Why Are Initial Velocity and Position the Constants in Motion Equations?

In summary, when integrating Newton's second law twice to find the position of a particle over time, the first constant of integration represents the initial velocity and the second constant represents the initial position. This is because the second derivative of position is acceleration, and if the acceleration is a polynomial of time, the constant of integration will equal the velocity at t=0. This can be seen logically by considering the value of the integrated polynomial at t=0. For a constant acceleration, the position of the object can be calculated using the equation S=S{_0}+U{_0}t-\frac{1}{2}at^2.
  • #1
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Homework Statement



So my main issue is with regards to when you integrate Newton's second law twice to get the position of a particle with respect to time. Why does everyone say that the first constant of your integration is initial velocity and second constant is initial position. Is there any logic behind that or is it just arbitary?


Homework Equations





The Attempt at a Solution

 
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  • #2
Because the second derivative of position is acceleration.
 
  • #3
It actually depends on what kind of function the acceleration is. But If the acceleration is a polynomial of time, then the constant of integration does equal the velocity at t=0.

You can realize this logically. If the acceleration is a polynomial, which you then integrate, then what must be the value of the polynomial at t=0?

P.S. welcome to physicsforums :)

Edit: I mean 'what is the value of the integrated polynomial, without the constant of integration, at t=0' Hmm, maybe I asked for too many steps at once. First, start off with a polynomial, then integrate it.
 
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  • #4
For a constant acceleration the position of the object is

[itex]S=S{_0}+U{_0}t-\frac{1}{2}at^2[/itex]

[itex]\frac {ds}{dt}=U{_0}-at[/itex]

[itex]\frac {dv}{dt}=-a[/itex]
 
  • #5


The reason for the first constant being the initial velocity and the second constant being the initial position is not arbitrary, but rather a result of the mathematics involved in solving the equations of motion. When integrating Newton's second law twice, the first constant arises as a result of the initial velocity term in the equation, while the second constant arises from the initial position term. These constants are necessary in order to fully determine the position of the particle with respect to time. Additionally, these constants have physical meaning in the context of the problem, as they represent the initial conditions of the particle's motion. Therefore, it is not arbitrary, but rather a result of the mathematical process and has a logical explanation.
 

FAQ: Why Are Initial Velocity and Position the Constants in Motion Equations?

What is an equation of motion problem?

An equation of motion problem is a physics problem that involves determining the mathematical relationship between an object's position, velocity, and acceleration over time. This relationship is described by a set of equations known as the equations of motion.

What are the three equations of motion?

The three equations of motion are:

  • 1. Velocity equation: v = u + at, where v is the final velocity, u is the initial velocity, a is the acceleration, and t is the time.
  • 2. Displacement equation: s = ut + 1/2at^2, where s is the displacement, u is the initial velocity, a is the acceleration, and t is the time.
  • 3. Acceleration equation: v^2 = u^2 + 2as, where v is the final velocity, u is the initial velocity, a is the acceleration, and s is the displacement.

How do you solve an equation of motion problem?

To solve an equation of motion problem, you first need to identify the known and unknown quantities. Then, use the appropriate equation of motion that relates these quantities and substitute the known values. Finally, solve for the unknown quantity using algebraic manipulation.

What are some common types of equation of motion problems?

Some common types of equation of motion problems include:

  • 1. Projectile motion problems, which involve objects moving through the air with a constant acceleration due to gravity.
  • 2. Simple harmonic motion problems, which involve objects oscillating back and forth with a constant acceleration.
  • 3. Uniformly accelerated motion problems, which involve objects moving in a straight line with a constant acceleration.

How can equations of motion be applied in real life?

Equations of motion have many real-life applications, such as predicting the motion of projectiles in sports, calculating the speed and distance of moving vehicles, and determining the time it takes for an object to fall from a certain height. They are also used in engineering to design and analyze the motion of machines and structures.

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