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Forces • Objectives • What is a force? • What is gravity? • What is friction? • What is pressure? Forces • Force is a push or a pull, that causes a change in the motion or shape of an object. • Words that indicate the use of a force: • Pushing , Pulling, Stretching, Squeezing, Bending, and Falling. • Force is measured in units of: Newtons (N) Forces • Force is a push or a pull. • Example: Electric Forces • Force is a push or a pull. • Example: Gravitational Forces • Force is a push or a pull. • Example: Magnetic Forces • Forces can be equal. (Balanced Forces): • If the two girls push with the same amount of force on the stack of books, • the books do not move due to balanced forces. • Other examples: • Book laying on a table, person sitting in a chair, or a light signal hanging from a power line. Forces • Forces can also be unequal. Unbalanced forces. • When an unbalanced force is applied to an object motion is produced. • Any object in motion is due to an unbalanced force. • Motion requires a force. • The overall amount of force used to move an object is called net force. Forces • Finding the net force applied to an object requires the sum of all applied forces. • Forces that are applied in the same direction are added. • Forces that are applied in opposite directions are subtracted. Forces • Remember No motion = NO net force! net Force examples • If Mary pushes with 260 newtons of force and Janet pushes 170 newtons of force, what is the net force on the books? 260 n -170 n 90 n To the right Mary Janet net Force examples • If Mary pushes with 210 newtons of force and Janet pushes 235 newtons of force, what is the net force on the books? 235 n -210 n 25 n To the left Mary Janet net Force examples • If a plane travels north with an engine thrust of 25N of force and hits a cross wind going east with 5N of force. What is the net force and overall direction of the plane? 5N East C= a2 + b2 C= 252 + 52 25 N North 25.5 N Northeast C= 625 + 25 C = 25.5 N Gravity • Gravity is a Universal Force. • Gravitational Force causes every object to attract every other object; • a ball falling to the ground • the moon orbiting the Earth Greater Mass = greater force Friction • Friction is a force that opposes motion. • Friction acts in a direction opposite to the object's direction in motion. • Without friction, the object would continue to move at a constant speed forever. • There are 3 types of friction. Friction • Static Friction • In static friction the two surfaces are not moving past each other. • Here the friction between the 2 surfaces is so great that there is no motion. Friction • Sliding Friction • The opposing force of the two surfaces slides past each other. • Friction is overcome so that there is motion. Friction • Rolling Friction • The opposing force of the two surfaces roll past each other. • Friction is overcome so that there is motion. Force: F=ma • Force is connected with Newton’s 2nd law of motion. F = ma Units: N = kg m s 2 Force: F=ma • What is the force of impact of a 800kg car that is traveling at 10 m/s2 when it hits another car? • Force = ? • Mass (m) = 800kg • Acceleration (a) = 10 m/s2 Formula: F = ma Substitution: F = (800kg) (10m/s2) Results: F = 8,000 N Force: F=ma • How much force is needed to accelerate a 7000kg car at a rate of 1.5 m/s2 ? • F= ? • m = 7000kg • a = 1.5 m/s2 Formula: F = ma Substitution: F = (7000kg) (1.5m/s2) Results: F = 10,500 N Force: F=ma • A weightlifter raises a 150kg barbell with an acceleration of 2.0 m/s2. How much force does he use to lift the weights? • F= ? • m = 150kg • a = 2.0 m/s2 Formula: F = ma Substitution: F = (150kg) (2.0m/s2) Results: F = 300 N Force: F=ma • A bowler takes a 32kg ball and rolls it with an acceleration of 2.8 m/s2. How much force does the bowler use? • F= ? • m = 32kg • a = 2.8 m/s2 Formula: F = ma Substitution: F = (32kg) (2.8m/s2) Results: F = 89.6 N Force: F=ma • A dancer lifts his partner above his head with a force of 100N and an acceleration of 4.0 m/s2. What is the mass of the dancer? • m= ? • F = 100 N • a = 4.0 m/s2 Formula: m = F/a Substitution: m = 100N /4.0 m/s2 Results: m = 25.0 kg Force: F=ma • If a car with a mass of 2,500 kg hits a tree with a force of 500,000 N. What is the acceleration of the car as it hits the tree? • a= ? • m = 2,500 kg • F = 500,000 N Formula: a = F/m Substitution: a = 500,000 N / 2,500 kg Results: a = 200 m/s2 Force: F=ma • A rocket, with a mass of 20 kg, is shot into the air with 2,000 N of force. What is the acceleration of the rocket? • a= ? • m = 20 kg • F = 2,000 N Formula: a = F/m Substitution: a = 2,000 N / 20 kg Results: a = 100 m/s2 Force: F=ma • Another rocket is shot into the air with 2,000 N of force, but it’s acceleration was only 10 m/s2. What is the 2nd rocket’s mass? • m= ? • F = 2,000 N • a = 10 m/s2 Formula: m = F/a Substitution: m = 2,000 N / 10 m/s2 Results: a = 200 kg Pressure • Pressure is the force per unit area. • Pressure is the amount of force acting on an area. • Each book exerts the same force on the table. • However, which book exerts the greater pressure? Why? Pressure • Is it possible for a 1 pound force to exert the same amount of pressure as a 50 pound force? • Yes, because P = F/A • Notice that when a force acts on a smaller area the pressure is larger. • And when a force is acting on a larger area, the pressure is smaller. Pressure • Pressure exerted by liquids and gases is known as Fluid pressure. • 2 common forms of fluid pressure: Air & Water pressure • Air pressure is measured with a barometer. • Water pressure is measured with a manometer. Pressure • Air pressure changes with altitude. • As the elevation increases the air pressure will decrease. • As the elevation decreases the air pressure will increase. Pressure • Water pressure changes with depth. • As the depth increases the water pressure will increase. • As the depth decreases the water pressure will decrease. Bernoulli’s Principle • This principle that states that as the speed of a fluid increases, the pressure decreases. • This principle is what allows the wing of an airplane to produce lift and makes flight possible. Bernoulli’s Principle • There are 4 forces on a plane. • Gravity pulls the airplane down while lift works against gravity to push the plane up. • The thrust of the plane’s engine pushes the plane forward, while drag tends to slow the plane down due to air resistance.

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posted: | 8/13/2011 |

language: | English |

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