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Forces

Forces. Equations in Chapter 3: Forces. a= F net /m Acceleration F=G[( m 1 m 2 )/ d 2 ] Gravitational Force F=mg Gravitational Acceleration W=mg Weight p= mv Momentum F=( mv f -mv i )/t Net Force & Momentum. Section 1 Newton’s Second Law.

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Forces

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  1. Forces

  2. Equations in Chapter 3: Forces • a= Fnet/m Acceleration • F=G[(m1m2)/d2] Gravitational Force • F=mg Gravitational Acceleration • W=mg Weight • p=mv Momentum • F=(mvf-mvi)/t Net Force & Momentum

  3. Section 1Newton’s Second Law Force & motion are connected Objects have greater acceleration if greater force is applied Mass of object & force applied affect acceleration

  4. Friction Friction depends on kinds of surfaces and force pressing surfaces together.

  5. Microwelds Areas where surface bumpers stick together The source of friction

  6. Friction Friction - force that opposes sliding motion of 2 touching surfaces. Rolling friction is force between a rolling object and the surface it rolls on.

  7. Friction Static friction is the force preventing two surfaces from sliding past each other. No motion occurs. Applied Force Friction Sliding friction is force that opposes the motion of two surfaces sliding past each other. This force overcomes friction and motion occurs.

  8. Air Resistance Air resistance is the force that opposesmotion of objects that move through the air. Acts in opposite direction to gravity.

  9. Air Resistance Air resistance depends on speed, size and shape.

  10. Terminal Velocity Gravity is downward force Air resistance is upward force • The faster you fall the greater the air resistance • Eventually these 2 forces will become equal. net force on object = 0 acceleration (changing speed) = 0 • Object falling at constant speed = terminal velocity Terminal Velocity is different for different objects Depends on size, shape and mass

  11. Newton’s Second Law of Motion The acceleration of an object is in the samedirection as the net force on the object. Equation a= Fnet/m a=acceleration (m/s2) Fnet=net force (newtons) m=mass (kilograms)

  12. Calculating Acceleration a= Fnet/m Use the triangle to manipulate! • If the mass of a helicopter is 4,500 kg & net force on it is 18,000 N. what is the helicopters acceleration? • What is the net force on a dragster with a mass of 900 kg if its acceleration is 32.0 m/s2? • A car is being pulled by a tow truck. What is the car’s mass if net force on car is 3,000 N and it has an acceleration of 2.0 m/s2?

  13. Section 2Gravity One of the four basicforces. This long-range force gives the universe structure. Due to inertia, all objects fall with same acceleration regardless of mass (g is constant)

  14. Gravity Gravity is an attractive force between any 2 objects. Depends on masses of objects & distance apart. Force will increase as distance decreases. Force will increase as mass increases

  15. Law of Universal Gravitation Equation F=G[(m1m2)/d2] F= gravitational force G=universal gravitational constant m=mass of objects d=distance between the objects

  16. Law of Universal Gravitation Gravitational force between 2 massesdecreases as distanceincreases. N Gravitational force can never = zero.

  17. Earth’s Gravitational Acceleration Equation F=mg F=force of gravity (Newtons) m=mass (kilograms) g=acceleration of gravity (m/s2) g= 9.8 m/s2 on Earth g= 1.6 m/s2on the Moon

  18. Space Objects in space shuttle float because they have noforcesupporting them. As you get farther away from Earth the gravitation force acting on you decreases

  19. Weight Weight is the gravitational force exerted on an object. (F=mg ) Equation W=mg W=weight (Newton’s) m=mass (kilograms) g=acceleration of gravity (m/s2)

  20. Mass Mass is a measure of the amount of matter an object contains. Weight = Force (changes as g changes) Mass = Measurement Weight Mass

  21. Calculations F=mg Force of gravity g= 9.8 m/s2 (Earth) g= 1.6 m/s2(Moon) W=mg Weight 1. What is the force of gravity on a sky diver with a mass of 60 kg on Earth? 2. What is the force of gravity on a sky diver with a mass of 60 kg on the Moon?

  22. Free Fall Free fall is simply the lack of a downwardforce upon an object thus noupwardforce exists. (all forces except gravity are ignored) Example from book—standing on a scale in an elevator, in freefall you weight zero.

  23. Projectile Motion Projectile is anything thrown through the air. Projectile motion is the curvedpath the object takes due to Earth’s gravity.

  24. Projectiles Have horizontal and vertical velocities

  25. Horizontal & Vertical Motion

  26. Horizontal & Vertical Distance Two identical items hit the ground at same time due to identical downward accelerations even if one is dropped and other is thrown horizontally.

  27. Catapult Lab&Projectile Motion You are going to use everyday objects to build a catapult. Catapults were often used as weapons of war during the Middle Ages. You can design your catapult however you please, but you may use only the materials that have been provided and nothing extra.

  28. Centripetal Force Centripetal acceleration is the accelerationtoward the center of a curved or circular path. Centripetal force is the net force exerted toward the center of a curved path.

  29. Demonstration Draw a circle, trace the line with a block or some other object. Notice you have to twist the object towards the center to stay on the path of the circle. This is centripetal—towards the center.

  30. Section 3Newton’s Third Law of Motion This is the law that rocket propulsion is based on. action-reaction

  31. Newton’s Third Law of Motion When one object exerts a force on a second object, the second one exerts a force that is equal in strength and opposite in direction. For every action there is an equal and opposite reaction.

  32. Newton’s Third Law of Motion The forces are equal but not balanced. They act on different objects. Action Reaction Direction seen as a result is forward

  33. Momentum Momentum is amount of force needed to changemotion of a movingobject. Equation p=mv p=momentum (kg m/s) m=mass (kilograms) v=velocity (m/s)

  34. Newton’s 2nd and Momentum Equation F=(mvf-mvi)/t F=Net force exerted mvf-mvi=change in momentum t=time Momentum has a direction.

  35. Momentum Momentum changes if mass, velocity or both change. Momentum can be transferred. Total amount of momentum is conserved. No loss & No gain! 2 objects moving toward each other p=0

  36. Momentum & Newton’s Cradle

  37. Momentum p=mv Calculations Page 86 1. What is the momentum of a car with a mass of 1,300 kg traveling at 28 m/s? 2. If a baseball has a mass of .15 kg and is thrown with a momentum of 6.0 kg*m/s what is speed? 3. What is mass of person walking .8 m/s if momentum is 52.0 kg*m/s?

  38. Equations Review • a= Fnet/m Acceleration • F=G[(m1m2)/d2] Gravitational Force • F=mg Gravitational Acceleration • W=mg Weight • p=mv Momentum • F=(mvf-mvi)/t Net Force & Momentum

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