Learn where the kinematic equations come from, and how you can use them to analyze scenarios involving constant acceleration.
What are the kinematic equations?
The kinematic equations relate the five kinematic variables listed below.
The in these formulas really represents the interval of time it took the object to be displaced . It would probably be conceptually clearer to write the time interval as but the kinematic formulas are already so cluttered that people usually leave off the sign in front of the time for the sake of cleanliness.
We just have to make sure we know that by writing , we really mean the time interval .
If we know three of these five kinematic variables for an object undergoing constant acceleration, we can use a kinematic equation to solve for one of the unknown variables.
The kinematic equations are listed below.
In this section, we're just stating the kinematic equations. In the sections below, we'll derive each kinematic equation individually so we can see where they come from and why they're only true assuming constant acceleration.
In the solved examples section below, we will also solve an example problem using each individual equation in order.
Since the kinematic equations are only accurate if the acceleration is constant during the time interval considered, we have to be careful to not use them when the acceleration is changing. Also, the kinematic equations assume all variables are referring to the same direction: horizontal , vertical , etc.
If you plug in a horizontal initial velocity into a kinematic equation, the rest of the variables you plug into that equation should also be for the horizontal direction.
If you plug in a vertical initial velocity into a kinematic equation, the rest of the variables you plug into that formula should also be for the vertical direction.
In other words, to be really clear, the kinematic formulas for a given direction—for example, —should really be written with a subscript to denote that direction:
However, including all these subscripts can start to get messy. So textbooks, courses, and professors often leave off the subscripts and just remember that the kinematic equations will work for any direction—even diagonal—in which the acceleration is constant, as long as you include only variables in that particular direction.
What is a free falling object—i.e., a projectile?
It might seem like the fact that the kinematic equations only work for time intervals of constant acceleration would severely limit the applicability of these equations. However one of the most common forms of motion, free fall, just happens to be constant acceleration.
All free falling objects—also called projectiles—on Earth, regardless of their mass, have a constant downward acceleration due to gravity of magnitude .
A free falling object is defined as any object that is accelerating only due to the influence of gravity. We often assume the effect of air resistance is small enough to ignore, in which case an object that is dropped, thrown, or otherwise falling freely has a constant downward acceleration of magnitude .
Note that is just the magnitude of the acceleration due to gravity. If upward is selected as positive, we must make the acceleration due to gravity negative for a projectile when we plug into the kinematic equations.
Warning: Forgetting to include a negative sign is one of the most common sources of error when using kinematic equations.
How do you select and use a kinematic equation?
We choose the kinematic equation that includes both the unknown variable we're looking for and three of the kinematic variables we already know. This way, we can solve for the unknown we want to find, which will be the only unknown in the equation.
For instance, say we knew a book on the ground was kicked forward with an initial velocity of , after which it took a time interval for the book to slide a displacement of . We could use the kinematic equation to algebraically solve for the unknown acceleration of the book—assuming the acceleration was constant—since we know every other variable in the equation besides
Problem solving tip: Note that each kinematic equation is missing one of the five kinematic variables—.
To choose the kinematic equation that's right for your problem, figure out which variable you are not given and not asked to find. For example, in the problem given above, the final velocity of the book was neither given nor asked for, so we should choose a equation that does not include at all. The kinematic equation is missing , so it's the right choice in this case to solve for the acceleration .
The fifth kinematic equation looks just like the third kinematic equation except with the initial velocity replaced with final velocity and the plus sign replaced with a minus sign. It can be derived by plugging the first kinematic equation into the third kinematic equation.
This fifth kinematic equation often isn't as useful since the initial velocity is typically known/given in many situations.
How do you derive the first kinematic equation, ?
This kinematic equation is the easiest to derive since it is really just a rearranged version of the definition of acceleration. We can start with the definition of acceleration,
Yes. If the time interval is large, the formula is the average acceleration over the time interval.
So, the derivation we are going through below really assumes that the in the equation is the average acceleration . However, if the acceleration is constant, the instantaneous acceleration will equal the average acceleration , in which case we don't have to worry about being specific and can just call it .
So, for the kinematic equation we will derive to be accurate, the acceleration should be constant. Or else the in the equation is referring to the average acceleration and not the instantaneous acceleration.
Now we can replace with the definition of change in velocity .
Finally if we just solve for we get
And if we agree to just use for , this becomes the first kinematic equation.
How do you derive the second kinematic equation, ?
A cool way to visually derive this kinematic equation is by considering the velocity graph for an object with constant acceleration—in other words, a constant slope—and starts with initial velocity as seen in the graph below.
The area under any velocity graph gives the displacement . So, the area under this velocity graph will be the displacement of the object.
We can conveniently break this area into a blue rectangle and a red triangle as seen in the graph above.
The height of the blue rectangle is and the width is , so the area of the blue rectangle is .
The base of the red triangle is and the height is , so the area of the red triangle is .
The total area will be the sum of the areas of the blue rectangle and the red triangle.
If we distribute the factor of we get
We can simplify by combining the terms to get
And finally we can rewrite the right hand side to get the second kinematic equation.
This equation is interesting since if you divide both sides by , you get . This shows that the average velocity equals the average of the final and initial velocities . However, this is only true assuming the acceleration is constant since we derived this equation from a velocity graph with constant slope/acceleration.
How do you derive the third kinematic equation, ?
There are a couple ways to derive the equation . There's a cool geometric derivation and a less exciting plugging-and-chugging derivation. We'll do the cool geometric derivation first.
Consider an object that starts with a velocity and maintains constant acceleration to a final velocity of as seen in the graph below.
Since the area under a velocity graph gives the displacement , each term on the right hand side of the equation represents an area in the graph above.
The term represents the area of the blue rectangle since .
The term represents the area of the red triangle since .
The base of the red triangle is given by . The height of the red triangle is given by .
But this height can be rewritten as since , from the definition of acceleration.
So,
That's it. The equation has to be true since the displacement must be given by the total area under the curve. We did assume the velocity graph was a nice diagonal line so that we could use the triangle formula, so this kinematic equation—like all the rest of the kinematic equations—is only true under the assumption that the acceleration is constant.
Here's the alternative plugging-and-chugging derivation. The third kinematic equation can be derived by plugging in the first kinematic equation, , into the second kinematic equation, .
If we start with second kinematic equation
and we use to plug in for , we get
We can expand the right hand side and get
Combining the terms on the right hand side gives us
And finally multiplying both sides by the time gives us the third kinematic equation.
Again, we used other kinematic equations, which have a requirement of constant acceleration, so this third kinematic equation is also only true under the assumption that the acceleration is constant.
How do you derive the fourth kinematic equation, ?
To derive the fourth kinematic equation, we'll start with the second kinematic equation:
We want to eliminate the time from this equation. To do this, we'll solve the first kinematic equation, , for time to get . If we plug this expression for time into the second kinematic equation we'll get
Multiplying the fractions on the right hand side gives
And now solving for we get the fourth kinematic equation.
What's confusing about the kinematic equations?
People often forget that the kinematic equations are only true assuming the acceleration is constant during the time interval considered.
Sometimes a known variable will not be explicitly given in a problem, but rather implied with codewords. For instance, "starts from rest" means , "dropped" often means , and "comes to a stop" means . Also, the magnitude of the acceleration due to gravity on all free falling projectiles is assumed to be , so this acceleration will usually not be given explicitly in a problem but will just be implied for a free falling object.
People forget that all the kinematic variables——except for can be negative. A missing negative sign is a very common source of error. If upward is assumed to be positive, then the acceleration due to gravity for a free falling object must be negative: .
The third kinematic equation, , might require the use of the quadratic formula, see solved example 3 below.
People forget that even though you can choose any time interval during the constant acceleration, the kinematic variables you plug into a kinematic equation must be consistent with that time interval. In other words, the initial velocity has to be the velocity of the object at the initial position and start of the time interval . Similarly, the final velocity must be the velocity at the final position and end of the time interval being analyzed.
What do solved examples involving the kinematic equations look like?
Example 1: First kinematic equation,
A water balloon is dropped from the top of a very tall building.
What is the velocity of the water balloon after falling for ?
Assuming upward is the positive direction, our known variables are
(Since the water balloon was dropped, it started at rest.)
(This is the time interval after which we want to find the velocity.)
(This is implied since the water balloon is a free falling object.)
If you wait long enough, yes, but we won't use that in our equation for two reasons.
First, we are only considering the time interval of , which is presumably less than the time required to reach the ground.
Second, we can only apply the kinematic equations for a time interval of constant acceleration. When the water balloon hits the ground the acceleration changes dramatically from the collision. So if we are going to claim that the acceleration in our kinematic equation is since the object is in free fall, then we can't claim that the final velocity is zero since that is a portion of the trip when the object was colliding with the ground (i.e. not in free fall) and we would be contradicting ourselves.
If we knew the acceleration upon impact, and if that acceleration was constant we could apply the kinematic equations during the time interval of impact. But that's not the question we're considering here, and in most free fall problems we'll only consider the time after release and the time before impact.
The motion is vertical in this situation, so we'll use as our position variable instead of . The symbol we choose doesn't really matter as long as we're consistent, but people typically use to indicate vertical motion.
Since we don't know the displacement and we weren't asked for the displacement , we'll use the first kinematic equation , which is missing .
Note: The final velocity was negative since the water balloon was heading downward.
Yes, we're free to call downward the positive direction for any problem, but we have to be consistent with that decision. If we call downward positive, then every vector pointing downward has to be considered positive.
So the acceleration would have to be , and we would end up getting a positive final velocity instead of a negative final velocity.
Example 2: Second kinematic equation,
A leopard is running at The leopard then speeds up to in a time of .
How much ground did the leopard cover in going from to
Assuming the initial direction of travel is the positive direction, our known variables are
(The initial speed of the leopard)
(The final speed of the leopard)
(The time it took for the leopard to speed up)
Since we do not know the acceleration and were not asked for the acceleration, we'll use the second kinematic equation for the horizontal direction , which is missing .
Example 3: Third kinematic equation,
A student throws her pencil straight upward at
How long does it take the pencil to first reach a point higher than where it was thrown?
Assuming upward is the positive direction, our known variables are
(The initial upward velocity of the pencil)
(We want to know the time when the pencil moves through this displacement.)
(The pencil is a free falling projectile.)
Since we don't know the final velocity and we weren't asked to find the final velocity, we will use the third kinematic equation for the vertical direction , which is missing .
Normally we would just solve our expression algebraically for the variable we want to find, but this kinematic equation cannot be solved algebraically for time if none of the terms are zero. That's because when none of the terms are zero and is the unknown variable, this equation becomes a quadratic equation. We can see this by plugging in known values.
To put this into a more solvable form of the quadratic equation, we move everything onto one side of the equation. Subtracting from both sides we get
At this point, we solve the quadratic equation for time . The solutions of a quadratic equation in the form of are found by using the quadratic formula . For our kinematic equation , , and .
So, plugging into the quadratic formula, we get
Since there is a plus or minus sign in the quadratic formula, we get two answers for the time : one when using the and one when using the . Solving the quadratic formula above gives these two times:
There are two positive solutions since there are two times when the pencil was high. The smaller time refers to the time required to go upward and first reach the displacement of high. The larger time refers to the time required to move upward, pass through high, reach a maximum height, and then fall back down to a point high.
So, to answer our question of "How long does it take the pencil to first reach a point higher than where it was thrown?" we would choose the smaller time .
If the quadratic formula gives a negative time and a positive time, then the object in question only reaches the specified displacement once after being thrown. In that case, we can simply choose the positive time and neglect the negative time.
The negative time is referring to a time before the object was thrown when it would have been at the specified displacement. In other words, assume the object had been on its trajectory before being thrown and just happened to reach the point of being thrown with the initial velocity specified.
Example 4: Fourth kinematic equation,
A motorcyclist starts with a speed of and, seeing traffic up ahead, decides to slow down over a length of with a constant deceleration of magnitude . Assume the motorcycle is moving forward for the entire trip.
What is the new velocity of the motorcyclist after slowing down through the
Assuming the initial direction of travel is the positive direction, our known variables are
(The initial forward velocity of the motorcycle)
(Acceleration is negative since the motorcycle is slowing down and we assumed forward is positive.)
(We want to know the velocity after the motorcycle moves through this displacement.)
Since we don't know the time and we weren't asked to find the time, we will use the fourth kinematic equation for the horizontal direction , which is missing .
Note that in taking a square root, you get two possible answers: positive or negative. Since our motorcyclist will still be going in the direction of motion it started with and we assumed that direction was positive, we'll choose the positive answer .
Now we can plug in values to get
FAQs
There are four basic kinematics equations:
- v = v 0 + a t.
- Δ x = ( v + v 0 2 ) t.
- Δ x = v 0 t + 1 2 a t 2.
- v 2 = v o 2 + 2 a Δ x.
What are the 5 kinematic equations used for? ›
The equations can be utilized for any motion that can be described as being either a constant velocity motion (an acceleration of 0 m/s/s) or a constant acceleration motion. They can never be used over any time period during which the acceleration is changing. Each of the kinematic equations include four variables.
What are the kinetics equations? ›
The kinetic equation is the evolution equation of the distribution function. It is to the distribution function what Newton's second law is to an individual particle. It is deduced from this fundamental law. It has the following general form: ∂ t f + v .
What is the 5 formula of kinematics? ›
The fifth kinematic equation looks just like the third kinematic equation Δ x = v 0 t + 1 2 a t 2 except with the initial velocity v 0 replaced with final velocity v and the plus sign replaced with a minus sign. It can be derived by plugging the first kinematic equation into the third kinematic equation.
How many basic kinematics equations are there? ›
The kinematic equations are a set of four equations that can be utilized to predict unknown information about an object's motion if other information is known.
What are the 6 equations of motion? ›
The equations are as follows: v=u+at,s=(u+v2)t,v2=u2+2as,s=ut+12at2,s=vt−12at2.
What are the three main concepts of kinematics? ›
There are three basic concepts in kinematics - speed, velocity and acceleration.
What are the basics of kinematics? ›
Kinematics is the study of motion, without any reference to the forces that cause the motion. It basically means studying how things are moving, not why they're moving. It includes concepts such as distance or displacement, speed or velocity, and acceleration, and it looks at how those values vary over time.
What are the big five of kinematics? ›
The 5 major kinematic quantities are displacement (x-x0), time (t), initial velocity (v0), final velocity (v), and constant acceleration (a). These quantities are commonly included when describing the position and motion of an object.
What is kinematics in simple terms? ›
Kinematics is a subfield of physics and mathematics, developed in classical mechanics, that describes the motion of points, bodies (objects), and systems of bodies (groups of objects) without considering the forces that cause them to move.
Kinematics equations are the constraint equations of a mechanical system such as a robot manipulator that define how input movement at one or more joints specifies the configuration of the device, in order to achieve a task position or end-effector location.
What are the principles of kinematics? ›
Kinematics has five fundamental concepts- Position, Displacement, Velocity, Speed and acceleration.
What are the 6 kinematic equations? ›
Linear Motion in the Second Dimension
Component | Component |
---|
v x = v 0 x + a x t | v y = v 0 y + a y t |
x = 1 2 ( v 0 x + v x ) t | y = 1 2 ( v 0 y + v y ) t |
x = x 0 + v 0 x t + 1 2 a x t 2 | y = y 0 + v 0 y t + 1 2 g t 2 |
v x 2 = v 0 x 2 + 2 a x ( x − x 0 ) | v y 2 = v 0 y 2 + 2 a y y |
What are the 5 variables in kinematic equations? ›
In kinematics there are five important quantities: displacement (change in position), initial velocity, final velocity, acceleration, and time.
What are the 4 basic quantities of kinematics? ›
Lesson Summary. Kinematics is the study of motion, without reference to the forces that cause the motion. In kinematics, there are five important quantities: displacement (change in position), initial velocity, final velocity, acceleration, and time.
What are the 3 forms of motion in kinematics? ›
Frequently Asked Questions – FAQs
- Linear Motion.
- Rotary Motion.
- Oscillatory Motion.
What are kinematics 3 examples? ›
Kinematics is used in everyday life for explaining motion without reference to the forces involved. Some examples of kinematics include measuring the distance of a walking trail, understanding how we can a car's velocity to calculate its acceleration, and seeing the effects of gravity on falling objects.