The period is completely independent of other factors, such as mass or amplitude. The only things that affect the period of a simple pendulum are its length and the acceleration due to gravity. įor small angle oscillations of a simple pendulum, the period is T = 2 π L g. This leaves a net restoring force back toward the equilibrium position that runs tangent to the arc and equals − m g sin θ θ. Tension in the string exactly cancels the component m g cos θ θ parallel to the string. The weight m g has components m g cos θ θ along the string and m g sin θ θ tangent to the arc. The displacement of the pendulum bob is the arc length s. Are they constant for a given pendulum? How does the mass impact the frequency? How does the initial displacement affect it? What happens if a small push is given to the pendulum to get it started? Does that change the frequency? In what way does the length affect the frequency? Ask students to measure their time periods or frequencies. Construct simple pendulums of different lengths. The relationship between frequency and period is The SI unit for frequency is the hertz (Hz), defined as the number of oscillations per second. Its units are usually seconds.įrequency f is the number of oscillations per unit time. The time to complete one oscillation (a complete cycle of motion) remains constant and is called the period T. Periodic motion is a motion that repeats itself at regular time intervals, such as with an object bobbing up and down on a spring or a pendulum swinging back and forth. Each vibration of the string takes the same time as the previous one. When you pluck a guitar string, the resulting sound has a steady tone and lasts a long time. From there, the motion will repeat itself. (e) In the absence of damping (caused by frictional forces), the ruler reaches its original position. (d) Now the ruler has momentum to the left. It stops the ruler and moves it back toward equilibrium again. (c) The restoring force is in the opposite direction. (b) The net force is zero at the equilibrium position, but the ruler has momentum and continues to move to the right. Therefore, Hooke’s law describes and applies to the simplest case of oscillation, known as simple harmonic motion.įigure 5.38 (a) The plastic ruler has been released, and the restoring force is returning the ruler to its equilibrium position. Recall that Hooke’s law describes this situation with the equation F = − kx. The simplest oscillations occur when the restoring force is directly proportional to displacement. It is then forced to the left, back through equilibrium, and the process is repeated until it gradually loses all of its energy. However, by the time the ruler gets there, it gains momentum and continues to move to the right, producing the opposite deformation. Once released, the restoring force causes the ruler to move back toward its stable equilibrium position, where the net force on it is zero. The deformation of the ruler creates a force in the opposite direction, known as a restoring force. Consider, for example, plucking a plastic ruler to the left as shown in Figure 5.38. Without force, the object would move in a straight line at a constant speed rather than oscillate. Newton’s first law implies that an object oscillating back and forth is experiencing forces. How does mass of the system affect them? How does the initial force applied affect them? Ask students to observe how the stiffness of the spring affects them. Introduce the terms frequency and time period. Ask students to attach weights to these to construct oscillators. Find springs or rubber bands with different amounts of stiffness. One of the most common uses of Hooke’s law is solving problems involving springs and pendulums, which we will cover at the end of this section. The units of k are newtons per meter (N/m). A stiffer system is more difficult to deform and requires a greater restoring force. The larger the force constant, the stiffer the system. The force constant k is related to the stiffness of a system. In the absence of force, the object would rest at its equilibrium position. It is a change in position due to a force. The deformation can also be thought of as a displacement from equilibrium. Note that the restoring force is proportional to the deformation x. The restoring force is the force that brings the object back to its equilibrium position the minus sign is there because the restoring force acts in the direction opposite to the displacement. Where x is the amount of deformation (the change in length, for example) produced by the restoring force F, and k is a constant that depends on the shape and composition of the object.
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