PHYS144 Chapter 2: phys144_partB_kinematics.pdf

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greenclam342

4 Feb 2015

School

University of Alberta

Department

Physics

Course

PHYS144

Professor

Darren Grant

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Document Summary

Kinematics is the study of objects in motion, and motion implies a change in position known as displacement. Displacement is a vector that points from the object"s initial position to the nal position. Large changes in a quantity are generally denoted using the capital greek letter. Position is often denoted as x so displacement may be written as x. In this course we will use x or s to represent displacement and x to represent position. In some books you will nd displacement also written as x - x0, where x0 is the initial position. Velocity describes motion in a given direction and speed, a scalar, is the magnitude of velocity. Consider an object starting at a position x0 at time t0: The object then moves so that at time t it is at position x. The average velocity of the object is given by:

Related Questions

Good Vibes: Introduction to Oscillations

The conditions that lead to simple harmonic motion are as follows:

There must be a position of stable equilibrium.

There must be a restoring force acting on the oscillating object. The direction of this force must always point toward the equilibrium, and its magnitude must be directly proportional to the magnitude of the object's displacement from its equilibrium position. Mathematically, the restoring force F⃗ is given by ⃗, where is the displacement from equilibrium and k is a constant that depends on the properties of the oscillating system.

The resistive forces in the system must be reasonably small.

Consider a block of mass m attached to a spring with force constant k, as shown in the figure(Figure 1) . The spring can be either stretched or compressed. The block slides on a frictionless horizontal surface, as shown. When the spring is relaxed, the block is located at x=0. If the block is pulled to the right a distance A and then released, A will be the amplitude of the resulting oscillations.

Assume that the mechanical energy of the block-spring system remains unchanged in the subsequent motion of the block.

Part A

After the block is released from x=A, it will

(a) remain at rest

(b) move to the left until it reaches equilibrium and stop there.

(d) move to the left until it reaches x=−A and then begin to move to the right.

The time it takes the block to complete one cycle is called the period. Usually, the period is denoted T and is measured in seconds.

The frequency, denoted f, is the number of cycles that are completed per unit of time: f=1/T. In SI units, f is measured in inverse seconds, or hertz (Hz).

Part B

If the period is doubled, the frequency is

(a) unchanged

(b) doubled

Part C

An oscillating object takes 0.10 s to complete one cycle; that is, its period is 0.10 s. What is its frequency f?

Express your answer in hertz.

Part D

If the frequency is 40 Hz, what is the period T?

Express your answer in seconds.

The following questions refer to the figure (Figure 2) that graphically depicts the oscillations of the block on the spring.

Note that the vertical axis represents the x-coordinate of the oscillating object, and the horizontal axis represents time.

Part E

Which points on the x-axis are located at a distance A from the equilibrium position?

(a) R only

(b) Q only

Part F

Suppose that the period is T. Which of the following points on the t-axis are separated by the time interval T?

(a) K and L

(b) K and M

(d) L and N

(e) M and P

Now assume for the remaining Parts G - J, that the x coordinate of point R is 0.12 m and the t coordinate of point K is 0.0050 s.

Part G

What is the period T?

Express your answer in seconds.

Part H

How much time t does the block take to travel from the point of maximum displacement to the opposite point of maximum displacement?

Express your answer in seconds.

Part I

What distance d does the object cover during one period of oscillation?

Express your answer in meters.

Part J

What distance d does the object cover between the moments labeled K and N on the graph?

Express your answer in meters.

emeraldyak680

Learning Goal: Understand how to determine the constants in thegeneral equation for simple harmonic motion, in terms of giveninitial conditions.
A common problem in physics is to match the particular initialconditions - generally given as an initial position x_0 andvelocity v_) at t=0 - once you have obtained the general solution.You have dealt with this problem in kinematics, where theformula

1. x(t) = x_0 + v_0t + at^2

has two arbitrary constants (technically constants of integrationthat arise when finding the position given that the acceleration isa constant). The constants in this case are the initial positionand velocity, so "fitting" the general solution to the initialconditions is very simple.

For simple harmonic motion, it is more difficult to fit the initialconditions, which we take to be

x_0, the position of the oscillator at t = 0, and
v_0, the velocity of the oscillator at t = 0.

There are two common forms for the general solution for theposition of a harmonic oscillator as a function of time t:

2. x(t) = A\cos(omega t + phi) and
3. x(t) = C\cos(omega t) + S\sin(omega t),

where A, phi, C, and S are constants, omega is the oscillationfrequency, and t is time.

Although both expressions have two arbitrary constants--parametersthat can be adjusted to fit the solution to the initialconditions--Equation 3 is much easier to use to accommodate x_0 andv_0. (Equation 2 would be appropriate if the initial conditionswere specified as the total energy and the time of the first zerocrossing, for example.)

Find C and S in terms of the initial position and velocity of theoscillator.
Give your answers in terms of x_0, v_0, and omega. Separate youranswers with a comma.

to change the rate of their rotation with virtually constant angular acceleration. Many introductory problems in rotational kinematics involve the motion of a particle with constant, nonzero angular acceleration. The kinematic equations for such motion can be written as

$\theta (t) = \theta_0 +\omega_0t + \frac{1}{2}\alpha t^2$

and

$\omega (t) = \omega_0 + \alpha t$ .

Here, the symbols are defined as follows:

is the angular position of the particle at a time t.
is the initial angular position of the particle.
is the angular velocity of the particle at a time t.
is the initial angular velocity of the particle.
is the angular acceleration of the particle.
is the time that has elapsed since the particle was located at its initial position.

In answering the following questions, assume that the angular acceleration is constant and nonzero: $\alpha \neq 0$ .

True or false: The quantity represented by theta

is a function of time (i.e., is not constant).

	true
	false

True or false: The quantity represented by theta_0

is a function of time (i.e., is not constant).

	true
	false

True or false: The quantity represented by omega_0

is a function of time (i.e., is not constant).

	true
	false

True or false: The quantity represented by omega

is a function of time (i.e., is not constant).

	true
	false

Which of the following equations is not an explicit function of time

? Keep in mind that an equation that is an explicit function of time involves

as a variable.

	$\theta=\theta_0+\omega_0 t+\frac{1}{2}\alpha t^2$
	$\omega=\omega_0+\alpha t$
	$\omega^2=\omega_0^2+2\alpha(\theta-\theta_0)$

In the equation $\omega=\omega_0+\alpha t$ , what does the time variable

represent?

Choose the answer that is always true. Several of the statements may be true in a particular problem, but only one is always true.

	the moment in time at which the angular velocity equals $\omega_0$
	the moment in time at which the angular velocity equals $\omega$
	the time elapsed from when the angular velocity equals $\omega_0$ until the angular velocity equals $\omega$

Consider two particles A and B. The angular position of particle A, with constant angular acceleration, depends on time according to $\theta_{\rm A}(t)=\theta_0+\omega_0t+\frac{_1}{^2}\alpha t^2$ . At time $t=t_1$ , particle B, which also undergoes constant angular acceleration, has twice the angular acceleration, half the angular velocity, and the same angular position that particle A had at time $t=0$ .

Which of the following equations describes the angular position of particle B?

	$\theta_{\rm B}(t)=\theta_0+2\omega_0t+\frac{1}{4}\alpha t^2$
	$\theta_{\rm B}(t)=\theta_0+\frac{1}{2}\omega_0t+\alpha t^2$
	$\theta_{\rm B}(t)=\theta_0+2\omega_0(t-t_1) +\frac{1}{4}\alpha (t-t_1)^2$
	$\theta_{\rm B}(t)=\theta_0+\frac{1}{2}\omega_0(t-t_1)+\alpha (t-t_1)^2$
	$\theta_{\rm B}(t)=\theta_0+2\omega_0(t+t_1) +\frac{1}{4}\alpha (t+t_1)^2$
	$\theta_{\rm B}(t)=\theta_0+\frac{1}{2}\omega_0(t+t_1)+\alpha (t+t_1)^2$

How long after the time t_1

does the angular velocity of particle B equal that of particle A?

	$\frac{\omega_0}{4\alpha}$
	$\frac{\omega_0+4\alpha t_1}{2\alpha}$
	$\frac{\omega_0+2\alpha t_1}{2\alpha}$
	The two particles never have the same angular velocity.

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PHYS144 Chapter 2: phys144_partB_kinematics.pdf

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