Projectile motion

Try flicking an eraser off the edge of a table at the same time as dropping one. You should notice that they reach the ground at the same time. This demonstrates the independence of vertical and horizontal motion for a projectile. 


Key Concepts

A projectile is any body in motion, on which the only force acting is weight. The free-body diagram is identical to that on the Acceleration due to gravity page.

This means that:

  • vertically, the object has constant acceleration and so suvat equations can be used
  • horizontally, the object has zero acceleration and so we can return to an old friend \(\text {speed} = {\text {distance} \over t}\)

We must assume that no horizontal forces (e.g. drag) act at any time. This means that the body is modelled as a particle.

We can think of projectile motion as two problems in one: constant velocity horizontally and constant acceleration vertically. The motions in both directions have one common feature... time!

Video analysis of a projectile

By analysing a video of projectile motion, we can see how the motion is made of two components:

  • x - constant velocity
  • y - accelerating (NB: constant negative acceleration)

This results in a parabolic trajectory.

Common time

The vertical component of a projectile is independent of the horizontal motion, so a ball thrown vertically will stay in the air for the same time as a ball thrown forwards with the same vertical component.

Essentials

Monkey and hunter

This classic "monkey and hunter" example demonstrates how the bullet fired by a hunter, with both a vertical and horizontal velocity component, falls at the same rate as a monkey falling with only a vertical velocity component.

Projectiles in sport

An American footballer moving with a horizontal velocity equal to that of a ball will catch it.

Test Yourself

Use quizzes to practise application of theory. 


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Exam-style Questions

Online tutorials to help you solve original problems

Question 1

Question 2

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How much of Projectile motion have you understood?