In this explainer, we will learn how to apply the laws of the uniform acceleration motion of a particle in a straight line.

A particle in uniform motion changes its displacement in the time that it moves. The displacement of a particle, , is the product of the time for which it moves, , and its velocity, . This can be expressed as

The initial displacement of the particle may have been zero or nonzero. If the particle has some initial displacement , then the final displacement of the particle, , is given by

These expressions are equivalent as

The change in time, , can be expressed as where is the initial time and is the final time.

This expression can be rearranged to define the velocity of a particle.

### Definition: Velocity of a Particle

The velocity, , of a particle is given by where is the displacement of the particle at an instant and is the displacement of the particle at an instant .

The velocity of a particle that accelerates uniformly changes uniformly as the particle moves. The change of the velocity of the particle is the product of the time for which it moves and its acceleration. This can be expressed as where is the initial velocity of the particle, is the final velocity of the particle, is the acceleration of the particle, and is the time interval over which the particle moves.

The change in time, , can be expressed as where is the initial time and is the final time.

This allows us to the write the expression

This expression can be rearranged to define the acceleration of a particle.

### Definition: Acceleration of a Particle

The acceleration, , of a particle is given by where is the velocity of the particle at a time and is the velocity of the particle at .

If we assume that is zero, we see that

We can denote as . Doing this allows us to express acceleration as and to express velocity as

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/s^{2},
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

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/s^{2}. 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/s^{2} 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/s^{2} to
0.22 m/s^{2}.

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:

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/s^{2}.
Given that its initial velocity was 60 cm/s, find the velocity of the body
to the nearest
centimetre per second 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 is given by where is the displacement of the particle at a time and is the displacement of the particle at .
- The acceleration, , of a particle is given by where is the velocity of the particle at a time and is the velocity of the particle at .
- 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.