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!
Use quizzes to practise application of theory.
START QUIZ!
The image shows 4 projectiles with equal speed.
Which will have the longest range?
45° gives the longest range.
The image shows 4 projectiles with equal speed.
Which will have the longest time of flight?
90° reaches the highest so will be in the air longest.
The image shows 4 projectiles with equal speed.
Which will have the highest horizontal component of momentum?
30° gives the greatest horizontal component of velocity and hence momentum.
The image shows 4 projectiles with equal speed.
Which will have the same KE when they land?
all have the same speed on landing as when the take off.
The image shows 4 projectiles with equal speed.
Which will have the greatest KE at the highest point?
30° gives the largest horizontal component of velocity.
The image shows 4 projectiles with equal speed.
In which order will they land?
The ones with the highest vertical component will have the longest time of flight.
The image represents projectiles on two planets
Which will have the longest range?
B and C have zero range, A has higher speed.
The image represents projectiles on two planets
Which will have the longest time of flight?
B has highest vertical component and lowest gravity
The image represents projectiles on two planets
In which order will they land?
The vertical ones with the lowest gravity will be in the air longest.
The image represents projectiles thrown off 4 different cliffs
Which will have the longest range?
A will be in the air longest so will travel furthest
How much of Projectile motion have you understood?
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