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30 Physics Dynamics Notes Kinematics is the science that describes the motion of objects. The "how of motion" (dynamics) explains why objects change their motion. The physical phenomena responsible for changing an object's motion is FORCE. A force is a push or pull. It's symbol is F and the unit for force is the Newton(N). A 1 Newton force is that force required to change the velocity of a 1.0 kg object by 1.0 m/s in 1.0 seconds. As such there is an equivalent unit for force: 1N 1kgm s2 or 1N 1kgms2 Newton's Law's of Motion 1. An object maintains constant velocity unless acted upon by an unbalanced (or non-zero net) force. This is sometimes called Galileo's law of inertia. One property of matter is its inertia....it's tendancy to remain at constant velocity. Matter is inert to chage as far as motion (velocity) is concerned. 2. The acceleration of an object is directly proportional to the net force acting on it aF and inversely proportional to its mass. a 1 m Together we get Newton's second law. F ma 3. For every action (force) that object A exerts on object B, object B exerts an equal force on A but in the opposite direction. Weight & Mass Weight and Mass are NOT THE SAME!! Mass is a measure of the amount of matter in an object (measured in kilograms) and weight is the force of gravity pulling down on an object (measured in Newtons). So why do so many people confuse the two and/or not differentiate the two? It just so happens that the more mass an object has the more it weighs and in fact mass and weight are proportional. So if an objects mass is doubled so is its weight....but they are NOT THE SAME!! Gravitational force depends on the mass of the two objects involved and the distance separating their centres. As long as one of the objects is the earth (which is typically the case when we are discussing the weight of an object) then this force of gravity (weight) is simplified to: Fg mg where Fg is the force of gravity (weight) m is the mass of the object and g is the gravitational field strength of (in this case) earth and is equally to 9.80 Nkg-1 Consider this. If you were in a spaceship in deep space and the space ship accelerated at a rate of 9.8 ms-2, what would it feel like? What if you were accelerating at 4.9 ms-2, or 19.6 ms-2?