Physics 215 – Fall 2014Lecture Welcome back to Physics 215 Today’s agenda: Review of motion with constant acceleration Vectors in Mechanics Motion in two dimensions Projectile motion

Physics 215 – Fall 2014Lecture Current homework assignment HW2: –Ch.2 (Knight textbook): 48, 54 –Ch.3 (Knight textbook): 28, 34, 40 –Ch.4 (Knight textbook): 42 –due Wednesday, Sept 10 th in recitation

Physics 215 – Fall 2014Lecture Motion with constant acceleration: v  = v 0 + at v av  = ( 1 / 2 ) (v 0 + v) x = x 0 + v 0 t + ( 1 / 2 ) a t 2 v 2 = v a (x - x 0 ) *where x 0, v 0 refer to time = 0 s; x, v to time t

Physics 215 – Fall 2014Lecture An object moves with constant acceleration, starting from rest at t = 0 s. In the first four seconds, it travels 10 cm. What will be the displacement of the object in the following four seconds (i.e. between t = 4 s and t = 8 s)? 1.10 cm 2.20 cm 3.30 cm 4.40 cm

Physics 215 – Fall 2014Lecture Sample problem A brick is dropped (zero initial speed) from the roof of a building. The brick strikes the ground in 2.50 s. Ignore air resistance, so the brick is in free fall. –How tall, in meters, is the building? –What is the magnitude of the brick’s velocity just before it reaches the ground? –Sketch a(t), v(t), and y(t) for the motion of the brick.

Physics 215 – Fall 2014Lecture Motion in more than 1 dimension Have seen that 1D kinematics is written in terms of quantities with a magnitude and a sign Examples of 1D vectors To extend to d > 1, we need a more general definition of vector

Physics 215 – Fall 2014Lecture Vectors: basic properties are used to denote quantities that have magnitude and direction can be added and subtracted can be multiplied or divided by a number can be manipulated graphically (i.e., by drawing them out) or algebraically (by considering components)

Physics 215 – Fall 2014Lecture Vectors: examples and properties Some vectors we will encounter: position, velocity, force Vectors commonly denoted by boldface letters, or sometimes arrow on top Magnitude of A is written |A|, or no boldface and no absolute value signs Some quantities which are not vectors: temperature, pressure, volume ….

Physics 215 – Fall 2014Lecture Drawing a vector A vector is represented graphically by a line with an arrow on one end. Length of line gives the magnitude of the vector. Orientation of line and sense of arrow give the direction of the vector. Location of vector in space does not matter -- two vectors with the same magnitude and direction are equivalent, independent of their location

Physics 215 – Fall 2014Lecture Adding vectors To add vector B to vector A Draw vector A Draw vector B with its tail starting from the tip of A The sum vector A+B is the vector drawn from the tail of vector A to the tip of vector B.

Physics 215 – Fall 2014Lecture Multiplying vectors by a number Direction of vector not affected (care with negative numbers – see below) Magnitude (length) scaled, e.g. – 1*A=A – 2*A is given by arrow of twice length, but same direction – 0*A = 0 null vector –-A = -1*A is arrow of same length, but reversed in direction A 1 x= A 2 x = A 0 x = A -1 x = A

Physics 215 – Fall 2014Lecture

Physics 215 – Fall 2014Lecture Find the vector B – A

Physics 215 – Fall 2014Lecture Projection of a vector “How much a vector acts along some arbitrary direction” Component of a vector Projection onto one of the coordinate axes (x, y, z)

Physics 215 – Fall 2014Lecture Components A x y i j A = A x + A y A = a x i + a y j i = unit vector in x direction j = unit vector in y direction a x, a y = components of vector A Projection of A along coordinate axes

Physics 215 – Fall 2014Lecture More on vector components Relate components to direction (2D): a x = |A|cos , a y = |A|sin  or Direction: tan  = a y /a x Magnitude: |A| 2 = a x 2 + a y 2  A x y AyAy AxAx i j

Physics 215 – Fall 2014Lecture A bird is flying along a straight line in a direction somewhere East of North. After the bird has flown a distance of 2.5 miles, it is 2 miles North of where it started. How far to the East is it from its starting point? 1.0 miles miles mile miles

Physics 215 – Fall 2014Lecture Why are components useful? Addition: just add components e.g. if C = A + B c x = a x + b x ; c y = a y + b y Subtraction similar Multiplying a vector by a number – just multiply components: if D = n*A d x = n*a x ; d y = n*a y Even more useful in 3 (or higher) dimensions A x y i j B

Physics 215 – Fall 2014Lecture D Motion in components s – vector position  s = x Q i + y Q j Note: component of position vector along x-direction is the x-coordinate! x y Q i j xQxQ yQyQ

Physics 215 – Fall 2014Lecture Displacement in 2D Motion s – vector position Displacement  s = s F - s I, also a vector! y x O sIsI sFsF ss

Physics 215 – Fall 2014Lecture Describing motion with vectors Positions and displacements s,  s = s F - s I Velocities and changes in velocity: v av = ––––,v inst = lim –––  v = v F - v I Acceleration: a av = ––––, a inst = lim –––– t0t0 t0t0

Physics 215 – Fall 2014Lecture Ball A is released from rest. Another identical ball, ball B is thrown horizontally at the same time and from the same height. Which ball will reach the ground first? A.Ball A B.Ball B C. Both balls reach the ground at the same time. D. The answer depends on the initial speed of ball B.

Physics 215 – Fall 2014Lecture D Motion in components x and y motions decouple v x =  x/  t; v y =  y/  t a x =  v x /  t; a y =  v y /  t If acceleration is only non-zero in 1 direction, can choose coordinates so that 1 component of acceleration is zero –e.g., motion under gravity

Physics 215 – Fall 2014Lecture A ball is ejected vertically upward from a cart at rest. The ball goes up, reaches its highest point and returns to the cart. In a second experiment, the cart is moving at constant velocity and the ball is ejected in the same way, where will the ball land? 1. In front of the cart. 2. Behind the cart. 3. Inside the cart. 4. The outcome depends on the speed of the cart.

Physics 215 – Fall 2014Lecture Motion under gravity a y = -g v y = v 0y - gt y = y 0 + v 0y t - (1/2)gt 2 a x = 0 v x = v 0x x = x 0 + v 0x t x y v0v0  v 0y = v 0 sin(  ) v 0x = v 0 cos(  ) Projectile motion...

Physics 215 – Fall 2014Lecture Projectile question A ball is thrown at 45 o to vertical with a speed of 7 m/s. Assuming g=10 m/s 2, how far away does the ball land?

Physics 215 – Fall 2014Lecture A battleship simultaneously fires two shells at enemy ships. If the shells follow the parabolic trajectories shown, which ship will be hit first? A. A B. Both at the same time C. B D. need more information

Physics 215 – Fall 2014Lecture Projectile motion R : when is y=0 ? t[v y1 -(1/2)gt] = 0 i.e., T = (2v)sin  /g  R (x-eqn.) max (y-eqn.)  h max (y-eqn.) y x

Physics 215 – Fall 2014Lecture Maximum height and range

Physics 215 – Fall 2014Lecture Reading assignment Kinematics in 2D , in textbook