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Calculus for Kinematics (DP IB Maths: AI HL)
Revision Note
Differentiation for Kinematics
How is differentiation used in kinematics?
- Displacement, velocity and acceleration are related by calculus
- In terms of differentiation and derivatives
- velocity is the rate of change of displacement
- or
- acceleration is the rate of change of velocity
- or
- so acceleration is also the second derivative of displacement
- or
- Sometimes velocity may be a function of displacement rather than time
- rather than
- in such circumstances, acceleration is
- this result is derived from the chain rule
- All acceleration formulae are given in the formula booklet
- velocity is the rate of change of displacement
- Even if a motion graph is given, if possible, use your GDC to draw one
- you can then use your GDC’s graphing features to find gradients
- velocity is the gradient on a displacement (-time) graph
- acceleration is the gradient on a velocity (-time) graph
- you can then use your GDC’s graphing features to find gradients
- Dot notation is often used to indicate time derivatives
- is sometimes used as displacement (rather than) in such circumstances
- , so is velocity
- , so is acceleration
Worked example
a)
The displacement, m, of a particle at seconds, is modelled by the function .
Find expressions for and .
b)
The velocity, m s-1, of a particle is given as, where m is the displacement of the particle.
Find an expression, in terms of , for the acceleration of the particle.
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Integration for Kinematics
How is integration used in kinematics?
- Since velocity is the derivative of displacement () it follows that
- Similarly, velocity will be an antiderivative of acceleration
- You might be given the acceleration in terms of the velocity and/or the displacement
- In this case you can solve a differential equation to find an expression for the velocity in terms of the displacement
How would I find the constant of integration in kinematics problems?
- A boundary or initial condition would need to be known
- phrases involving the word “initial”, or “initially” are referring to time being zero, i.e.
- you might also be given information about the object at some other time (this is called a boundary condition)
- substituting the values in from the initial or boundary condition would allow the constant of integration to be found
How are definite integrals used in kinematics?
- Definite integrals can be used to find the displacement of a particle between two points in time
- would give the displacement of the particle between the times and
- This can be found using a velocity-time graph by subtracting the total area below the horizontal axis from the total area above
- gives the distance a particle has travelled between the times and
- This can be found using a velocity velocity-time graph by adding the total area below the horizontal axis to the total area above
- Use a GDC to plot the modulus graph
- would give the displacement of the particle between the times and
Examiner Tip
- Sketching the velocity-time graph can help you visualise the distances travelled using areas between the graph and the horizontal axis
Worked example
A particle moving in a straight horizontal line has velocity () at time seconds modelled by.
- Given that the initial position of the particle is at the origin, find an expression for its displacement from the origin at time seconds.
- Find the displacement of the particle from the origin in the first five seconds of its motion.
- Find the distance travelled by the particle in the first five seconds of its motion.
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