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# Motions under non-constant acceleration - PowerPoint PPT Presentation

Motions under non-constant acceleration. How do we know the position from the velocity function v(t) ?. Displacement from a Velocity - Time Graph.

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### Motions under non-constant acceleration

How do we know the position from the velocity function v(t)?

• The displacement of a particle during the time interval ti to tf is equal to the area under the curve between the initial and final points on a velocity-time curve

• First, we divide the time range from Ta to Tf into N equal intervals

• Displacement Dxn during the n-th interval

Total displacement from Ta to Tf

In the limit as N approaches to infinite, the time interval Dt becomes smaller and smaller.

Displacement is the area under the curve of v(t).

Position xf at Tf with initial position xa at Ta

If the function v(t) is constant in time,

Velocity vf at Tf with initial velocity va at Ta

If the function a(t) is constant in time,

Position xf at Tf under a constant acceleration with initial velocity va at Ta=0

### Chapter 3

Motion in Two Dimensions

• The position of an object is described by its position vector,

• The displacement of the object is defined as the change in its position

Fig 3.1

• The average velocity is the ratio of the displacement to the time interval for the displacement

• The direction of the average velocity is the direction of the displacement vector,

Fig 3.2

• The average velocity between points is independent of the path taken

• This is because it is dependent on the displacement, which is also independent of the path

• If a particle starts its motion at some point and returns to this point via any path, its average velocity is zero for this trip since its displacement is zero

• The instantaneous velocity is the limit of the average velocity as ∆t approaches zero

• The direction of the instantaneous velocity vector at any point in a particle’s path is along a line tangent to the path at that point and in the direction of motion

• The magnitude of the instantaneous velocity vector is the speed

• The average acceleration of a particle as it moves is defined as the change in the instantaneous velocity vector divided by the time interval during which that change occurs.

• As a particle moves, can be found in different ways

• The average acceleration is a vector quantity directed along

Fig 3.3

• The instantaneous acceleration is the limit of the average acceleration as approaches zero

• Various changes in a particle’s motion may produce an acceleration

• The magnitude of the velocity vector may change

• The direction of the velocity vector may change

• Even if the magnitude remains constant

• Both may change simultaneously

• When the two-dimensional motion has a constant acceleration, a series of equations can be developed that describe the motion

• These equations will be a generlization of those of one-dimensional kinematics to the vector form.

• Position vector

• Velocity

• Since acceleration is constant, we can also find an expression for the velocity as a function of time:

• The velocity vector can be represented by its components

• is generally not along the direction of either or

Fig 3.4(a)

• The position vector can also be expressed as a function of time:

• This indicates that the position vector is the sum of three other vectors:

• The initial position vector

• Thedisplacement resulting from

• The displacement resulting from

• The vector representation of the position vector

• is generally not in the same direction as or as

• and are generally not in the same direction

Fig 3.4(b)

• The equations for final velocity and final position are vector equations, therefore they may also be written in component form

• This shows that two-dimensional motion at constant acceleration is equivalent to two independent motions

• One motion in the x-direction and the other in the y-direction

• becomes

• vxf = vxi + axt and

• vyf = vyi + ayt

• becomes

• xf = xi + vxit + 1/2 axt2 and

• yf = yi + vyit + 1/2 ayt2

• The motion of an object under the influence of gravity only

• The form of two-dimensional motion

• The free-fall acceleration is constant over the range of motion

• And is directed downward

• The effect of air friction is negligible

• With these assumptions, the motion of the object will follow

• The final position is the vector sum of the initial position, the displacement resulting from the initial velocity and that resulting from the acceleration

• This path of the object is called the trajectory

Fig 3.6

• Consider the motion as the superposition of the motions in the x- and y-directions

• Constant-velocity motion in the x direction

• ax = 0

• A free-fall motion in the y direction

• ay = -g

• Reference frame chosen

• y is vertical with upward positive

• Acceleration components

• ay = -g and ax = 0

• Initial velocity components

• vxi = vi cos qi and vyi = vi sin qi

• The velocity components for the projectile at any time t are:

• vxf = vxi = vi cos qi = constant

• vyf = vyi – g t = vi sin qi – g t

• Displacements

• xf = vxit = (vi cos qi)t

• yf = vyit + 1/2ay t2 = (vi sinqi)t - 1/2 gt2

• Combining the equations gives:

• This is in the form of y = ax – bx2 which is the standard form of a parabola

• The range, R, is the maximum horizontal distance of the projectile

• The maximum height, h, is the vertical distance above the initial position that the projectile can reaches.

Fig 3.7

Fig 3.5

• The y-component of the velocity is zero at the maximum height of the trajectory

• The accleration stays the same throughout the trajectory

• The maximum height of the projectile can be found in terms of the initial velocity vector:

• The time to reach the maximum:

• The range of a projectile can be expressed in terms of the initial velocity vector:

• The time of flight = 2tm

• This is valid only for symmetric trajectory

• The maximum range occurs at qi = 45o

• Complementary angles will produce the same range

• The maximum height will be different for the two angles

• The times of the flight will be different for the two angles