The Nature of Force Force In science, the word force has a simple and specific meaning. Force is a push or a pull When one object pushes or pulls another object, you say that the first object exerts a force on the second object. Force is a vector quantity. Described by magnitude and direction. An arrow represents the direction and strength of a force and the longer the arrow, the greater the force. The SI unit for force is a Newton. Combining Forces Often, more than a single force acts on an object at one time. Net Force is the combination of all forces acting on an object. Balanced Forces Balanced Forces are equal forces acting on one object in opposite directions. When equal forces are exerted in opposite directions there is no net force. Balanced forces acting on an object do not change the object's velocity. Friction, Gravity and Elastic Forces Friction Friction is a force that two surfaces exert on each other when they rub against each other. Friction acts in the direction opposite to motion. Without friction or other unbalanced forces, an object will not stop until it hits another object. Causes of Friction Smooth surface has less friction than rough surface. Strength of force of friction depends on types of surfaces involved and how hard the surfaces push together. (i.e. rubbing hands together) 4 Types of Friction Static Friction - Friction that acts on objects that are not moving Example: moving a piece of furniture across room. Must use extra force to start friction of stationary objects. Sliding Friction - Friction where two solid surfaces slide over each other. When an object is pulled at a constant velocity across a level surface, the pulling force equals the sliding friction. Rolling friction - Friction where an object rolls across a surface. Rolling friction is less than sliding friction. Fluid friction - Friction where a solid object moves through a fluid. Less than sliding friction. Parts of machines bathed in oil. Gravity Gravity is a force that pulls objects toward each other. Newton realized gravity acts everywhere in universe. Law of Universal Gravitation states that the force of gravity acts between all objects in the universe. Any two objects in the universe, without exception, attract each other. Mass and Distance Force of gravity between object increases with greater mass and decreases with greater distance. The further from Earth it is, the less gravitational force. Mass is the measure of the amount of matter in an object. (Kg) Gravity & Weight Weight is gravitational force exerted on a person or object at the surface of a planet. Stepping on scale (weight) shows the gravitational force Earth exerting on you. Objects with greater mass have greater weight. Weight = Mass x Acceleration due to gravity Weight – Newtons (N). Mass – Kilograms (KG). Acceleration due to gravity = 9.8 meters per second per second (m/s²) (at Earth's surface) Weight varies with strength of gravitational force – mass doesn't. Strength of gravitational force exerted on an object or person by the moon is 1/6 of the force exerted by Earth. Gravity and Motion Free fall – when the only force acting on an object is gravity. In free fall, the force of gravity alone causes an object to accelerate in the downward direction. All objects in free fall accelerate at the same rate. Air Resistance Air resistance is a type of fluid friction. Air resistance is an upward force exerted on falling objects. Objects with greater surface area experience more air resistance as they fall. (Flat piece of paper vs. crumpled piece of paper) Projectile Motion A projectile is an object that is thrown. A ball thrown horizontally falls at the same rate as ball dropped. Elastic Forces Matter is considered elastic if it returns to its original shape after is squeezed or stretched. Compression is an elastic force that squeezes or pushes matter together. Tension an elastic force that stretches or pulls matter. Newton's Laws Isaac Newton In the early 1600s Galileo said, that when an object is in motion, a force is needed to change the motion of an object. In the late 1600s, Isaac Newton built upon Galileo's idea and proposed three basic laws of motion. First Law of Motion An object will remain at rest or moving at a constant velocity unless it is acted upon by an unbalanced force. An unbalanced force will cause an object to speed up, slow down or change direction. Inertia Inertia is the tendency of an object to resist a change in motion. Newton's first law is also called the law of inertia. The greater the mass of an object - the greater its inertia - the greater the force required to change its motion. VIDEO Second Law of Motion Acceleration depends on the net force acting on the object and on the object's mass. Acceleration = Net Force / Mass (Acceleration is the rate at which velocity changes) Third Law of Motion If one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object. Action and reaction forces do not cancel out because they are acting on different objects. Momentum Momentum is a quantity of motion. Characteristic of a moving object that depends on both the mass and velocity of the object. Momentum = Mass x Velocity Measured in kilogram-meters per second (kg-m/s) Conservation of Momentum In science, conservation refers to the conditions before and after an event. The Law of Conservation of Momentum states that, in the absence of outside forces, the total momentum of objects that interact does not change. The total momentum of any group of objects remains the same, or is conserved, unless outside forces act on the objects. Rockets and Satellites How Do Rockets Lift Off? Rockets and space shuttles lift into space using Newton’s third law of motion. They burn fuel and push the exhaust gases downward at a high velocity as they lift off. In turn, the gases push upward on the rocket. A rocket can rise into the air because the gases it expels with a downward action force exert an equal but opposite reaction force on the rocket. As long as this upward force, called thrust, is greater than the downward pull of gravity, there is an unbalanced force in the upward direction that causes a change in the rocket's velocity. As a result, the rocket accelerates upward into space. What is a Satellite? Satellite any object that orbits another object in space. (e.g. Moon or Artificial Satellite) Circular Motion Artificial satellites travel around Earth in an almost circular path. An object traveling in a circle is constantly changing direction, so it is accelerating. If an object is accelerating, an unbalanced force must be acting on it. Centripetal Force is any force that causes an object to move in a circular path. (Center seeking, and acts perpendicular to the direction the object is moving) Satellite Motion Satellites in orbit around Earth continuously fall toward Earth, but because Earth is curved they travel around it. The speed with which an object must be thrown in order to orbit Earth is 7,900 meters per second. Satellite Location Some travel in low orbits. (Mapping, or Observation) These satellites complete a rotation around the Earth in a couple of hours. Others in higher orbits travel more slowly. (Communications) These travel 36,000 kilometers above Earth's surface. How to cite this note (MLA)
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