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Forces

Explore the fundamental concepts of forces, motion, and energy with Newton's laws. Learn about vectors, momentum, conservation of momentum, friction, gravity, and more.

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Forces

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  1. Forces By Neil Bronks

  2. The unit is called the Newton (N) Force causes a body to change velocity…….. accelerate

  3. This car has a mass of 2000kg This car has a force of 500N Scalar vs. Vector Scalar has only magnitude…..speed, mass Vector has magnitude and direction …….. Velocity, Force, acceleration or displacement.

  4. Distance and Displacement Scalar- Distance travelled 200m Vector- Displacement 120m

  5. 80 60 40 20 0 0.5x10x20=100 Velocity m/s 0.5x20x60=600 40x20=800 0.5x10x40=200 T/s 10 20 30 40 50 Total Distance Traveled =200+100+800+600=1700m

  6. Motion Formula A car starts from rest and accelerates for 12s at 2ms-2. Find the final velocity. v = u + at Using V = U + at = 0 + 2x12 = 24m/s v2 = u2 + 2as A car traveling at 30m/s takes 200m to stop what is it’s deceleration? Using V2 = U2 + 2as 0 = 900 + 2a (200) a = -900/400=-2.25ms-2

  7. Motion Formula S = ut + 0.5at2 A train accelerates from rest at 10ms-2 for 12s find the distance it has traveled. Using S = ut + 0.5at2 = 0x12 +0.5x10x144 =720m

  8. Vector Addition Speed in still air 120m/s  Wind 50m/s Resultant R2 = 1202 + 502 = 14400 + 2500 = 16900 R = 130m/s Tan  = 50/120  = 22.60

  9. The unit is called the Newton (N) Friction is the force that opposes motion Lubrication reduces friction Friction is the force between two bodies in contact.

  10. 5m/s V=? m/s 3 m/s Momentum 3kg 2kg 3kg 2kg In a closed system the linear momentum is always conserved Momentum Before = Momentum After Mass Moving x velocity before = Mass moving x velocity after 2kg x 5m/s = 2kg x (-3m/s) + 3kg x v 3v = 10 + 6 V = 5.333m/s

  11. VERIFICATION OF THE PRINCIPLE OF CONSERVATION OF MOMENTUM l Card Vehicle 2 Dual timer t1 t2 Photogate Light beam Air track Vehicle 1 Velcro pad

  12. m=2kg u=400m/s Momentum of the Shoot Momentum of Recoil = 2 x 400 150 x v = Recoil Mass of canon=150kg Mass Canon x Velocity Canon = Mass of Ball x Velocity of Ball 5.3m/s V= 800/150=

  13. Newton’s Laws • 1 /. Every body stays in it’s state of rest or constant motion until an outside force acts on it • 2/. The rate of change of momentum is proportional to the applied force and in the direction of the applied force. • F=ma • 3/. To every action there is an equal and opposite reaction

  14. Newton 2 force Rate of change of Momentum Forcem.a Or Force=k.m.a where k=constant As this is the basic constant so we say k=1 andForce=m.a

  15. Dual timer Photogate l Pulley Light beam Card Slotted weights s Air track TO SHOW THAT aµ F t2 t1

  16. F M A Force and acceleration If the forces acting on an object are unbalanced then the object will accelerate, like these wrestlers: Force (in N) = Mass (in kg) x Acceleration (in m/s2)

  17. Friction=? Feng=5000N a=3m/s2 Acceleration gives Net Force 900kg As net force causes acceleration F=m.a Fnet = 900kg. 3m/s2 Fnet= 2700N So Friction = Feng – 2700 = 2300N

  18. Archimedes Principle • A body in a fluid experiences an up-thrust equal to the weight of liquid displaced. 12N 20N 8N

  19. Floatation • A floating body displaces its own weight in water.

  20. MEASUREMENT OF g Electromagnet Electronic timer Switch h=½gt2 Ball bearing h Trapdoor

  21. F  m1m2 d2 Where F = Gravitational Force m1.m2 = Product of masses d = Distance between their center of gravity Newton's Law of Gravitation • This force is always positive • Called an inverse square law

  22. Hookes Law Example Force =Constant (k) x Extension Example a/. A mass of 3kg causes an extension of 0.3m what is the spring constant? 3x9.8 = k x 0.3 K=98N/m B/. What is the extension if 40N is put on the same spring? Force = Spring Constant x Extension 40 = 98 x s S = 40/98 = 0.41 m

  23. W F D Work done When any object is moved around work will need to be done on it to get it to move (obviously). We can work out the amount of work done in moving an object using the formula: Work done = Force x Distance Moved in J in N in m

  24. Work Done = Energy Converted Work Done raising an object = PE Stored

  25. h At the top of the oscillation the pendulum bob stops. All it’s energy is PE PE at top=KE at bottom PE = mgh At the bottom the bob has no PE only KE KE = ½ mv2

  26. Power • The rate at which work is done • POWER = Work Done time taken Example A jet takes 2mins to climb to 4000m. If the jet has mass 200tonnes find the work done and the power? Work Done = Force x Distance = 200x1000x9.81x4000 =7 x 109 Joules Power = Work Done / Time = 7 x 109 Joules / 120 = 6.54 x 107 Watts

  27. F Pressure (in N/m2) = Force (in N) Area (in m2) P A Pressure Pressure depends on two things: • How much force is applied, and • How big (or small) the area on which this force is applied is. Pressure can be calculated using the equation:

  28. The Barometer • The weight of the air holds up the mercury. • If we use water the column is 10.4m high. • 1 Atmosphere is 760mm of Hg.

  29. Volume scale Tube with volume of air trapped by oil Bicycle pump Reservoir of oil Valve Pressure gauge VERIFICATION OF BOYLE’S LAW 1..

  30. P 1/V Plot a graph of P against 1/V. A straight-line graph through the origin will verify that, for a fixed mass of gas at constant temperature, the pressure is inversely proportional to the volume, i.e. Boyle’s law.

  31. INVESTIGATION OF THE LAWS OF EQUILIBRIUM FOR A SET OF CO-PLANAR FORCES (2) Support Newton balance Newton balance Paperclips w4 w2 w1 w3

  32. ?N 25N 90 50 70 60 ? 10 • First law coplanar forces • Forces Up = Forces Down 25 + x = 15 + 5 +10 + 5 x = 10 N 15N 5N 5N 10N

  33. 10N 25N A 90 50 70 60 ? 10 • Second law coplanar forces • Take moments about A Clockwise Moments = Anticlockwise Moments 10x15 + 50x5 + 70x50 + 90x5 = 60x25 + dx10 15N 5N 5N 10N

  34. Angular Velocity =θ/t Units of Radians per second Angle  time A particle goes round a circle in 4s what is it’s angular velocity? Circular Motion t 

  35. Circular Motion • Linear Velocity(V) • m/s • V=  r • r=radius of motion • Always changing as direction is always changing this creates acceleration • If the radius is 6m

  36. Centripetal Acceleration a = r 2 Always towards the centre So the acceleration in the previous example a= 6(/2)2 =14.8m/s2

  37. Satellites balance forces • Balance of Gravity and Centripetal • ((GMm)/d2)=mv2/d Gravity F=-GmM/r2

  38. Equate The Forces T=Period (Time for Orbit) Cancel Mass of satellite V=Distance time Period of Orbit ((GMm)/d2)=mv2/d (GM)/d=v2 (GM)/d=(2d/T)2 T2=42 d3/GM

  39. Simple Harmonic Motion • Is a vibration where the acceleration is proportional to the displacement a  -s • Further from centre =more acceleration

  40. Hooke’s Law as SHM Force  Extension F  -s m.a  -s If mass is constant a  -s So motion under hookes law is SHM

  41. l Pendulum Split cork • If we displace the bob by a small angle it vibrates with SHM Timer Bob 20:30

  42. T2 l

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