the line of the force applied (B) and the pivot point (P) 1: Multiply (A) x (X) Where (X) is the distance between the pivot point (P)and the point of action (Q) (A) is the component of the force which is perpendicular to (X) All Torque no Action • Couples A couple is a pair of forces equal in magnitude, opposite in direction but not in line which cause a turning effect on an object. torque of a couple = magnitude of one of the forces x the perpendicular distance between them • Equilibrium An object is in equilibrium when there are resultant forces or torques acting on it. • Centre of Gravity The point on an object where all of the moments of that object are balanced. Car Safety • Stopping distance = thinking distance + braking distance Thinking distance = thinking time x speed Braking distance found using v2 = u2 + 2as (where v2 = 0 and a is a deceleration and therefore -ve) • Factors affecting thinking distance: (Speed, age, alcohol or drugs, tiredness) • Factors affecting braking distance (If you double the speed you quadruple the braking distance, as well as the deceleration being affected by road surface, tyre wear, weather conditions) Deforming solids • • • Two forces that extend an object are called tensile • • • A ductile material stretches a lot beyond the elastic limit Two forces that shorten an object are called compressive The elastic limit is the force beyond which a material will not return to its original shape A brittle material snaps when it reaches the elastic limit A polymeric material does not show linear material Symbols Words W = Fs Work done = Force x displacement = Energy Transferred ρ = m/v Density = mass/volume P = W/t Power = work done/time η = Pout/Pin Efficiency = Useful energy out/total energy in Ek = ½mv² Kinetic Energy = half mass times velocity squared Ep = mgh Gravitational potential energy = mass x acceleration of free fall x height F = ma Force = mass x acceleration W = mg Weight = mass x acceleration of free fall F = kx Force = extension x force constant Eep = ½Fx Elastic potential = half force times extension ε = ∆l / l Strain = extension / original length σ=F/A Stress = load / area E=σ/ε Youngs Modulus = Stress / Strain