Lecture III Curvilinear Motion

Topics Covered in Curvilinear Motion Plane curvilinear motion Coordinates used for describing curvilinear motion Rectangular coords n-t coords Polar coords

Plane curvilinear Motion Studying the motion of a particle along a curved path which lies in a single plane (2D). This is a special case of the more general 3D motion. 3D

Plane curvilinear Motion – (Cont.) If the x-y plane is considered as the plane of motion; from the 3D case, z and j are both zero, and R becomes as same as r. The vast majority of the motion of particles encountered in engineering practice can be represented as plane motion.

Coordinates Used for Describing the Plane Curvilinear Motion Normal-Tangential coordinates Polar coordinates Rectangular coordinates PC Path t y y q Path r Path n n PA P P n r t t PB q x x O O

Plane Curvilinear Motion – without Specifying any Coordinates (Displacement) Actual distance traveled by the particle (it is s scalar) Note: Since, here, the particle motion is described by two coordinates components, both the magnitude and the direction of the position, the velocity, and the acceleration have to be specified. The vector displacement of the particle Ds (Dt) Note: If the origin (O) is changed to some different location, the position r(t) will be changed, but Dr(Dt) will not change. or r(t)+Dr(Dt)

Plane Curvilinear Motion – without Specifying any Coordinates (Velocity) Note: vav has the direction of Dr and its magnitude equal to the magnitude of Dr divided by Dt. Average velocity (vav): Instantaneous velocity (v): as Dt approaches zero in the limit, Note: the average speed of the particle is the scalar Ds/Dt. The magnitude of the speed and vav approach one another as Dt approaches zero. Note: the magnitude of v is called the speed, i.e. v=|v|=ds/dt= s.. Note: the velocity vector v is always tangent to the path.

Plane Curvilinear Motion – without Specifying any Coordinates (Acceleration) Average Acceleration (aav): Instantaneous Acceleration (a): as Dt approaches zero in the limit, Note: aav has the direction of Dv and its magnitude is the magnitude of Dv divided by Dt. Note: in general, the acceleration vector a is neither tangent nor normal to the path. However, a is tangent to the hodograph. P V1 C V1 Hodograph P V2 V2 a1 a2

The description of the Plane Curvilinear Motion in the Rectangular Coordinates (Cartesian Coordinates)

Plane Curvilinear Motion - Rectangular Coordinates y v vy q vx P Path j r x O i a ay Note: the time derivatives of the unit vectors are zero because their magnitude and direction remain constant. ax P Note: if the angle q is measured counterclockwise from the x-axis to v for the configuration of the axes shown, then we can also observe that dy/dx = tanq = vy/vx.

Plane Curvilinear Motion - Rectangular Coordinates (Cont.) The coordinates x and y are known independently as functions of time t; i.e. x = f1(t) and y = f2(t). Then for any value of time we can combine them to obtain r. Similarly, for the velocity v and for the acceleration a. If is a given, we integrate to get v and integrate again to get r. The equation of the curved path can obtained by eliminating the time between x = f1(t) and y = f2(t). Hence, the rectangular coordinate representation of curvilinear motion is merely the superposition of the components of two simultaneous rectilinear motions in x- and y- directions.

Plane Curvilinear Motion - Rectangular Coordinates (Cont Plane Curvilinear Motion - Rectangular Coordinates (Cont.) – Projectile Motion y v vy vx vx vo vy g v Path (vy)o = vo sinq q x (vx)o = vo cosq

Exercises

Exercise # 1 A particle which moves with curvilinear motion has coordinates in millimeters which vary with the time t in seconds according to x = 2t2 - 4t and y = 3t2 – (1/3)t3. Determine the magnitudes of the velocity v and acceleration a and the angles which these vectors make with the x-axis when t = 2 s.

Exercise # 2 At any instant the horizontal position of the weather balloon is defined by x = (9t) m, where t is in second. If the equation of the path is y = x2/30, determine the distance of the balloon from the station at A, the magnitude and direction of the both the velocity and acceleration when t = 2 s.  

Exercise # 3 A sack slides off the ramp with a horizontal velocity of 12 m/s. If the height of the ramp is 6 m from the floor, determine the time needed for the sack to strike the floor and the range R where the sacks begin to pile up.

Exercise # 4 The chipping machine is designed to eject wood at chips vO = 7.5 m/s. If the tube is oriented at 30° from the horizontal, determine how high, h, the chips strike the pile if they land on the pile 6 m from the tube. 

Exercise # 5 A helicopter is flying with a constant horizontal velocity (V) of 144.2 km/h and is directly above point (A) when a loose part begins to fall. The part lands 6.5 s later at point (B) on inclined surface. Determine ;   a) The distance (d) between points (A) and (B). b) The initial height (h).