1 / 43

GPS: Measure and calculate frictional forces and Newton’s three laws of motion

Explore the difference between contact and field forces, how to measure forces with Newton's laws, and the significance of vectors in force analysis.

spillers
Download Presentation

GPS: Measure and calculate frictional forces and Newton’s three laws of motion

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. GPS: Measure and calculate frictional forces and Newton’s three laws of motion EQ: What is the difference between contact forces and field forces?

  2. Force: Any push or pull • Unit of force is a Newton (N) • Is a vector quantity • Symbolized using arrows • Arrows denote direction and magnitude (size) • Multiple forces act on objects • Newton’s laws deal with net force – the leftover force after all forces have acted on an object

  3. Categorizing Forces • Contact forces – object from the external world applies a force to a system by touching it • Examples: • Friction • Normal force • Tension

  4. Categorizing Forces • Field force – object from external world applies a force to a system WITHOUT touching it • Example: • Gravitational force (weight) • Electromagnetic force

  5. Free body diagrams • Forces result from interactions between masses • Mass which causes the force is the “agent” • The object acted on by the force is the “system” • Example: falling book • Book’s mass is system • Earth’s mass is agent

  6. Free Body Diagrams • Free body diagrams allow analysis of how forces affect motion • ID system – draw as circle – or particle (dot) • ID agents – draw as vectors (arrows) • ID everywhere system touches external world – contact forces • ID field forces • Make vectors proportional in size • Draw force vectors away from system (circle) • Choose positive direction – usually in direction of greatest force.

  7. Forces as Vectors • Determining net force • Forces in same direction – add • Forces in opposite direction – subtract • Total is called net force • Object moves in direction of largest force

  8. 10N 10N 10m/s 10m/s 10N 10N 10N 10N Forces as Vectors • Net Force up and down = 0N • Net Force left and right = 20N to right • Object will increase velocity and will move to the right – positive acceleration 10N • Net Force up and down = 0N • Net Force right and left = 0N • Object will maintain velocity 10N

  9. 10N 10N 10N 10N Forces as Vectors • Balanced forces = 0 net force • Equal in size and opposite in direction • Result in no change in velocity – no acceleration

  10. Forces as Vectors • Forces that act in opposite directions act against each other • Magnitudes are subtracted • Motion is in the direction of greatest force 20N 30N • Net force is 10 N • Object will move to the right 20N 30N • Net force is 10 N • Object will move to the left

  11. Forces as Vectors • Forces in same direction work together to affect motion • Magnitudes are added 20N 30N 5N 5N • Net Force acting on object = 50N • Object will move to right • Net Force acting on the object is 10N • Object will move downward

  12. On p. 89 in text: Complete 1-5 Practice gets you closer to perfect

  13. GPS: Measure and calculate frictional forces and Newton’s three laws of motion EQ: How do net forces affect acceleration?

  14. Newton’s 1st Law: Law of Inertia • Objects with no net force acting on them will not change their motion • Objects at rest will stay at rest • Objects in motion will maintain that motion – no acceleration will occur • Forces are in equilibrium • Resistance to change in motion is due to inertia

  15. Newton’s 1st Law: Law of Inertia • Equilibrium: Objects at rest • Net force on object = 0N • Objects sitting on a surface • WEIGHT pushes downward FORCE • NORMAL force pushes upward • A contact force applied by a surface • Total normal is equal andopposite of weight • Hanging objects • Weight pulls down • Tensionpulls upward • A contact force applied by a rope/string/cable • Total tension is equal and opposite of weight

  16. An object at rest on a surface w = weight n = normal force An object at rest hanging from a surface t = tension t =5N t =5N w = 1N n = 1N w =10N

  17. Newton’s 1st Law: Law of Inertia • Mass is NOT volume • Volume is the amount of space an object occupies • Which has more mass, a pillow or a car battery? • Car battery • Which has more volume? • Pillow • Which has more inertia?

  18. Newton’s 1st Law: Law of Inertia • Mass is NOT weight • Weight is a measure of the force of gravity on an object • Weight can change with gravity, mass doesn’t • Weight is a force – units are Newtons • The more mass, the more weight – b/c more material for gravity to act on

  19. Dynamic Equilibrium • Zero net force • Nonzero constant velocity – NOT AT REST • Which property allows objects to keep moving at constant velocity? • INERTIA • Which force acts against dynamic equilibrium? • FRICTION • Friction only comes into play when there is motion between two surfaces

  20. GPS: Measure and calculate frictional forces and Newton’s three laws of motion EQ: What is the relationship between mass, force and acceleration?

  21. Newton’s 1st Law: Law of Inertia • Inertia is measured as mass • Mass is the amount of material in an object • More mass, more inertia, more resistance to change • More massive objects are harder to get moving and harder to stop

  22. Friction: A force between two surfaces that ALWAYS OPPOSES motion • Has to be acted against to get things moving • Has to be reduced to keep things moving • Is always between two surfaces and always makes a moving object slow down • Friction converts moving energy to heat • Causes surfaces to heat up • Causes surfaces to wear down

  23. Newton’s 2nd Law • Changing Velocity – • Net force acting on a body causes acceleration • If velocity changes, there’s a net force acting on it • Any change in speed or direction aka ACCELERATION

  24. Newton’s 2nd Law • The net force (Fnet) causing acceleration can be calculated: • Net Force = (mass) acceleration • Fnet = ma or a = Fnet or m = Fnet m a

  25. Newton’s 2nd Law • Fnet = ma • 1kg ball to accelerate 10m/s2 requires 10N of net force • m = 1kg a = 10m/s2 Fnet = ? Fnet =ma • Fnet = 1kg(10m/s2) Fnet = 10kg٠m/s2 • 1kg ٠m/s2= 1N = unit of force

  26. Increasing mass requires a greater net force to get the same acceleration • m = 1kg a = 10m/s2 Fnet = 10N • m = 2kg a = 10m/s2 Fnet = 20 N • Mass doubled, net force needed doubled • INERTIA doubled – more resistance • Semi-Truck has much bigger engine than mini-cooper – bigger engine, more Force

  27. Increasing acceleration requires a greater net force • m = 1kg a = 10m/s2 Fnet = 10N • m = 1kg a = 20m/s2 Fnet = 20 N • Acceleration doubled, net force needed doubled Mustang vs Mustang GT : Same mass • GT accelerates faster – bigger engine (V-8) – more force

  28. Maintaining net force on a greater mass will result in LESS acceleration • a = Fnet/m • m = 1kg Fnet = 10N a = 10N/1kg = 10m/s2 • m = 2kg Fnet = 10 N a = 10N/2kg = 5m/s2 • Mass was doubled, acceleration cut in ½ • Mass cut in ½, acceleration doubled

  29. There is an inverse relationship between “m” and “a” • IF Fnetis not changed, ↑mass causes ↓acceleration • IF Fnetis not changed, ↓mass causes ↑ acceleration • Camaro (↓m) accelerates faster than Cadillac SUV (↑m) with same 3.6L engine size (same Fnet)

  30. 2010 Chevrolet Camaro Performance SpecsMass is about 1542kg Engine: 3.6L V6Power:300 HP SAE @ 6,400 rpmTorque:273 ft. per lb. @ 5,200 rpm 2009 Cadillac SRX Performance SpecsMass is about 2028kg Engine: 3.6L V6Power:255 HP SAE @ 6,500 rpmTorque:254 ft. per lb. @ 2,800 rpm

  31. WEIGHT = Fw= m(“a” due to gravity) • On Earth “a” due to gravity = g = 10m/s2 Fw = mg • If a 1kg ball hangs from a spring scale. What is its weight? • m = 1kg g = 10m/s2 Fw = ? Fw= mg • Fw= 1kg(10m/s2) Fw= 10N

  32. GPS: Measure and calculate frictional forces and Newton’s three laws of motion EQ: How can Force,mass, and acceleration be derived using Newton’s 2nd Law ?

  33. Practice • Calculate the acceleration of a 2000kg airplane just before takeoff when it’s thrust is 500N.

  34. Practice • Calculate the acceleration of a 300,000kg jumbo jet just before takeoff when the thrust of each of its 4 engines is 30,000N

  35. Practice • Calculate the horizontal force that must be applied to produce an acceleration of 1g for a 1.3kg puck on a horizontal friction free table.

  36. Practice A 10kg screen hangs from the ceiling on two wires. What is its weight? What is the tension force on each wire?

  37. Falling with AIR RESISTANCE (friction) • Air resistance increases with speed • Air resistance increases with greater surface area • Increased air resistance means increased friction --- opposes motion

  38. Terminal Velocity • Air resistance ↑ = weight ↓ • 0 net force – falling body reaches constant velocity • 0 acceleration • Why does body keep falling down? INERTIA! -- resists change in motion

  39. Newton’s 3rd Law • Newton’s 3rd Law • When one object exerts a force on another, the second object exerts a force equal and opposite direction on the first object • “ For every action there is an equal and opposite reaction”

  40. Newton’s 3rd Law • Examples:

  41. Newton’s 3rd Law

  42. Newton’s 3rd Law • If a horse pulls on a cart and the cart pulls on the horse with equal and opposite force, How does the cart ever move forward? • Remember: Motion depends on NET FORCE acting on a body.

  43. Newton’s 3rd Law If a brick is dropped from a bridge into a valley. Earth pulls on the rock. Newton’s 3rd Law states the rock pulls on Earth with equal and opposite force. Why don’t we see Earth’s acceleration? massR = .2kg Find the acceleration of the ball and the Earth: F = ma g = 10m/s2 massE = 6 x 1024 kg

More Related