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## Force

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**Unit of Force**• Force is measured in Newtons (N) • 1 Newton (N) is the amount of force required to accelerate a 1 kg object at a rate of 1 m/s2**Newton’s Three Laws**• I. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it. • II. The relationship between an object's mass m, its acceleration a, and the applied force F is F = ma. • III. For every action there is an equal and opposite reaction.**What forces can do to an object**• 1. Can cause an object to start moving or increase the speed of an already moving object. This is called Acceleration. • 2. Can cause an object to stop moving or slow down the speed of a moving object. This is called Deceleration. • 3. Can cause a moving object to change direction.**Types of Forces**• Gravitational • Electromagnetic • Frictional • Strong and Weak Nuclear**Gravitational Forces**• Force of attraction between all objects as a result of their masses and distances between them. • The higher the masses and the shorter the distances between two objects, the greater the force of gravity. • The smaller the masses and the larger the distances between two objects, the weaker the force of gravity.**Why things fall to the ground**• When we drop an object, the force of gravity is strong because the earth has a large mass and is close by. The object gets pulled to the ground. • Near the Earth’s surface, objects accelerate to the ground at a rate of 9.8 m/s2, regardless of their masses.**The gravitational force of the Earth decreases as you move**away from the planet. • Different planets and stars will have different forces of gravity at their surfaces due to their different masses. The bigger the object the greater the force of gravity. • Gravitational forces also explain the Earth’s tides**Mass vs. Weight**• Mass is a measure of the quantity of matter in an object. This does not depend on where the object is. • Weight is a measure of the gravitational force acting on an object. This DOES depend on where the object is.**Acceleration**• Acceleration = Change of velocity Time Taken • Example – A F1 car starts at 0 m/s and gets to 27 m/s in 3 seconds, what is the F1’s acceleration?A = 27m/s – 0m/s = 9m/s2 3 s**A useful formula**Weight = Gravitational force = Mass X gravitational field intensity OR w = Fg = mg or F=ma m is mass measured in kilograms (kg) g gravitational field intensity measured in Newtons/kilogram (N/kg); this is also known as acceleration. F is the force measured in Newtons (N)**Example**• What is the gravitational force (weight) of a 60 kg astronaut on the earth’s surface? Intensity of gravitation on earth is 9.8 N/kg F = mg F = 60 X 9.8 F = 588 N**Example**• What is the gravitational force (weight) of a 60 kg astronaut on the Moon’s surface? F=mg F= 60 X 1.67 (force of gravity on the Moon) F= 100.2 N