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Causes of motion
THE BALL AND THE PENDULUM:
- Galileo did the pendulum experiment and found that it rose nearly the same height each time.
- He reasoned that if a ball rolls down a slope onto an infinitely long flat surface, it will continue moving until something else causes it to stop.
- Galileo realized the importance of distinguishing between motions horizontally and vertically in a gravitational field.
Newton’s first law of motion
In AS/A-Level Physics, motion on earth is opposed by frictional forces
- Newton formed three laws of motion (which sometimes break down under certain conditions) which are very nearly correct under all circumstances.
- So, an object has a constant velocity until a force acts on it. This law defines what a force IS and DOES – a force is something which can cause acceleration
- If a body has a number of forces, F1, F2 ... Fn acting on it, it will remain in a state of constant motion only if: (That is the sum on all the forces from F1 to Fn) is equal to zero
- This can be calculated separately for HORIZONTAL and VERTICAL FORCES
- Because a force can cause acceleration, it is a vector quantity, with both magnitude and direction. It therefore requires a way of representing both direction and magnitude on a diagram. A diagram which shows all the forces acting on a body in a certain situation is called a FREE BODY DIAGRAM. This doesn’t show forces acting on objects other than the one being considered.
- CENTER OF GRAVITY – the weight of an object acting through a single point (the center) The center is the point at which gravity appears to act, similar to an objects center of mass
- For uniform objects, the center of the mass will be at the intersection of all lines of symmetry, especially in the middle of the object
Drag forces
- Drag forces are made up of two types of forces – FRICTION and AIR RESISTANCE – a result of matter in contact with matter
- FRICTION always occurs when two surfaces rub on each other. Although appearing smooth, they are slightly rough, causing friction.
- Friction OPPOSES any motion, but cannot actually CAUSE motion. When an object stops, so does friction
- Friction can be measured using a force meter and moving something across a surface at a constant velocity
- For an object which is not accelerating, the sum on all the forces from F1 to Fn = 0 meaning that the frictional force resisting motion must be exactly balanced by the pulling force of the hand
- AIR RESISTANCE (or aerodynamic drag) – caused when a body moves through air.
- Caused by an object having to push air out of the way in order to move through it. Air resistance depends on speed, the faster the object moves, the greater the aerodynamic drag (think of a car)
- As aerodynamic drag increases, objects with a constant driving force tend to reach a max. velocity when they accelerate
- Free fall and terminal velocity – if someone jumps from a height, they will accelerate due to their own weight (N) and air resistance will affect them Acceleration of free fall or acceleration due to gravity (9.81 ms-2)
And we're all done for today!
