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Motion and Forces Newton at work Isaac Newton • Came up with 3 Laws of Motion based on the works of Galileo and Johannes Kepler . • “If I have seen further, it is by standing on the shoulders of giants.” • Invented Calculus. (Archimedes actually had laid out the basic principles of calculus in 3rd century BC, but work was lost in Middle Ages and not rediscovered until late 1990s) • Published his Laws in 1687 in the book Mathematical Principles of Natural Philosophy. Force? What is a force? – Think silently for 12 s Write your definition in your notebook and share with 2 people = a push or pull that one body exerts on another •Can exist during physical contact – CONTACT FORCES •Can exist with NO physical contact, called FIELD FORCES Forces are measured in units called Newtons 𝑘𝑔 ∙𝑚 N=( 𝑠2) Tool for measuring the magnitude of a force = SPRING SCALE Forces and Motion • Forces can cause an object to accelerate… • To speed up • To slow down • To change direction Some specific types of forces to know: CONTACT FORCES • Generic applied forces FA • Weight - W or mg • Normal – FN or N • Tension – FT or T • Friction -- Ff • Air resistance -- FAR • Buoyant -- FB FIELD FORCES • Gravitational Force – FG • Magnetic Force -- FM • Electrical Forces -- FE Force Normal(FN) A surface force always drawn perpendicular to a surface. Flat surfaces – 1 D FN = mg for objects resting on horizontal surfaces. N mg Applied forces affect normal force. friction applied force normal weight N = applied force Tension(T or FT) • A pulling force. • Generally exists in a rope, string, or cable. • Arises at the molecular level, when a rope, string, or cable resists being pulled apart. Friction(Ff) A force that acts in a direction opposite to the motion of two surfaces in contact with each other. Friction works to slow things down or hold them in place. Friction is caused by “microwelds”. Every surface, even smooth ones, have bumps. These bumps rub and lodge against each other. Friction Friction can cause an object to slow down and stop. It sometimes seems as if the object has lost energy, but in reality the energy was converted into There are two types of friction • Friction and the Normal Force • The frictional force which exists between two surfaces is directly proportional to the normal force. • That’s why friction on a sloping surface is less than friction on a flat surface. Friction Lubricants can be used to reduce friction. examples : oil, grease, wax, water, mud, etc… Air resistance • Friction between an object moving through the air and the air itself. • When an object falls through the air, air resistance pushes UP on the object, working to slow it down. Buoyancy • Is the ability of a fluid to exert an upward force on anything immersed in it. • The upward force is called the buoyant force. • The denser the fluid, the greater the buoyant force. • This force determines if things sink or float. • Explains how boats float. • Water has a density of 1.0 g/mL. • Anything below 1.0 g/mL will float to the top. • Anything greater than 1.0 g/mL will sink. • If it is exactly 1.0, it will remain in place, neither going up or down. • LESS dense moves UP. • MORE dense moves down How is Mass Different Than Weight? (Hold that thought!!!) Your task Investigate the relationship between mass and weight. • You must gather data about 5 objects found in the classroom or lab. 3 of known mass, 2 of random unknown mass • After gathering the data, you must create a table and graph/chart • Your table must be organized and clearly labeled with correct units shown in the column headings • Your graph must be clearly titled as “Weight vs Mass” with the appropriate information on the correct axis. Your axis must also include units for the quantities. • You need to draw a line of best fit and figure out the slope of it. How is Mass Different Than Weight? Weight is measurement of FORCE. 2.2 pounds 3.3 pounds 9.8 N 14.8 N (1 kg x 9.86 m/s2) (1.5 kg x 9.8 Acceleration due to gravity (9.8 m/s2) 1.0 Kg Mass 4.4 pounds 19.7 N (2 kg x 9.86 m/s2) 6m/s2) Acceleration due to gravity (9.8 m/s2) Acceleration due to gravity (9.8 m/s2) 1.5 Kg Mass 2.0 Kg Mass DO NOT USE kilograms (kg) as a measurement for weight (force). USE N for force. Weight(mg) = Mass (m) X gravity (g) • Unit of mass = kg • Unit of acceleration = m/s2 • Unit of weight = Newton • 1 Newton= about ¼ pound Balanced Forces • A force that produces no change in an object’s motion because it is balanced by an equal, opposite net force. Unbalanced Forces Are net forces that results in an object’s motion being changed. + Consider onaaforce table.isThe forces Now, considerthis ballball b. If applied tondit, It will act the same way. However, what if a 2 force Now consider ball is c which hasinthe force on it are balanced. it will move. This a change itssame motion due is applied to it at another point in its path? applied to it. to an unbalanced force Its motion will change AGAIN due to the UNBALANCED FORCE on it Free-body diagrams Free-body diagrams are used to show the relative magnitude and direction of all forces acting on an object. This diagram shows four forces acting upon an object. There aren’t always four forces, For example, there could be one, two, or three forces. Problem 1 A book is at rest on a table top. Diagram the forces acting on the book. Problem 1 Solution In this diagram, there are normal and gravitational forces on the book. Problem 2 An egg is free-falling from a nest in a tree. Neglect air resistance. Draw a free-body diagram showing the forces involved. Problem 2 Solution Gravity is the only force acting on the egg as it falls. Problem 3 A flying squirrel is gliding (no wing flaps) from a tree to the ground at constant velocity. Consider air resistance. A free body diagram for this situation looks like… Problem 3 Solution Gravity pulls down on the squirrel while air resistance keeps the squirrel in the air for a while. Problem 4 A rightward force is applied to a book in order to move it across a desk. Consider frictional forces. Neglect air resistance. Construct a free-body diagram. Let’s see what this one looks like. Problem 4 Solution Note the applied force arrow pointing to the right. Notice how friction force points in the opposite direction. Finally, there is still gravity and normal forces involved. Problem 5 A skydiver is descending with a constant velocity. Consider air resistance. Draw a free-body diagram. Problem 5 Solution Gravity pulls down on the skydiver, while air resistance pushes up as he falls. Problem 6 A man drags a sled across loosely packed snow with a rightward acceleration. Draw a free-body diagram. Problem 6 Solution The rightward force arrow points to the right. Friction slows his progress and pulls in the opposite direction. Since there is not information that we are in a blizzard, normal forces still apply as does gravitational force since we are on planet Earth. Problem 7 A football is moving upwards toward its peak after having been booted by the punter. Draw a free-body diagram. Problem 7 Solution The force of gravity is the only force described. It is not a windy day (no air resistance). Problem 8 A car runs out of gas and is coasting to the right down a hill and slowing down. Problem 8 Solution Even though the car is coasting down the hill, there is still the dragging friction of the road (left pointing arrow) as well as gravity and normal forces. Net Force Now let’s take a look at what happens when unbalanced forces do not become completely balanced (or cancelled) by other individual forces. An unbalanced forces exists when the vertical and horizontal forces do not cancel each other out. Example 1 Notice the upward force of 1200 Newtons (N) is more than gravity (800 N). The net force is 400 N up. Example 2 Notice that while the normal force and gravitation forces are balanced (each are 50 N) the force of friction results in unbalanced force on the horizontal axis. The net force is 20 N left. Another way to look at balances and unbalanced forces A force diagram illustrating Static Friction Normal Force Frictional Force Applied Force Gravity A force diagram illustrating Static Friction Normal Force Bigger Applied Force Bigger Frictional Force Gravity A force diagram illustrating Static Friction The forces on the book are now UNBALANCED! Normal Force Frictional Force Gravity Static friction cannot get any larger, and can no longer completely oppose the applied force. Even Bigger Applied Force