Constant Acceleration
Graphs to Functions A simple graph of constant velocity corresponds to a position graph that is a straight line. The functional form of the position is This is a straight line and only applies to straight lines. x t v t v0v0 x0x0
Constant Acceleration Constant velocity gives a straight line position graph. Constant acceleration gives a straight line velocity graph. The functional form of the velocity is v t a t a0a0 v0v0
Acceleration and Position For constant acceleration the average acceleration equals the instantaneous acceleration. Since the average of a line of constant slope is the midpoint: v t v0v0 ½t½t a 0 ( ½ t) + v 0
Acceleration Relationships Algebra can be used to eliminate time from the equation. This gives a relation between acceleration, velocity and position. For an initial or final velocity of zero. This becomes x = v 2 / 2ax = v 2 / 2a v 2 = 2 a xv 2 = 2 a x from
Accelerating a Mass A loaded 747 jet has a mass of 4.1 x 10 5 kg and four engines. It takes a 1700 m runway at constant thrust (force) to reach a takeoff speed of 81 m/s (290 km/h). What is the force per engine? The distance and final velocity are used to get the acceleration. The acceleration and mass give the force.
Pulley Acceleration The normal force on m 1 equals the force of gravity. The force of gravity is the only external force on m 2. Both masses must accelerate together. m1m1 m2m2 FTFT F g = m 2 g Consider two masses linked by a pulley m 2 is pulled by gravitym 2 is pulled by gravity m 1 is pulled by tensionm 1 is pulled by tension frictionless surfacefrictionless surface FTFT
Atwood’s Machine In an Atwood machine both masses are pulled by gravity, but the force is unequal. The heavy weight will move downward at ( kg)(9.8 m/s 2 )/( kg) = 1.8 m/s 2.( kg)(9.8 m/s 2 )/( kg) = 1.8 m/s 2. Using y = (1/2)at 2, it will take t 2 = 2(1.80 m)/(1.8 m/s 2 ) t = 1.4 s.t = 1.4 s. next