Motion Graphs (DP IB Physics): Revision Note

Motion Graphs

• The motion of objects can be analysed in terms of position, velocity and acceleration

  • These are all related to each other by gradients and areas under curves

• Three types of graphs that can represent the motion of an object are:

  • Displacement-time graphs

  • Velocity-time graphs

  • Acceleration-time graphs

Displacement-Time Graphs

• On a displacement-time graph:

  • Slope equals velocity

  • The y-intercept equals the initial displacement

  • A straight (diagonal) line represents a constant velocity

  • A curved line represents an acceleration

  • A positive slope represents motion in the positive direction

  • A negative slope represents motion in the negative direction

  • A zero slope (horizontal line) represents a state of rest

  • The area under the curve is meaningless

    

Displacement-time graphs displacing different values of velocity

Velocity-Time Graphs

• On a velocity-time graph:

  • Slope equals acceleration

  • The y-intercept equals the initial velocity

  • A straight (diagonal) line represents uniform acceleration

  • A curved line represents non-uniform acceleration

  • A positive slope represents acceleration in the positive direction

  • A negative slope represents acceleration in the negative direction

  • A zero slope (horizontal line) represents motion with constant velocity

  • The area under the curve equals the change in displacement

    

Velocity-time graphs displacing different values of acceleration

Acceleration-Time Graphs

• On an acceleration-time graph:

  • Slope is meaningless

  • The y-intercept equals the initial acceleration

  • A zero slope (horizontal line) represents an object undergoing constant acceleration

  • The area under the curve equals the change in velocity

How displacement, velocity and acceleration graphs relate to each other

• Acceleration can either be

  • Uniform i.e. a constant value. For example, acceleration due to gravity on Earth

  • Non-uniform i.e. a changing value. For example, an object with increasing acceleration

Motion of a Bouncing Ball

• For a bouncing ball, the acceleration due to gravity is always in the same direction (in a uniform gravitational field such as the Earth's surface)

  • This is assuming there are no other forces on the ball, such as air resistance

• Since the ball changes its direction when it reaches its highest and lowest point, the direction of the velocity will change at these points

• The vector nature of velocity means the ball will sometimes have a:

  • Positive velocity if it is travelling in the positive direction

  • Negative velocity if it is traveling in the negative direction

• An example could be a ball bouncing from the ground back upwards and back down again

  • The positive direction is taken as upwards

  • This will be either stated in the question or can be chosen, as long as the direction is consistent throughout

• Ignoring the effect of air resistance, the ball will reach the same height every time before bouncing from the ground again

• When the ball is traveling upwards, it has a positive velocity which slowly decreases (decelerates) until it reaches its highest point

• At point A (the highest point):

  • The ball is at its maximum displacement

  • The ball momentarily has zero velocity

  • The velocity changes from positive to negative as the ball changes direction

  • The acceleration, g, is still constant and directed vertically downwards

• At point B (the lowest point):

  • The ball is at its minimum displacement (on the ground)

  • Its velocity changes instantaneously from negative to positive, but its speed (magnitude) remains the same

  • The change in direction causes a momentary acceleration (since acceleration = change in velocity / time)