Projectile Motion l l An object may move

Projectile Motion l l An object may move in both the x and y directions simultaneously The form of two-dimensional motion we will deal with is called projectile motion

Assumptions of Projectile Motion l The free-fall acceleration is constant over the range of motion l l It is directed downward It is reasonable as long as the range is small compared to the radius of the Earth The effect of air friction is negligible With these assumptions, an object in projectile motion will follow a parabolic path l This path is called the trajectory

Projectile Motion Diagram

Analyzing Projectile Motion l l l Consider the motion as the superposition of the motions in the x- and y-directions The actual position at any time is given by: The initial velocity can be expressed in terms of its components l l The x-direction has constant velocity l l vxi = vi cos q and vyi = vi sin q ax = 0 The y-direction is free fall l ay = -g

Effects of Changing Initial Conditions l The velocity vector components depend on the value of the initial velocity l l Change the angle and note the effect Change the magnitude and note the effect

Analysis Model l The analysis model is the superposition of two motions l l Motion of a particle under constant velocity in the horizontal direction Motion of a particle under constant acceleration in the vertical direction l Specifically, free fall

Projectile Motion Vectors l l The final position is the vector sum of the initial position, the position resulting from the initial velocity and the position resulting from the acceleration

Projectile Motion – Implications l l The y-component of the velocity is zero at the maximum height of the trajectory The acceleration stays the same throughout the trajectory

Range and Maximum Height of a Projectile l l l When analyzing projectile motion, two characteristics are of special interest The range, R, is the horizontal distance of the projectile The maximum height the projectile reaches is h

Height of a Projectile, equation l The maximum height of the projectile can be found in terms of the initial velocity vector: l This equation is valid only for symmetric motion

Range of a Projectile, equation l The range of a projectile can be expressed in terms of the initial velocity vector: l This is valid only for symmetric trajectory

More About the Range of a Projectile

Range of a Projectile, final l l The maximum range occurs at qi = 45 o Complementary angles will produce the same range l l The maximum height will be different for the two angles The times of the flight will be different for the two angles

Projectile Motion – Problem Solving Hints l Conceptualize l l Categorize l l l Establish the mental representation of the projectile moving along its trajectory Confirm air resistance is neglected Select a coordinate system with x in the horizontal and y in the vertical direction Analyze l l If the initial velocity is given, resolve it into x and y components Treat the horizontal and vertical motions independently

Projectile Motion – Problem Solving Hints, cont. l Analysis, cont l l Analyze the horizontal motion using constant velocity techniques Analyze the vertical motion using constant acceleration techniques Remember that both directions share the same time Finalize l l Check to see if your answers are consistent with the mental and pictorial representations Check to see if your results are realistic

Non-Symmetric Projectile Motion l l Follow the general rules for projectile motion Break the y-direction into parts l up and down or l symmetrical back to initial height and then the rest of the height Apply the problem solving process to determine and solve the necessary equations May be non-symmetric in other ways

Uniform Circular Motion l l l Uniform circular motion occurs when an object moves in a circular path with a constant speed The associated analysis motion is a particle in uniform circular motion An acceleration exists since the direction of the motion is changing l l This change in velocity is related to an acceleration The velocity vector is always tangent to the path of the object

Changing Velocity in Uniform Circular Motion l The change in the velocity vector is due to the change in direction l The vector diagram shows

Centripetal Acceleration l l l The acceleration is always perpendicular to the path of the motion The acceleration always points toward the center of the circle of motion This acceleration is called the centripetal acceleration

Centripetal Acceleration, cont l The magnitude of the centripetal acceleration vector is given by l The direction of the centripetal acceleration vector is always changing, to stay directed toward the center of the circle of motion

Fc- centripetal vs centrifugal Question Centripetal Is it “real”? Yes. Directly caused by some No it’s “fictitious”, only in the interaction (e. g. pull of sense that there is no “real” gravity or tension in a string) interaction. It’s just another way of expressing inertia, or Newton’s 3 rd law* What direction is it in for circular motion? Towards the centre Away from the centre What’s its magnitude for circular motion? Mv 2/r What symbol is usually used? Fc So why are there TWO forces? Centrifugal Fc There aren’t really 2 forces. It’s the same thing seen from 2 different perspectives. *Newton’s 3 rd law: For every action, there is an equal and opposite reaction.

Period l l l The period, T, is the time required for one complete revolution The speed of the particle would be the circumference of the circle of motion divided by the period Therefore, the period is defined as

Tangential Acceleration l l l The magnitude of the velocity could also be changing In this case, there would be a tangential acceleration The motion would be under the influence of both tangential and centripetal accelerations l Note the changing acceleration vectors

Total Acceleration l l The tangential acceleration causes the change in the speed of the particle The radial acceleration comes from a change in the direction of the velocity vector

Total Acceleration, equations l Tangential acceleration: l Centripetal acceleration: l Total acceleration: l Magnitude l Direction l Same as velocity vector if v is increasing, opposite if v is decreasing