Chapter 4: Problem 38
(a) Use the chain rule to show that for a particle in rectilinear motion \(a=v(d v / d s)\). (b) Let \(s=\sqrt{3 t+7}, t \geq 0 .\) Find a formula for \(v\) in terms of \(s\) and use the equation in part (a) to find the acceleration when \(s=5\).
Short Answer
Expert verified
(a) The acceleration is given by \( a = v \frac{d v}{d s} \). (b) When \( s = 5 \), acceleration \( a = -\frac{9}{500} \).
Step by step solution
01
Understand the Chain Rule for Derivatives
The chain rule is a formula to compute the derivative of a composite function. In this problem, we want to find the relationship between acceleration \( a \), velocity \( v \), and position \( s \). We start by noting that \( a = \frac{d v}{d t} \) and \( v = \frac{d s}{d t} \). Using the chain rule, we can relate \( \frac{d v}{d t} \) with \( \frac{d v}{d s} \).
02
Apply the Chain Rule
Since \( v = \frac{d s}{d t} \), the acceleration \( a \) can also be expressed using \( s \): \[a = \frac{d v}{d t} = \frac{d v}{d s} \cdot \frac{d s}{d t} = \frac{d v}{d s} \cdot v.\] Thus, the acceleration \( a \) is related to velocity \( v \) and the derivative \( \frac{d v}{d s} \) by the expression \( a = v \frac{d v}{d s} \). This verifies part (a) of the exercise.
03
Relate Time \( t \) to Position \( s \)
We're given \( s = \sqrt{3t + 7} \). To find velocity \( v \) in terms of \( s \), we first express \( t \) in terms of \( s \). Solve for \( t \) from the equation: \[s^2 = 3t + 7 \Rightarrow t = \frac{s^2 - 7}{3}.\]
04
Derive Velocity \( v(t) \)
The velocity is the derivative of \( s \) with respect to \( t \): \[v = \frac{d s}{d t} = \frac{d}{d t}\left(\sqrt{3t + 7}\right) = \frac{1}{2\sqrt{3t + 7}} \cdot 3 = \frac{3}{2\sqrt{3t + 7}}.\]
05
Substitute Expression of \( t \) in Velocity Formula
Substitute \( t = \frac{s^2 - 7}{3} \) into the velocity formula: \[v = \frac{3}{2\sqrt{3(\frac{s^2 - 7}{3}) + 7}} = \frac{3}{2\sqrt{s^2}} = \frac{3}{2s}.\] So, \( v = \frac{3}{2s} \).
06
Use Chain Rule Equation from Part (a) to Find Acceleration
Using the relationship \( a = v \frac{d v}{d s} \), first find \( \frac{d v}{d s} \) given \( v = \frac{3}{2s} \): \[\frac{d v}{d s} = -\frac{3}{2s^2}.\]Therefore, \[a = \left(\frac{3}{2s}\right) \left(-\frac{3}{2s^2}\right) = -\frac{9}{4s^3}.\]
07
Calculate Acceleration when \( s = 5 \)
Substitute \( s = 5 \) into the acceleration formula: \[a = -\frac{9}{4 \times 5^3} = -\frac{9}{500}.\]Thus, the acceleration when \( s = 5 \) is \(-\frac{9}{500}\).
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Understanding Acceleration
In physics, acceleration is a measure of how quickly the velocity of an object changes. It's not just about speeding up; slowing down or changing direction is also a sign of acceleration. Mathematically, acceleration (\(a\)) is the derivative of velocity with respect to time (\(t\)). So, if the velocity of a particle is expressed as a function of time, the change in that velocity is calculated as its derivative using the formula \(a = \frac{dv}{dt}\).
- Positive acceleration indicates an increase in velocity.
- Negative acceleration, often termed deceleration, indicates a decrease in velocity.
Understanding Velocity
Velocity is a vector quantity that describes not just the speed of an object, but its direction as well. When working with rectilinear motion, which is motion in a straight line, it's crucial to consider both aspects of velocity.
Typically, we denote velocity as \(v\), which is the rate of change of position with respect to time: \(v = \frac{ds}{dt}\).
However, to solve some advanced physics problems, it is useful to understand velocity in terms of position (\(s\)). This involves expressing the velocity function in such a way that it reflects how quickly position changes as a function of itself.
Typically, we denote velocity as \(v\), which is the rate of change of position with respect to time: \(v = \frac{ds}{dt}\).
However, to solve some advanced physics problems, it is useful to understand velocity in terms of position (\(s\)). This involves expressing the velocity function in such a way that it reflects how quickly position changes as a function of itself.
- In the given exercise, we used the chain rule to express velocity in relation to position, which revealed \(v = \frac{3}{2s}\)
- This showed that as the position changes, so does the velocity.
Comprehending Position
Position is a fundamental concept in motion. It tells us where an object is located in space at a given time. When we study motion along a straight line, such as in rectilinear motion, position is often described by a single variable, \(s\).
For the exercise, position was defined by the equation \(s = \sqrt{3t + 7}\), which means position changes with time \(t\).
For the exercise, position was defined by the equation \(s = \sqrt{3t + 7}\), which means position changes with time \(t\).
- This function conveys that as time progresses, the position of the object evolves.
- Understanding how \(s\) depends on time allows for further exploration into how velocity and acceleration are related to position.