In this explainer, we will learn how to apply the laws of the uniform acceleration motion of a particle in a straight line.
Let us begin by recalling how the displacement of a particle in uniform motion changes with time. We may already be familiar with the formula
This can also be expressed in terms of variables. Suppose that we start measuring a particle’s position at time and finish measuring it at time . Then, the velocity vector, , is equal to the time for which it moves, , divided by the displacement vector, . This can be expressed as
Additionally, the displacement can be written in terms of position vectors (i.e., vectors pointing from the origin to a particle’s position). Assuming that the initial position vector of the particle is and the final position vector is , we have
That is, by adding the displacement vector to the initial position vector, we obtain the final position vector. This can be rearranged to be in terms of the displacement:
Combining this equation with the equation for the velocity, we get the following formulas.
Formula: Velocity of a Particle in Uniform Motion
The velocity, , of a particle in uniform motion is given by where is the displacement, is the time interval, and and are the starting and finishing position vectors of the particle respectively.
Alternatively, it might be the case that a particle does not undergo uniform motion but does have uniform acceleration (i.e., acceleration that is constant). This is a very typical situation in the real world, since gravity causes all objects to have a uniform acceleration downward. Recall that acceleration is defined to be the change in velocity over time, given by
Just as before, this can be expressed in terms of variables.
Formula: Uniform Acceleration of a Particle
The acceleration, , of a particle, if it is uniform, is given by where is the initial velocity (which can also be referred to as ), is the final velocity, is the change in velocity, and is the time interval.
This formula can also be rearranged to be in terms of the final velocity, . Starting from we can multiply both sides by to get or, rearranged in terms of ,
This equation can be simplified in some additional ways. Typically, we will take the start time to be 0, meaning is just . We can then just write this without the subscript as . This gives us
Another simplification we can make is that since the acceleration is uniform, all of the motion will be in the same direction (or in the exact opposite direction). Thus, we can write this equation without the use of vectors, using a negative sign if the motion points in the opposite direction. In fact, this is an assumption we can make throughout this explainer. If we make this assumption, then we get the first of three kinematic formulas we will consider in this explainer.
Formula: First Kinematic Equation
For a particle moving with a constant acceleration, its velocity after a time period is given by where is its initial velocity and is its acceleration.
It is worth noting that if the acceleration is 0, then , which shows us that the initial velocity will be equal to the final velocity, as we would expect. Also, in the case that the initial velocity , then , which would mean that the velocity is directly proportional to the acceleration and the time period.
In any case, the above equation can be used to solve any problem in which three of the four variables are known, and we are required to find the fourth.
Let us now look at an example in which the velocity of a uniformly accelerating particle is determined.
Example 1: Finding the Final Velocity of a Uniformly Accelerating Particle
If a particle started moving in a straight line with an initial velocity of 25.1 cm/s and a uniform acceleration of 2.4 cm/s2, determine its velocity after 9 seconds.
Answer
The velocity of the particle after it accelerates can be determined by the formula
We can substitute the values given for , , and into the formula.
We then find that
The displacement of a particle is the product of its velocity and the time for which it moves. The velocity of a particle depends on its acceleration and the time for which it accelerates, so the displacement of a particle that is initially at rest can be expressed in terms of its acceleration and the time that it accelerates for.
For a particle with a constant velocity, its displacement can be expressed as
If we assume that is zero and denote as , this becomes
For a uniformly accelerating particle that is initially at rest and has a final velocity , the mean of the initial and final velocities is given by
The displacement of the particle in a time interval is then given by
For a particle initially at rest, it is the case that
Substituting this expression for into the expression for , we obtain
For a uniformly accelerating particle that initially has a velocity and has a final velocity , the mean of the initial and final velocities is given by
The displacement of the particle in a time interval is given by
This expression can also be shown using a graph, as in the following figure.
The graph shows that the area under the blue line consists of the sum of the area of a rectangle, , that is given by and the area of a right triangle, , that is given by
We can denote the velocity at as . The following figure shows that the area of the blue-shaded right triangle of side length equals the area of the white-shaded right triangle of side length .
The area below the blue line is, therefore, equal to that of the rectangle shown in the following figure.
This area is equal to the displacement of the particle at the instant , given by
For a particle with an initial velocity , it is the case that
Substituting this expression for into the expression for , we obtain
This is our second kinematic equation, and it is useful for any problem in which we need to determine the displacement of a particle with uniform acceleration.
Formula: Second Kinematic Equation
For a particle moving with a constant acceleration, its displacement after a time period is given by where is its initial velocity and is its acceleration.
We note that, much like our previous formula, this has some useful special cases we can consider. If the particle starts its motion at rest, meaning that the initial velocity , then we just have . Furthermore, if there is no acceleration, meaning , then we have , which describes a particle in uniform motion just as we saw at the beginning of the explainer.
Let us now look at an example in which the displacement of an accelerating body is determined.
Example 2: Finding the Distance Traveled by a Uniformly Accelerating Particle
A small ball started moving horizontally at 16.3 m/s. It moved in a straight line with a uniform deceleration of 3 m/s2. Determine the distance the ball covered in the first 2 seconds.
Answer
The ball is moving in a straight line, accelerating uniformly. The displacement of a uniformly accelerating body moving in a straight line is given by the formula where is the initial velocity of the body and is the acceleration of the body.
The ball in the question is stated to be decelerating uniformly. A decelerating body accelerates in the opposite direction to the direction of its velocity when it starts to accelerate. The sign of the acceleration therefore has the opposite sign to that of the initial velocity.
Substituting the values given in the question, we have
Let us look at another such example.
Example 3: Calculating the Initial and Final Velocity of a Uniformly Accelerating Particle
A particle, moving in a straight line, was accelerating at a rate of 22 cm/s2 in the same direction as its initial velocity. If the magnitude of its displacement 10 seconds after it started moving was 29 m, calculate the magnitude of its initial velocity and its velocity at the end of this period.
Answer
The displacement of a uniformly accelerating body is given by the formula where is the initial velocity of the body and is the acceleration of the body. In this question, is denoted by .
We are given the displacement of the body, its acceleration, and the time that it accelerates for. Substituting these values into the formula, we have
The displacement is given in metres, so the acceleration is converted from 22 cm/s2 to 0.22 m/s2.
This expression can be rearranged to make the subject:
The final velocity, , of the body is given by
Substituting known values, we find that
If the time that a body moves for is not known, but the displacement and the initial velocity of the body are known, then the final velocity can be determined. Equivalently, if the displacement and the final velocity of the body are known, then the initial velocity can be determined.
The relation between the initial and final velocity where the time is unknown involves the kinematic formulas and
The formula can be rearranged to express in terms of velocity and acceleration:
This expression for can be substituted into
This gives us
This expression can be rearranged as follows:
This is the final kinematic equation, and it is ideal in cases when we are given the displacement but not the time period.
Formula: Third Kinematic Equation
For a particle moving with a constant acceleration, its velocity following a displacement is given by where is its initial velocity and is its acceleration.
Let us now look at an example of how to model the motion of a particle that occurs in an unknown time interval.
Example 4: Finding the Final Velocity of a Uniformly Accelerating Particle
A particle was moving in a straight line with a constant acceleration of 2 cm/s2. Given that its initial velocity was 60 cm/s, find the velocity of the body when it was 15 m from the starting point.
Answer
As the time for which the particle moved is not known, the velocity of the particle is determined using the formula where is the final velocity, is the initial velocity, is the acceleration, and is the displacement.
The velocity and acceleration are given in centimetres per second and centimetres per second squared, respectively, so the displacement is converted from 15 metres to 1 500 centimetres. Substituting the known values into the formula, we obtain
To the nearest centimetre per second, is 98 cm/s.
Let us now look at an example of the motion of a particle that requires analyzing its motion in two separate time intervals.
Example 5: Using Kinematic Equations to Solve a Multistep Problem
A body was uniformly accelerating in a straight line such that it covered 72 m in the first 3 seconds and 52 m in the next 4 seconds. Find its acceleration and its initial velocity .
Answer
The distance covered by the particle in the first 3 seconds is greater than in the subsequent 4 seconds; hence, the particle is decelerating from an unknown initial velocity.
The mean velocity of a uniformly accelerating particle in a time interval is given by where is the initial velocity and is the final velocity. The mean velocity is also given by where is the displacement and is the time interval length. We have, therefore, that
This expression can be rearranged to give
In the first 3 seconds that the particle moves, we see that
In the next 4 seconds that the particle moves, we see that
The initial velocity equals the final velocity ; hence, we see that and
We have, therefore, that
The final velocity, after 7 seconds of acceleration, is 22 m/s less than the initial velocity. The acceleration of the particle in the direction of its initial velocity is therefore given by
The initial velocity, , can now be determined using the formula
Substituting known values, we obtain
Another way in which this question can be solved is by using simultaneous equations.
We can use the formula for the first 3 seconds of motion, obtaining
We can use the same formula for the subsequent 4 seconds of motion, obtaining
Both of these equations contain two unknowns. To eliminate one of the unknowns, we can multiply one of the equations by a factor to make the coefficient for that unknown equal to the coefficient of the unknown in the other equation.
We multiply by , obtaining
We can now subtract the equation from the equation
This gives us
This allows to be found by substitution, the same way as in the first method of solving the question.
Let us now summarize what we have learned in these examples.
Key Points
- The velocity of a particle (accelerating at a constant rate) can be expressed in terms of acceleration and time by the formula where is the final velocity, is the initial velocity, is the acceleration, and is the time for which the particle accelerates.
- The displacement of a particle can be expressed in terms of acceleration and time by the formula where is the displacement of the particle, is the acceleration of the particle, is the initial velocity of the particle, and is the time for which the particle accelerates.
- The velocity of a particle before and after acceleration can be expressed in terms of acceleration and displacement by the formula where is the initial velocity of the particle, is the final velocity of the particle, is the acceleration of the particle, and is the displacement of the particle.
- If the acceleration of a body is in the opposite direction to the direction of its initial velocity, then the acceleration and initial velocity have opposite signs.
