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Forces and the Laws of Motion

Forces and the Laws of Motion. Changes in Motion. Force is a push or a pull exerted on an object. Force causes a change in velocity. A force acting on a mass causes an acceleration change in velocity. If 1 kg of mass is accelerated by 1 m/s 2 then 1 Newton (N) of force is needed.

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Forces and the Laws of Motion

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  1. Forces and the Laws of Motion

  2. Changes in Motion Force is a push or a pull exerted on an object Force causes a change in velocity A force acting on a mass causes an acceleration change in velocity If 1 kg of mass is accelerated by 1 m/s2 then 1 Newton (N) of force is needed

  3. Some forces act when two objects are in contact. These are contact forces

  4. Some forces act from a distance (the objects are not in contact). These are Field Forces

  5. Newton’s First Law of Motion Inertia – The tendency of an object to maintain its present state of motion Mass – Measure of matter in an object Galileo conducted experiments and concluded that if no friction is present, an object in motion will remain in motion at constant velocity forever

  6. Inertia, Mass and Motion Big Mass, big inertia Small mass, small inertia Inertia is a property of mass. The more mass an object has the greater its inertia.

  7. Newton’s 1st Law of Motion “Law of Inertia” An object will remain at rest or in motion in a straight line at constant velocity (not accelerating) unless an UNBALANCED FORCE acts on the object. Unless an unbalanced force acts on the object At Rest In Motion

  8. Because of Inertia, the car stops (force of the wall) but the driver keeps on moving! Seat belt anyone?

  9. Gravity, Mass and Weight The weight of an object is the force that is applied to an object by gravity FWeight = m x g Since g has been measured to be the same for all objects, neglecting air resistance, the greater the mass of an object the greater the objects weight

  10. A scale measures the amount of support force needed to balance the downward force of gravity Your weight (or anything’s weight), is the product of your mass times the acceleration of gravity at your location FWeight = m x gAt That Location

  11. Special Applications of Newton’s Laws Weight and Friction Weight – The force between too bodies, usually between a large mass and a much smaller mass Weight is not an inherent property of an object (like mass or inertia) but is location dependent The farther an object is from the center of the mass, the less the objects weight The acceleration of gravity by a body is the same for all objects

  12. What keeps the arrow moving after it is released from the bow? Early scientist thought that an “invisible force” called impetus pushed the arrow as it moved through the air.

  13. Force Diagrams  Free Body Diagram A force diagram shows all of the forces acting on an object using force vectors First, analyze the problem

  14. Second, draw a simple diagram of the object Third, put a point in the middle of the diagram and draw the first vector in the direction it is acting Draw the gravitational force pointing directly toward the center of the earth

  15. Determine the upward or normal force acting on the object The last force for this problem is the frictional force between the tires and the road

  16. Net or Unbalanced Force – The force that remains when all of the force vectors are added Now there is a net force since Fresistance is less than Fforward Adding all of the forces on the car equals 0 (No net force)

  17. Finding the Net or Unbalanced Force 1. Draw a force free body diagram and identify all of the forces acting on the object 2. Determine an appropriate coordinate system and sketch the force vectors using that coordinate system 3. Find the x and y components of the vectors 4. Find the net force acting in the x and y direction 5. Determine the net force (magnitude and direction) 6. If the net force is 0 the object is in equilibrium.

  18. Newton’s Second and Third Laws

  19. Force, Mass and Acceleration Newton’s Second Law – The net force acting on an object equals the mass of the object times the objects acceleration. F = m x a Force is a vector. The vector property of direction is in the direction of the acceleration. Mass is a scalar and changes the magnitude of the force.

  20. Solving Newton’s Second Law for acceleration gives: a = F/m The acceleration of an object is directly proportional to the net force on an object and inversely proportional to the mass of the object Net Force causes acceleration

  21. What TOTAL mass is being accelerated? The weight of the hanging mass is accelerating the total mass of the system. Adding a mass on the cart increases the mass of the system. Did the force increase? How will it affect the acceleration. The weight of the mass is the force that accelerates the system

  22. Newton’s Second Law and Falling Objects Neglecting air resistance, all objects fall at the same rate of acceleration, g F/m = g = F/m The greater the mass, the greater force needed to accelerate the object. So the ratio of F/m is g

  23. Newton’s Third Law of Motion F F

  24. Newton’s Third Law of Motion – Forces always act in pairs. For every force there is an equal and opposite force F F a a a =F/m a =F/M The cannon applies a force to the cannon ball, and the cannon ball applies an equal but opposite force to the cannon

  25. The earth exerts a force on the apple that causes the apple to accelerate down. But the apple also exerts an equal but opposite force on the earth, causing the earth to accelerate up! Gravity, Force and Newton’s Third Law Since the earth’s mass is MUCH greater than the mass of the apple, the acceleration of the earth up is VERY small -FAppleon Earth = FEarthon Apple

  26. Application of Newton’s Laws & Free Body Diagram The force of gravity (FWeight) always acts toward the center of the earth and is constant for that object The force of the surface (FNormal) always acts PERPENDICULAR to the surface FNormal FWeight q = 0O When the angle of the incline is 0O FW = FN

  27. FN is the force of the incline that supports the wagon and DOES NOT equal FW This component is FPerpindiuclar (F ) and is equal in magnitude but opposite direction of FN y FW is the weight of the wagon and is toward the center of the earth This is the parallel component down the incline F= FW q is not 0O x

  28. F is the adjacent component of FW q for the vectors is the same as q for the incline FW F= is the opposite component of FW A Closer Look At The Forces on an Incline

  29. The Static and Kinetic Forces of Friction and Free Body Diagram Fn Fn Fn Fa Fa Fg Fg Fg In figure a, there is no frictional force because there is no force trying to set the jug in motion In figure b, a force is being applied, but not enough to set the jug in motion In figure c the applied force is greater than the maximum frictional force so the jug accelerates

  30. Fn Fa Fg Friction acts to oppose the applied force. The frictional force increases until it reaches a maximum. The frictional force BEFORE the jug moves is called the Static Force of Friction FS, the force that is opposing the motion.

  31. Fn Fk FS Fk= Fa Fg After the jug is in motion, the force needed to maintain constant motion is the Kinetic Force of Friction FK For most surfaces FS > FK

  32. FS The nature of friction is due to the irregularities of the surfaces in contact. Friction is really a microscopic effect

  33. The frictional force equals the applied force until the maximum frictional force between the surfaces is reached. At that point the object begins to move. For most surfaces, the applied force to maintain constant motion is less than needed to begin the motion A graph of the applied force vs. frictional force. The peak in the graph represents the maximum static frictional force and the horizontal line is the kinetic force that maintains constant motion

  34. Calculating the Frictional Force The frictional force is determined by the Normal force of the surface on the object and the type of surfaces that are in contact Ff = mFN and m =Ff / FN Where m (mu) is the coefficient of static (mS) or kinetic (mK) friction. Since m is a ratio of two forces, it lacks a unit

  35. Objects in Equilibrium When the FNet on an object is 0, it is in Equilibrium Objects in Equilibrium are not accelerating Objects in Equilibrium may or may not be moving

  36. If M1 and M2 have the same mass, then the system is in equilibrium If M1 and M2do not have the same mass, then the system is not in equilibrium and the masses will accelerate M2 > M1

  37. FW is the weight of the wagon and is toward the center of the earth FN is the force of the incline that supports the wagon and DOES NOT equal FW Now there is a force acting DOWN the incline that is PARALLEL to the surface of the incline F= This component is FPerpinduclar (F ) and is equal in magnitude but opposite direction of FN y SF = FNet The Frictional Force, Ff is acting on the wagon and is opposite the motion FW q is not 0O x

  38. Ff is acting down the incline because the motion is now up the incline FApplied is up the incline Pulling a Wagon Up an Incline  Free Body Diagram Fn is the normal upward force from the incline. SF = FNet FW is the gravity acting on the wagon’s mass.

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