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Automotive engineers refer to the time rate of change of acceleration as the "jerk." If an object moves in one dimension such that its jerk \(J\) is constant, (a) determine expressions for its acceleration \(a_{x}(t),\) velocity \(v_{x}(t),\) and position \(x(t),\) given that its initial acceleration, velocity, and position are \(a_{x i}, v_{x i},\) and \(x_{i},\) respectively. (b) Show that \(a_{x}^{2}=\) \(a_{x i}^{2}+2 J\left(v_{x}-v_{x i}\right) .\)

Short Answer

Expert verified
\(a_x(t) = Jt + a_{xi}\), \(v_x(t) = \frac{1}{2}Jt^2 + a_{xi}t + v_{xi}\), \(x(t) = \frac{1}{6}Jt^3 + \frac{1}{2}a_{xi}t^2 + v_{xi}t + x_i\); also, \(a_x(t)^2 = a_{xi}^2 + 2J(v_x(t) - v_{xi})\) is proven.

Step by step solution

01

Understanding Jerk and Setting It as a Constant

Jerk is defined as the rate of change of acceleration over time. Since the jerk, J, is constant, we can represent it as a function of time t: \(J = \frac{d^2a_x}{dt^2} = \text{constant}\). This relationship is the starting point for finding expressions for acceleration, velocity, and position.
02

Integrating Jerk to Find Acceleration

To find the acceleration as a function of time, we integrate the jerk once with respect to time t, which gives us the acceleration plus a constant of integration (which represents the initial acceleration, \(a_{xi}\)): \(a_x(t) = \int J\, dt = Jt + C_1\). Using the initial condition that at \(t=0\), \(a_x = a_{xi}\), we solve for \(C_1\) and get \(C_1 = a_{xi}\), thus, \(a_x(t) = Jt + a_{xi}\).
03

Integrating Acceleration to Find Velocity

We take the acceleration function found in Step 2 and integrate it with respect to time to find the velocity function: \(v_x(t) = \int (Jt + a_{xi})\, dt = \frac{1}{2}Jt^2 + a_{xi}t + C_2\). By applying the initial condition that at \(t=0\), \(v_x = v_{xi}\), we solve for \(C_2\) to obtain \(C_2 = v_{xi}\), so the velocity function becomes \(v_x(t) = \frac{1}{2}Jt^2 + a_{xi}t + v_{xi}\).
04

Integrating Velocity to Find Position

To find the position function, we integrate the velocity over time: \(x(t) = \int (\frac{1}{2}Jt^2 + a_{xi}t + v_{xi})\, dt = \frac{1}{6}Jt^3 + \frac{1}{2}a_{xi}t^2 + v_{xi}t + C_3\). Given the initial condition that at \(t = 0\), \(x = x_i\), we can find the constant \(C_3\) to be \(C_3 = x_i\), resulting in the position function \(x(t) = \frac{1}{6}Jt^3 + \frac{1}{2}a_{xi}t^2 + v_{xi}t + x_i\).
05

Proving the Relationship between Acceleration and Velocity

Squaring both sides of the acceleration function we obtained, we get \(a_x(t)^2 = (Jt + a_{xi})^2 = J^2t^2 + 2Ja_{xi}t + a_{xi}^2\).We also know from the velocity function that \(v_x(t) - v_{xi} = \frac{1}{2}Jt^2 + a_{xi}t\).Multiplying both sides of the latter by 2J, we obtain \(2J(v_x(t) - v_{xi}) = 2J(\frac{1}{2}Jt^2 + a_{xi}t) = J^2t^2 + 2Ja_{xi}t\), which matches the right side of the equation for \(a_x(t)^2\) minus the constant term \(a_{xi}^2\).Therefore, \(a_x(t)^2 = a_{xi}^2 + 2J(v_x(t) - v_{xi})\), proving the given relationship.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Rate of Change of Acceleration
The concept of 'jerk' in physics relates to the rate of change of acceleration. It's like acceleration but one level deeper. While acceleration tells us how velocity changes over time, jerk describes how acceleration itself is changing. In simpler terms, if acceleration deals with the speed of an object picking up speed, jerk deals with how quickly that pick-up in speed is increasing or decreasing.

Imagine you're in a car that starts to accelerate; if the car suddenly speeds up, you're experiencing a high jerk. Conversely, if the accelerator is pressed gently and the speed increases smoothly, the jerk is low. In mathematical terms, if the jerk (J) is constant, it indicates a uniform change in acceleration. This has significant implications for the kinematic equations, as it allows us to predict an object's future velocity and position with precision, based on its acceleration behavior over time.
Integration in Physics
Integration is a fundamental operation in calculus that allows us to find quantities like displacement or velocity when we have a function describing its rate of change. In essence, when the jerk (a constant value in our case) is integrated, it gives us the acceleration function. That's because integrating the rate of change gives us the quantity that was changing—similar to adding up small amounts of change over time to get the total change.

In the context of our exercise, we used integration thrice: first to find acceleration from jerk, then velocity from acceleration, and finally, position from velocity. Each integration step required applying initial conditions to solve for constants of integration. These constants are crucial because they anchor our general solution to the specific scenario we're analyzing, like attaching specific numbers to a story we're telling.
Kinematic Equations
Kinematic equations are the bread and butter of introductory physics; they relate the variables of motion—position, velocity, acceleration, and time—without accounting for the forces causing the motion. For an object in one-dimensional motion with constant acceleration, kinematics provides a set of equations to connect these variables. However, if the jerk is also a factor, as in this exercise, we need to work with kinematic equations derived from calculus rather than the standard ones.

By systematically integrating the constant jerk, we create a cascade effect through kinematics: each variable is built upon the previous, from jerk to acceleration, acceleration to velocity, and velocity to position. This way, we can determine an object’s entire motion history and predict its future motion, provided we know the initial conditions and the constant jerk.
Initial Velocity and Acceleration
Initial conditions are what set the stage in any motion problem. They give us the values for velocity and acceleration at the starting point, time zero. In a jerk-influenced scenario, the initial acceleration and velocity serve as starting blocks for our integrations. These values allow us to define constants when we integrate jerk to get acceleration, and further when we integrate this changing acceleration to get velocity and then position.

The role of initial velocity () and initial acceleration () is more than just plugging in numbers—they inform us of how the motion begins. This information is indispensable because it helps us understand how the story of the object's motion unfolds from the very start. It's akin to knowing the speed and heading of a spacecraft at launch to predict its journey through space.

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Most popular questions from this chapter

An object moving with uniform acceleration has a velocity of 12.0 \(\mathrm{cm} / \mathrm{s}\) in the positive \(x\) direction when its \(x\) coordinate is \(3.00 \mathrm{cm} .\) If its \(x\) coordinate 2.00 \(\mathrm{s}\) later is \(-5.00 \mathrm{cm},\) what is its acceleration?

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