Forces By Neil Bronks

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

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

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

Velocity m/s T/s 40x20=800 Total Distance Traveled = =1700m 0.5x10x40= x10x20= x20x60 =600

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

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

Vector Addition Speed in still air 120m/s Wind 50m/s Resultant R 2 = = = R = 130m/s  Tan  = 50/120  =

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

Momentum 2kg 3kg 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 = V = 5.333m/s 5m/s V=? m/s 3 m/s 2kg 3kg

VERIFICATION OF THE PRINCIPLE OF CONSERVATION OF MOMENTUM Velcro pad Dual timer Photogate Air track t1t1 t2t2 Light beam Card l Vehicle 1 Vehicle 2

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

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

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 and Force=m.a

TO SHOW THAT a  F Card l t1t1 t2t2 s Dual timer Light beam Photogate Pulley Air track Slotted weights

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/s 2 ) F AM

Acceleration gives Net Force F eng =5000N a=3m/s 2 Friction=? 900kg As net force causes acceleration F=m.a F net = 900kg. 3m/s 2 F net = 2700N So Friction = F eng – 2700 = 2300N

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

Floatation A floating body displaces its own weight in water.

MEASUREMENT OF g h Switch Electromagnet Ball bearing Trapdoor Electronic timer h=½gt 2

Newton's Law of Gravitation This force is always positiveThis force is always positive Called an inverse square lawCalled an inverse square law F  m 1 m 2 d 2 Where F = Gravitational Force m 1. m 2 = Product of masses d = Distance between their center of gravity

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

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 W DF

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

PE = mgh KE = ½ mv 2 At the top of the oscillation the pendulum bob stops. All it’s energy is PE At the bottom the bob has no PE only KE PE at top=KE at bottom h

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 10 9 Joules Power = Work Done / Time = 7 x 10 9 Joules / 120 = 6.54 x 10 7 Watts

Pressure Pressure depends on two things: 1)How much force is applied, and 2)How big (or small) the area on which this force is applied is. Pressure can be calculated using the equation: Pressure (in N/m 2 ) = Force (in N) Area (in m 2 ) F AP

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.

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

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. P 1/V

INVESTIGATION OF THE LAWS OF EQUILIBRIUM FOR A SET OF CO-PLANAR FORCES (2) Newton balance Support Paperclips w1w1 w2w2 w3w3 w4w4

First law coplanar forces Forces Up = Forces Down 25 + x = x = 10 N ? 15N 25N 5N 10N 5N ?N

Second law coplanar forces Take moments about A Clockwise Moments = Anticlockwise Moments 10x x5 + 70x x5 = 60x25 + dx ? 15N 25N 5N 10N 5N 10N A

Circular Motion 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? tt

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

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

Satellites balance forces Balance of Gravity and Centripetal ((GMm)/d 2 )=mv 2 /d Gravity F=-GmM/r 2

Period of Orbit ((GMm)/d 2 )=mv 2 /d (GM)/d=v 2 (GM)/d=(2  d/T) 2 T 2 =4  2 d 3 /GM Equate The Forces V=Distance time T=Period (Time for Orbit) Cancel Mass of satellite

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

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

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

l T2T2