Projectiles Flashcards
Projectiles
Deals with motion that has constant acceleration (“Uniformly accelerated motion”) -> SUVAT
Assume that the acceleration due to gravity close to the Earths surface is constant -> g = 9.81 ms^-2 vertically downwards (add arrows to show direction for vectors)
‘Air resistance or drag can be assumed to be zero or negligible’
No Horizontal forces acting, so no Horizontal acceleration -> Horizontal velocity is constant
Vertical Projectiles
No Horizontal motion -> Ignoring drag -> Only force is weight
The object has constant acceleration a = g = 9.81 ms^-2 (vertically downward)
Define positive direction, create a SUVAT: A = 9.81 ms^-2
Horizontal Projectiles
Mass is projected horizontally and moves freely under gravity (drag is zero)
The only force acting once it is moving is weight (acceleration is constant)
The motion must be separated into horizontal and vertical components
Horizontal: Acceleration is 0. Constant Velocity
Independence of Horizontal and Vertical Motion
If one object is dropped and an identical object is launched horizontally from the same height, both objects (no air resistance) they will hit the ground at the same time
Horizontal motion is independent of the vertical motion
Trajectory forms a part parabola
Velocity Analysis
Horizontal component of velocity is constant (describe) as no forces act horizontally (explain)
Vertical Component: From the start to max height, the vertical velocity decreases (describe)
Acceleration is in the opposite direction to vertical velocity: vertically downward (explain)
At max height, the vertical velocity is zero
From maximum height until jest before hitting the ground, the vertical velocity increases in the opposite direction (describe)
Acceleration is in the same direction as velocity (explain)
Energy Analysis
Start: Max KE, Min GPE
Start to Max Height: KE decreases, GPE increases
Max Height: Minimum KE (But not 0 as there is motion), Max GPE
Max Height to End: KE increases, GPE decreases
End: Max KE, Min GPE
Experimental Techniques for finding g
When an object is accelerating under gravity, with no other force acting on it, we say it is in free fall
The acceleration of free fall (or acceleration due to gravity) has the label g, units ms^-2 as it is acceleration
The value for g, close to the surface of earth is 9.81 ms^-2
Affected by altitude, latitude and geology
In the experiments, air resistance is ignored -> A large vertical crop reduces percentage uncertainty on length (roughly 2.000m)
Electromagnet
Switch in top circuit is opened so electromagnet is off and the ball bearing drops
Opening the top circuit also starts the electronic timer
When the ball bearing opens the trap door, the timer stops
No human reaction time at it is automated but there may be a delay in timer being triggered on or off
Use s = ut + 0.5at^2 to find g
u = 0ms^-1
Connections to the timer are ‘break to start’
Connections to time are also ‘break to stop’
Light gates
Set up two light gates, one at the top and on at the bottom
The light gates will record the time taken for the object to pass through
You can work out u and v if you know the dimensions of the object
Then use v^2 = u^2 + 2as to find g
If an object is dropped from rest, a light gate is only needed at the bottom -> u = 0
Ticker timer and tape
A ticker timer marks a strip of paper at fixed intervals
By attaching the paper to a falling mass, we can measure the distance moved during each time interval so get average velocity
Then plot a graph of average velocity (Y-axis) against time
(X-axis)
The gradient of the graph is the acceleration, g
Detailed Experimental Method to find ‘g’: Electromagnet
Use a steel ball bearing attached to an electromagnet which is attached to a timer
Current is switched off to electromagnet, steel ball is released
Steel ball falls a height, h and opens a trapdoor which stops the timer
Measure height, h with a metre ruler
Record time
Repeat for various heights recording time for each height
Plot a graph of s against t^2
From s = 0.5at^2 -> m = 0.5a since u = 0
a = 2 x gradient
Compare value of a to g - 9.81ms^-2
Graph Possible Errors
The electromagnet may retain some of its magnetism for a short while so the time recorded may be longer than if the ball fell freely
The measurement of the height may not be accurate
Air resistance is ignored
