# More theoretical questions about acceleration and velocity

• HealthPhysics
In summary, the equations for constant acceleration of motion, such as a*t= velocity or d/t=v, are derived from mathematical summaries of a velocity-time graph. By calculating the gradient of the graph, we can determine the rate of change of the quantity, defined as acceleration "a". The final velocity and displacement can then be calculated using the equations v = u + at and s = ut + 0.5at^2. These equations are derived from the area under the velocity-time graph, with the displacement being equal to the area of a trapezium, or the difference between a rectangle and a triangle.
HealthPhysics

## Homework Statement

This question is from a theory stand point.
I understand the definitions of velocity and acceleration separately. For some reason, I can not conceptualize them in equation form. How do you really know a*t= velocity or d/t=v ? Is there a proof for this or something? I need to understand the reasoning better. The professor for my class is like it just is. Any suggestions on how to make understanding the equations and relationships in physics more concretely in my head? I need more background info then my book is giving to help me to better understand how to derive the constant acceleration equations.

## Homework Equations

constant acceleration of motion

## The Attempt at a Solution

Last edited:
HealthPhysics said:

## Homework Statement

This question is from a theory stand point.
I understand the definitions of velocity and acceleration separately. For some reason, I can not conceptualize them in equation form. How do you really know a*t= velocity or d/t=v ? Is there a proof for this or something? I need to understand the reasoning better. The professor for my class is like it just is. Any suggestions on how to make understanding the equations and relationships in physics more concretely in my head? I need more background info then my book is giving to help me to better understand how to derive the constant acceleration equations.

## Homework Equations

a*t= velocity or d/t=v

## The Attempt at a Solution

If a crowd of 60 000 people entered a sports stadium in 40 minutes, then they entered at an average rate of 90 000 per hour.
Is that description OK
Rate of Entry = number of people / time

If a car covers 60 km in 40 minutes, then it was traveling at a rate of 90 km each hour. Rate of covering distance = distance covered / time.

We call THAT rate speed, symbol V, the distance covered is given symbol d and time is given symbol t; so v = d/t

So rather than a proof - it is a definition.

Try this: Prove that the four legged piece of furniture people sit on at a restaurant is a chair. Of course the long legged chair at the bar is actually a stool !

Also if a car increases its speed from 20 km/h to 80 km/h in 40 seconds, it was increasing its speed at the rate of 60 km/h per minute.
We call the rate of increase of speed acceleration (a) so a = Δv/t or Δv = a*t

Now if the initial speed was 0, then the change in speed (Δv) will equal the final speed v so that would look like a*t = v

PeterO said:
If a crowd of 60 000 people entered a sports stadium in 40 minutes, then they entered at an average rate of 90 000 per hour.
Is that description OK
Rate of Entry = number of people / time

If a car covers 60 km in 40 minutes, then it was traveling at a rate of 90 km each hour. Rate of covering distance = distance covered / time.

We call THAT rate speed, symbol V, the distance covered is given symbol d and time is given symbol t; so v = d/t

So rather than a proof - it is a definition.

Try this: Prove that the four legged piece of furniture people sit on at a restaurant is a chair. Of course the long legged chair at the bar is actually a stool !

Also if a car increases its speed from 20 km/h to 80 km/h in 40 seconds, it was increasing its speed at the rate of 60 km/h per minute.
We call the rate of increase of speed acceleration (a) so a = Δv/t or Δv = a*t

Now if the initial speed was 0, then the change in speed (Δv) will equal the final speed v so that would look like a*t = v

I understand this already. My question is more about constant acceleration of motion equations. Is there a good way to visualize this?

HealthPhysics said:
I understand this already. My question is more about constant acceleration of motion equations. Is there a good way to visualize this?

The equations of motion are mathematical summaries of a simple velocity-time graph.

If a body is accelerating, its velocity-time graph is a sloping, straight line. The gradient of the line is the rate of change of the quantity graphed, defined as acceleration "a".

If the initial velocity is called "u", then that us the intercept on the vertical axis.
After a period of time "t", the velocity will have increased to a final velocity "v".
Thus there is a co-ordinate on the line (t,v).

If you draw such a graph, you add a pair of reference lines: a horizontal line at the initial velocity, and also at the final velocity.
The gradient of the line [rise / run] tells us that the increase in velocity over this time interval is given by a*t.
Thus the final velocity is given by:

v = u + at

The displacement during this time is given by the area under the graph.
Those horizontal lines show that area in 3 ways.

#1 - a lower rectangle plus a triangle [length x width plus 1/2 x base x height]
The rectangle has length t, and width u --> area ut
The triangle has base t, and height a*t so are 0.5 * t * a*t = 0.5at2

using symbol s for displacement we have

s = ut + 0.5at2

#2 - a large rectangle, with a triangle removed from the top left.
The large rectangle has length, t, and width, v, so area vt

as above we get

s = vt - 0.5at2

#3 - before the horizontal lines were drawn, the graph represented a trapezium on its side - the parallel sides are the vertical lines along the vertical axis, and the ordinate t.

The area of a trapezium is given by area = height * (base + top)/2
using our symbols that means

s = t(v+u)/2

That gives us 4 equations of motion - all mathematical summaries of one simple graph.

We can do one more algebraic manipulation using the first and last of them.

s = t(v+u)/2 and v = u + at

The second can be transformed to show t = (v-u)/a

substituting for t in the first we get

s = (v-u)/a * (v+u)/2

multiply both sides by 2a to eliminate the "fractions"

2as = (v-u)(v+u)

The right hand side is the classic "difference of two squares" expression
so

2as = v2 - u2

re-arranging so that there are no minus signs we get the more familiar

v2 = u2 + 2as

I would suggest approaching this question by breaking down the equations and understanding the individual components. Velocity is the rate of change of an object's position over time, while acceleration is the rate of change of an object's velocity over time. In other words, acceleration is the change in velocity divided by the change in time.

The equations a*t= velocity and d/t=v are simply representations of this relationship. The first equation, a*t= velocity, is derived from the definition of acceleration (change in velocity over time) and the second equation, d/t=v, is derived from the definition of velocity (change in position over time). These equations are based on the fundamental principles of motion and are widely accepted because they have been repeatedly tested and proven to accurately describe the relationship between acceleration, velocity, and time.

To better understand these equations, I would suggest reviewing the concepts of derivatives and integrals in calculus, as they are the mathematical tools used to derive these equations. Additionally, practicing with different examples and scenarios can also help solidify your understanding.

It is also important to note that these equations are specifically for objects moving with constant acceleration. In cases where acceleration is not constant, more complex equations may be needed to accurately describe the motion.

Overall, understanding the fundamental principles and concepts behind these equations can help make them more concrete in your mind. I would also recommend seeking additional resources or seeking clarification from your professor if needed.

## 1. What is the difference between acceleration and velocity?

Acceleration is the rate of change of velocity over time, while velocity is the rate of change of displacement over time. In simpler terms, acceleration measures how quickly an object's speed is changing, while velocity measures how fast an object is moving and in what direction.

## 2. Can an object have a constant velocity and changing acceleration?

Yes, an object can have a constant velocity and changing acceleration if it is moving in a circular path. In this case, the object's speed remains constant, but its direction of motion is changing, resulting in a changing acceleration.

## 3. How is acceleration related to force and mass?

According to Newton's second law of motion, acceleration is directly proportional to the net force acting on an object and inversely proportional to its mass. This means that the greater the force acting on an object, the greater its acceleration will be, and the greater the mass of an object, the smaller its acceleration will be.

## 4. What is the difference between average acceleration and instantaneous acceleration?

Average acceleration is the change in velocity over a given time interval, while instantaneous acceleration is the acceleration at a specific moment in time. Average acceleration provides an overall understanding of an object's motion, while instantaneous acceleration gives a more detailed picture of how an object is moving at a particular instant.

## 5. How does positive and negative acceleration affect an object's motion?

Positive acceleration indicates that an object is speeding up, while negative acceleration (also known as deceleration or retardation) indicates that an object is slowing down. The direction of acceleration depends on the direction of the net force acting on the object. For example, if the net force is in the same direction as the object's motion, it will have a positive acceleration, and if the net force is in the opposite direction, it will have a negative acceleration.

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