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FORCES • The term force refers to the interaction of objects and their environment. • All forces are exerted on one object by another object. • Forces have both size and direction and are normally classified as “pushes or pulls”. • All forces have both size and direction Types of Forces • Buoyancy - Is the tendency of an object to stay afloat. It is the upward force that all liquids exert on all matter (a push). • Electricity is the flow of electrons producing a current (a push). Negatively charged particles repel each other. • Friction is the force that retards the motion of two touching objects (a push). Gravity – most familiar force • Gravity is the basic force of attraction that is spread throughout the universe. Gravity pulls objects towards each other. • Gravity on earth pulls you and all objects towards the earth. • You must overcome gravity each time you lift something. • Gravitational force on earth is 9.8m/s2 Calculating Force The relationship between an object's mass m, its acceleration a, and the applied force F Force =(mass)(acceleration) or F = ma • The SI units for force is the Newton (N) A Newton is equivalent to the units: N = kg x m s2 Example • An object with a mass of 15.0 kg is moving with an acceleration of 25.0 m/s2. What is the force acting on that object? F = ma = (15.0 kg) x (25.0m/s2) = 375 kg• m/s2 = 375 N Another Force - Weight • Weight is a force applied to an object as a result of gravity. • Weight = mass x (gravitational force) Fw = (m) (g) • On earth, the force of gravity is nearly constant = 9.8 m/s2 Mass vs. weight Understand the difference Mass is: Weight is: A measure of the amount of A measure of the matter in an object. gravitational force on an object. Always constant for an Varied depending on where object no matter where the the object is in relation to object is in the universe. the Earth or any other large body in the universe. Expressed in kilograms, Expressed in Newtons (N). grams, and milligrams. Weight • It is different depending on where the object is located and the amount of gravity acting on it. • Weight is expressed in Newtons (N) • Weight of an object can be determined by the following formula Weight = (mass) (gravity) OR Fw = (m)(g) Example If an object has a mass of 75 kg on earth, what is it’s weight? Fw = (m)(g) = (75 kg) x (9.8 m/s2) = 735 kg • m/s2 = 735 N = 740 N How Energy Relates to Work Energy - the ability to do work Work - a measure of how productive an applied force is Work • Work is the product of the force applied to an object time the distance through which the force acts • EXAMPLES OF WORK – Lifting a book – Pulling a cart – Pushing a door open • Sometimes there are easy ways and hard ways to do the same amount of work. Work • The formula for work is: Work = (force) (distance) or W = Fd The unit for work is the Joule J = N * m = kg *m2 s2 It is important that you understand that all units used in the equation are in Kg, m and seconds. The problem will not be accurate (or correct) if the units are not in this form. Example • A woman pushes a shopping cart with a force of 15.0 N a distance of 25.0m. How much work was done? • W = Fd • W = (15N) (25m) • W = 375 J = 380J Example • A book weighing 3.0N is lifted 5.0m. How much work is done? • W = Fd • W = (3.0N) (5.0m) • W = 15J Example • It took 45J to push a chair along the floor a distance of 7.0m. With how much force was the chair pushed? • • • • • W = Fd You need to rearrange the equation to get force. F=W÷d F = 45J ÷ 7.0m F = 6.4N Example • A force of 2400N was necessary to lift a rock. A total of 450J of work was done. How far was the rock lifted? • • • • • W = Fd Rearrange for distance d=w÷F d = 450J ÷ 2400N d = 0.19m