Section 3 – Projectile Motion
A projectile is an object which has been given a forward motion through the air, but which is also pulled downward by the force of gravity. This results in the path of the projectile being curved.
A projectile has two separate motions at right angles to each other.
Each motion is independent of the other.
The horizontal motion is at a constant velocity since there are no forces acting horizontally (air resistance can be ignored).
The vertical motion is one of constant acceleration, equal to g (10 m/s2 on earth).
For projectiles which are projected horizontally, the initial vertical velocity is zero.
For vertical calculations, use
Velocity time graphs for horizontal and vertical motion
Example
An aircraft flying horizontally at 200 m/s, drops a food parcel which lands on the ground 12 seconds later.
Calculate:
a) the horizontal distance travelled by the food parcel after being dropped
b) the vertical velocity of the food parcel just before it strikes the ground
c) the height that the food parcel was dropped from.
Solution
(a useful layout is to present the horizontal and vertical data in a table or in two data columns)
(then select the data you need for the question, and clearly indicate whether you are using horizontal or vertical data).
(a) (Draw a velocity time graph of vertical motion)
Satellites and Projectile Motion
Newton’s thought experiment allowed us to understand satellite orbits.
If a projectile is launched with sufficient horizontal velocity, it will travel so far that the curvature of the Earth must be taken into account.
Now imagine a projectile launched with such a great horizontal velocity that it never reaches the ground! It will continue to circle the Earth until its horizontal velocity decreases.
The satellite orbits around the Earth because it is in constant free-fall due to the Earth’s gravity.
Apparent Weightlessness
Astronauts in orbit around the Earth are in a constant state of free-fall. The spaceship, the astronauts and everything inside are all accelerating towards the Earth due to gravity.
This is known as apparent weightlessness. The effects are because of gravity, and not because they have escaped from the gravitational field of the Earth.
