CHAPTER 3 Two-Dimensional Motion and Vectors

Representations: x y (x, y) (r,  ) VECTOR quantities: Vectors have magnitude and direction. Other vectors: velocity, acceleration, momentum, force …

Vector Addition/Subtraction 2nd vector begins at end of first vector Order doesn’t matter Vector addition Vector subtraction A – B can be interpreted as A+(-B)

Vector Components Going backwards, Cartesian components are projections along the x- and y-axes

Example 3.1 True or False: a) The magnitude of A-B = 0 b) The x-component of A-B > 0 c) The y-component of A-B > 0 a) F b) F c) T

Example 3.2 Alice and Bob carry a bottle of wine to a picnic site. Alice carries the bottle 5 miles due east, and Bob carries the bottle 10 miles traveling 30 degrees north of east. Carol, who is bringing the glasses, takes a short cut which goes directly to the picnic site. How far did Carol walk? What was Carol’s direction? miles, at degrees Alice Bob Carol

Arcsin, Arccos and Arctan: Watch out! same sine same cosine same tangent Arcsin, Arccos and Arctan functions can yield wrong angles if x or y are negative.

2-dim Motion: Velocity Graphically, v =  r /  t It is a vector (rate of change of position) Trajectory

Multiplying/Dividing Vectors by Scalars, e.g. r / t Vector multiplied/divided by scalar is a vector Magnitude of new vector is magnitudeo the orginial vector multiplied/divided by the |scalar| Direction of new vector is the same or opposite to original vector

Principles of 2-d Motion X- and Y-motion are independent Can be treated as two separate 1-d problems To get trajectory (x vs. y) 1.Solve for x(t) and y(t) 2.Invert one Eq. to get t(x) 3.Insert t(x) into y(t) to get y(x)

Projectile Motion X-motion is at constant velocity a x =0, v x =constant Y-motion is at constant acceleration a y =-g Note: we have ignored air resistance rotation of earth (Coriolis force)

Projectile Motion Acceleration is constant

Pop and Drop Demo

The Ballistic Cart Demo

Finding Trajectory, y(x) 1. Write down x(t) 2. Write down y(t) 3. Invert x(t) to find t(x) 4. Insert t(x) into y(t) to get y(x) Trajectory is parabolic

Example 3.3 An airplane drops food to two starving hunters. The plane is flying at an altitude of 100 m and with a velocity of 40.0 m/s. How far ahead of the hunters should the plane release the food? X 181 m h v0v0

Example The Y-component of v at A is ( 0) 2.The Y-component of v at B is ( 0) 3.The Y-component of v at C is ( 0) 4.The total velocity is greatest at (A,B,C) 5.The X-component of v is greatest at (A,B,C) h D  v0v0 1.> <0 4. A 5. Equal at all points

Range Formula  Good for when y f = y i

Range Formula  Maximum for  =45 

Example 3.5a 100 m A softball leaves a bat with an initial velocity of m/s. What is the maximum distance one could expect the ball to travel?

Example 3.5b 75.4 m/s A cannon aims a projectile at a target located on a cliff 500 m away in the horizontal direction and 75 meters above the cannon. The cannon is pointed 50 degrees to the horizontal. What muzzle velocity should the cannon employ to hit the target? h D  v0v0

A. If the arrow traveled with infinite speed on a straight line trajectory, at what angle should the hunter aim the arrow relative to the ground? B. Considering the effects of gravity, at what angle should the hunter aim the arrow relative to the ground? Example 3.7, Shoot the Monkey  =Arctan(h/L)=25.6  A hunter is a distance L = 40 m from a tree in which a monkey is perched a height h=20 m above the hunter. The hunter shoots an arrow at the monkey. However, this is a smart monkey who lets go of the branch the instant he sees the hunter release the arrow. The initial velocity of the arrow is v = 50 m/s.

Solution: Must find v 0,y /v x in terms of h and L 1. Write height of arrow when x=L 2. Require y to be position of monkey at t=L/v x 3. Require monkey and arrow to be at same place Aim directly at Monkey!

Shoot the Monkey Demo

Relative velocity Velocity always defined relative to reference frame. All velocities are relative Relative velocities are calculated by vector addition/subtraction. Acceleration is independent of reference frame For high, v ~c, rules are more complicated (Einstein)

Example hours = 1 hr. and 4 minutes mph A plane that is capable of traveling 200 m.p.h. flies 100 miles into a 50 m.p.h. wind, then flies back with a 50 m.p.h. tail wind. How long does the trip take? What is the average speed of the plane for the trip?

Relative velocity in 2-d Sum velocities as vectors velocity relative to ground = velocity relative to medium + velocity of medium. v be = v br + v re boat wrt river river wrt earth Boat wrt earth

2 Cases pointed perpendicular to stream travels perpendicular to stream

Example 3.9 An airplane capable of moving 200 mph in still air. The plane points directly east, but a 50 mph wind from the north distorts his course. What is the resulting ground speed? What direction does the plane fly relative to the ground? mph 14.0 deg. south of east

Example 3.10 An airplane capable of moving 200 mph in still air. A wind blows directly from the North at 50 mph. If the airplane accounts for the wind (by pointing the plane somewhat into the wind) and flies directly east relative to the ground. What is his resulting ground speed? In what direction is the nose of the plane pointed? mph 14.5 deg. north of east