Are the SUVAT equations always valid in IB Physics kinematics?

No. The equations commonly called SUVAT equations apply only when acceleration is constant over the interval being analysed. They work for idealized free fall near Earth's surface when gravitational acceleration is treated as constant, but not for an object whose acceleration changes substantially because of fluid resistance. If motion has several constant-acceleration stages, apply the equations separately to each stage. Before using one, list the variables and state why the acceleration can be considered uniform.

How do I decide whether gravitational acceleration is positive or negative?

The sign of gravitational acceleration depends on the positive direction you choose. If upward is positive, gravitational acceleration is $-g$; if downward is positive, it is $+g$. Either choice produces the same physical result when displacement and velocity signs are treated consistently. Do not change the sign of gravity merely because an object reaches its highest point and begins falling. Mark the positive direction on your diagram before entering values into an equation.

What is the difference between distance and displacement on a velocity-time graph?

The signed area between a velocity-time graph and the time axis gives displacement. Area above the axis is positive and area below it is negative according to the graph's coordinate convention. To obtain distance travelled, find the magnitude of every area and add those magnitudes so that no section cancels another. If the graph never crosses the time axis, the magnitude of displacement equals the distance travelled. Always check the wording because questions may deliberately contrast these two quantities.

Why is the velocity zero but acceleration non-zero at the top of a projectile's path?

At the highest point, the projectile's vertical velocity is momentarily zero because it is changing from upward to downward motion. Gravity is still acting, so the vertical acceleration remains directed downward with magnitude approximately $g$ in the idealized model. If the projectile also has horizontal motion, its total velocity is not zero because its horizontal component remains. Zero instantaneous velocity in one direction does not imply zero resultant force or acceleration. Confusing these quantities often leads students to claim incorrectly that gravity stops acting at the top.

How should I show working in an IB Physics kinematics calculation?

Begin with the relevant equation or physical relationship, then substitute values with their correct signs. Keep enough intermediate precision to avoid changing the final result through premature rounding. State the final unit and include a direction or sign when the requested quantity is a vector. If the motion has multiple stages or components, label each part so the examiner can follow which values belong together. This structure also makes it easier to identify an error and may preserve method credit when the final number is incorrect.

What is the best way to use kinematics video solutions when revising?

Attempt the question fully before watching its solution so that there is a real method to compare. Pause the video after the diagram, sign convention and variable list, then identify the first point where your approach differs. Record the cause of the mistake rather than copying the correct calculation, such as “used distance instead of displacement” or “applied constant acceleration across two stages.” Rework the entire question without the video immediately afterward. Finish by attempting a related question later, because successful transfer to a new problem is stronger evidence of understanding than recognizing a familiar solution.

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IB Physics Kinematics: Common Mistakes and Fixes

• 5 Aug 2026•14

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IB Physics Kinematics: Common Mistakes and Fixes

• 5 Aug 2026•14

IB Physics kinematics common mistakes usually come from sign conventions, graph interpretation, equation selection, vectors, or incomplete working rather than difficult mathematics. The most effective correction is to compare your method with a worked solution that makes every decision visible: defining a positive direction, identifying variables, selecting a model, substituting values, and checking the result.

In the current IB Physics course, kinematics appears as A.1 Kinematics within Theme A, Space, time and motion. Students at both SL and HL need to describe motion using displacement, velocity and acceleration, apply constant-acceleration equations appropriately, interpret motion graphs, and analyse projectile motion. This article explains the recurring errors, why they happen, and how to use worked video solutions to prevent them.

Why kinematics errors repeat in IB Physics

Kinematics questions often look different while testing the same small set of decisions. A ball may be thrown vertically, a vehicle may be represented on a velocity-time graph, or a projectile may leave a platform, but each question still requires a consistent model of motion.

The current official specimen assessment illustrates that these skills can appear in multiple-choice, data-based, short-response, and extended-response contexts. A calculator and clean physics data booklet are required in the specimen papers, but access to equations does not remove the need to understand their conditions and vector signs.

Before calculating, ask four questions:

What object and time interval am I analysing?
Which direction is positive?
Is the acceleration constant during that interval?
Does the question ask for a scalar or a vector quantity?

Common IB Physics kinematics mistakes and their fixes

Common mistake	Why the method fails	Reliable fix
Confusing distance with displacement	Distance is total path length, while displacement is the change in position	Sketch the path and mark initial and final positions before calculating
Confusing speed with velocity	Speed has magnitude only; velocity includes direction	Include a sign or stated direction whenever velocity is required
Treating negative acceleration as slowing down	Speed decreases only when velocity and acceleration have opposite signs	Compare the signs of velocity and acceleration
Using constant-acceleration equations automatically	The standard equations assume uniform acceleration	State why acceleration is constant before selecting an equation
Mixing signs during vertical motion	Changing the sign of gravitational acceleration halfway creates contradictions	Choose one positive direction and keep it throughout
Reading graph height instead of gradient	The vertical coordinate and gradient represent different quantities	Write what the axes show before interpreting the graph
Ignoring signed area below a velocity-time axis	Areas below the axis represent negative displacement	Add signed areas for displacement and magnitudes for distance
Mixing projectile components	Horizontal and vertical motion follow different equations	Create separate horizontal and vertical variable lists
Rounding too early	Intermediate rounding can shift the final answer	Keep calculator values and round only at the end
Giving only a numerical answer	Method marks may depend on visible reasoning	Show the equation, substitution, result and unit

Mistake 1: confusing distance, displacement, speed and velocity

Distance is the total path length and is a scalar. Displacement is the change from initial to final position and is a vector. Similarly, speed is the magnitude of velocity, whereas velocity includes direction.

Suppose a student walks 30 m east and then 10 m west. The distance travelled is 40 m, but the displacement is 20 m east. Dividing 40 m by the elapsed time gives average speed, while dividing 20 m east by the elapsed time gives average velocity.

The fix is to underline the requested quantity and represent the motion on a one-dimensional axis. When reviewing a worked video solution, pause before the calculation and predict whether the solution should use total path length or net position change.

Mistake 2: interpreting negative acceleration as deceleration

A negative acceleration does not necessarily mean an object is slowing down. The sign describes direction relative to the chosen coordinate system, while slowing down describes a decrease in the magnitude of velocity.

An object slows down when velocity and acceleration have opposite signs. It speeds up when they have the same sign. For example, if upward is positive, a falling ball has negative velocity and negative acceleration, so its speed increases.

A strong solution therefore separates three ideas:

Positive or negative velocity indicates the direction of motion.
Positive or negative acceleration indicates the direction of acceleration.
Increasing or decreasing speed depends on the relationship between the two.

Mistake 3: using kinematic equations without checking the model

The familiar equations

$v=u+at$
$s=ut+\frac{1}{2}at^2$

apply to motion with constant acceleration. They should not be applied across an interval in which acceleration changes significantly, such as an object approaching terminal speed under changing fluid resistance.

Before selecting an equation, list $s$ , $u$ , $v$ , $a$ , and $t$ . Record known values with signs, circle the required unknown, and select an equation that excludes the remaining unknown quantity. This is more dependable than choosing the equation that looks most familiar.

Worked solutions are especially useful here because they reveal the model selection that happens before substitution. The RevisionDojo A.1 Kinematics Questionbank can be used to practise this decision across different question formats.

Mistake 4: changing sign conventions during vertical motion

In vertical-motion problems, either upward or downward may be chosen as positive. Both conventions work, but all displacements, velocities and accelerations must follow the same choice.

If upward is positive, gravitational acceleration near Earth's surface is written as $a=-g$ . A ball thrown upward has positive initial velocity, reaches $v=0$ momentarily at its highest point, and then has negative velocity as it falls. Its acceleration remains negative throughout the idealized flight.

A frequent mistake is to change (-g) to $+g$ after the ball passes its highest point. Gravity has not reversed direction, so its sign should not change. Write a small arrow labelled “positive” beside the diagram before doing any algebra.

Mistake 5: confusing gradient, height and area on motion graphs

Graph questions test whether students understand the relationship among position, velocity and acceleration. The graph's shape is not a picture of the object's physical path.

Graph	Gradient represents	Signed area represents
Displacement-time	Velocity	No standard kinematics quantity
Velocity-time	Acceleration	Displacement
Acceleration-time	Rate of change of acceleration	Change in velocity

For a curved displacement-time graph, instantaneous velocity is found from the gradient of a tangent, not from a line joining two distant points. On a velocity-time graph, area below the time axis is negative displacement. If the question asks for total distance, calculate the magnitude of each region and add the magnitudes.

Units provide a quick check. The gradient of a velocity-time graph has units $(\text{m s}^{-1})/\text{s}=\text{m s}^{-2}$ , confirming that it represents acceleration. The area has units $\text{m s}^{-1}\times\text{s}=\text{m}$ , confirming displacement.

Mistake 6: mixing horizontal and vertical projectile motion

In ideal projectile motion without fluid resistance, horizontal and vertical motion share the same time but otherwise require separate analysis. Horizontal acceleration is zero, while vertical acceleration is gravitational.

If a projectile is launched with speed $u$ at angle $\theta$ above the horizontal, resolve the initial velocity first:

$u_x=u\cos\theta$
$u_y=u\sin\theta$

A common mistake is to place the full launch speed into both directions. Another is to assume vertical velocity is zero throughout the flight; it is zero only at the highest point. At that point, the projectile generally retains horizontal velocity, so its total velocity is not zero.

Create two columns labelled horizontal and vertical. Use the vertical motion to determine time when appropriate, then substitute that time into the horizontal relationship. The RevisionDojo kinematics notes provide a useful reference before attempting mixed projectile questions.

Mistake 7: overlooking multiple stages of motion

Some questions cannot be solved with one equation across the entire event. A vehicle may accelerate and then move at constant velocity, or an object may rise before falling to a different height.

Divide the motion wherever acceleration changes or a new condition begins. The final velocity of one stage becomes the initial velocity of the next, and total time or displacement is found by combining stage values carefully. A labelled timeline prevents values from different stages being inserted into the same equation.

When watching a solution, note exactly where the instructor resets the variable list. That transition is often the conceptual step the marks depend on.

Mistake 8: treating calculator output as a complete answer

IB Physics calculations should communicate a physical argument, not just a number. The official command term calculate expects a numerical answer with relevant working, so an unsupported calculator result is risky even when correct.

A clear response normally includes:

The governing equation or physical relationship.
Substitution with signs and consistent units.
An unrounded intermediate calculation where needed.
A final value with a unit and sensible precision.
A direction when the requested quantity is a vector.

Also check whether the result is physically plausible. A negative time usually indicates a sign or root-selection problem, while an unexpectedly large speed may reveal a unit conversion error.

How to learn from worked video solutions

Watching solutions passively rarely changes exam performance. Use the A.1 Kinematics worked video solutions as a correction tool rather than as a substitute for attempting questions.

For each question:

Attempt it without assistance and preserve your working.
Watch only until the first important setup decision.
Pause and compare the positive direction, diagram and variable list with yours.
Continue one step at a time, marking the first point where your reasoning diverged.
Classify the error as conceptual, algebraic, graphical, vector-related or presentational.
Rework the question from a blank page without the video.
Attempt a similar question several days later.

The important question is not merely “What was the correct answer?” Ask “What decision would have prevented my error?” Students can combine the videos with the A.1 Kinematics topic hub and use Jojo AI to clarify a specific step without skipping the reasoning.

A reliable exam routine for kinematics

Use the same routine even when a question appears straightforward:

Draw a diagram or inspect the graph axes.
Define the positive direction.
Separate stages or vector components.
List known and unknown quantities with units and signs.
Confirm whether acceleration is constant.
Select the relationship before substituting.
Solve, retain sufficient precision, and include the final unit.
Check sign, direction and physical plausibility.

Practise the routine under time pressure using the broader IB Physics resources. Consistency reduces the cognitive load of deciding how to begin, leaving more attention for the physics that distinguishes one question from another.

Conclusion

Most IB Physics kinematics mistakes are predictable: students confuse scalar and vector quantities, mishandle signs, misread graphs, use constant-acceleration equations outside their conditions, or mix projectile components. These errors are corrected by making the setup explicit and checking the physical model before calculating.

RevisionDojo's most relevant resources are the topic Questionbank and step-by-step kinematics videos. Attempt each question independently, compare your decisions with the worked method, record the first error, and then solve a similar problem without support.

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