# Physics - Mechanics - Dynamics

Detailed notes that will help when revising for mechanics

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Chapter 3: Dynamics ­ explaining motion
Force and acceleration
To calculate the acceleration produced by a net force we must also know the objects mass
The word equation for calculated the net force is:
Force (N) = mass (kg) x acceleration (m/s^2)
This equation is a simplified version of Newtons second law of motion
It only applies to objects which have a constant mass
From this equation we can see that acceleration is directly proportional to the net Force, and for a
constant Force acceleration is inversely proportional to the mass of the object
Defining the Newton
One Newton is the force that will give a 1kg mass an acceleration of 1m/s^2 in the direction of the
force
1N = 1kgms^-2
Acceleration caused by gravity
If you drop a stone from a distance s, the ball will accelerate towards the ground
This can be shown using a multi flash photograph
The photographs are taken at the same time interval, however the distance between the stone in
the photo's will increase, hence an acceleration
The force causing the ball to fall is the pull of the Earth's gravity (weight)
The centre of gravity of an object is defined as the point where its entire weight appears to act
Without air resistance objects of any mass will fall at the same rate
However air resistance is greater when acting on a lighter object therefore making it accelerate at a
smaller rate
Acceleration of free fall = 9.81 m/s^2
Weight = mass x acceleration of free fall
Acceleration on the moon
The moon is smaller and therefore has less mass and so its gravity is weaker
Because of the smaller effect of gravity the acceleration due to gravity is smaller
Acceleration of free fall on the moon = 1.6 m/s^2
Objects also weigh less on the moon due to the smaller value of acceleration due to free fall
Mass and weight
Mass will not vary from place to place as it is a measure of the amount of matter within an object
Weight will change as it is a force and is affected by the acceleration due to free fall
Acceleration due to free fall is equivalent to the gravitational field strength

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Determining the gravitational field strength
Electronic timer (method 1)
A steel ball is held by an electromagnet
When the current is switched off, the ball begins to fall and the timer starts
When the ball falls through the trap door the timer stops
Substituting the values we have found into s = ut + (at^2)/2 we can find g
h = (gt^2)/2
We could also have drawn a graph of h against t^2 and calculated the gradient
Uncertainty
The ball may still contain some magnetism…read more

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When they jump out of the plane the accelerate freely as the only force acting is the weight of the
person
At the start the acceleration of the skydiver is therefore g
Air resistance begins to increase, decreasing the acceleration due to free fall
Eventually they reach a maximum velocity, known as terminal velocity
Terminal velocity is determined by the weight and orientation of the skydiver
When the parachute is released it greatly increases the air resistance and so the terminal velocity is
reduced