BHS PHYSICAL SCIENCE

Newtons First Law A body will................................ of ...... or ................. .................... in a straight line, unless acted on by a ......................................" This law describes - ................IMPORTANT POINTS: (A moving object)

Continues in its state

Rest or uniform motion in a straight line

Unless acted upon by (external) forces Keith Warnewww.TeachBomb.com1Newtons Laws Second LawA ..............................on an object causes it to ...................in the direction of that force. The magnitude of the acceleration will be directly proportional to the ...................and inversely proportional to the ..............of the object.Fres = F + fandFres = m x a

Fma = ?Fm=1kgT = ?FgFres = Fg + TFres = m x aAndFg = mgThese are the two most basic scenarios - any of the variables can be calculated if all the others are known.a=2m.s-2InertiaA body has a natural tendency to resist any changes to its state of motion.This resistance is known as Inertia.Examples: What will happen if the card in the picture is flicked? Explain why.The moon moves around the sun in a circular orbit. Explain why.Every object in a state of uniform motion tends to remain in that state of motion unless an external (unbalanced)force is applied to it. GravityMotionEarth1.2.MotionInertiaFrictionInertiaA body has a ............................................ any changes to its state of motion.This resistance is known as ........................If the card in the picture is flicked ................. ........................................ Inertia keeps the peg stationary when the card is moved quickly. The pegs Inertia is overcomes .................. ................ which try to keep its position on the card.The moon was moving past the earth in a straight line but became ................. by the ............................ Gravity does not act against the direction of motion (90o) so the motion continues because ............................................... to ..................... (The question is who threw it in the first place!)Every object in a state of uniform motion tends to remain in that state of motion unless an external (unbalanced)force is applied to it. GravityMotionEarth1.2.MotionInertiaFrictionThe Normal or Reaction ForceThis is the Reaction Force (Newton ..) exerted by a surface on any object that rests (exerts a force on) that surface.Always EQUAL in magnitude and to the initial force and at 900 to the SURFACE.FN = .Normal Reaction Force (N)Weight (Fg)Normal Reaction Force ()N = ..Calculate the normal force in each example here if the ball has a 2 kg mass.FRICTIONFriction forces exists between two surfaces in contact.

There are two types FRICTION and (moving) FRICTION.

Static friction is . than dynamic friction.

The maximum force exerted on a stationary object ..it begins to move (accelerate) is equal to the maximum static frictional force.

The force required to . an object moving with velocity on a surface is equal to the dynamic friction.Max forceStatic friction V = 0Forcedynamic friction constant velocity = = Coefficient of FRICTIONSTATIC FRICTION and Can vary from zero to a MAX IMUM of Fs = s........s = coefficient of .......... Friction

DYNAMIC (moving) FRICTION.Always constant value given by Fd = dNd = coefficient of .......... friction

....... forceStatic friction V = ....Constant ForceDynamic friction constant velocityN = -FgN = -FgFf NFA Fgs = = gradient FN (N)..... .....s = = gradient Ff (N)..... .....DRY surfaceWET surface........................Graphs of Frictional force against normal force.0 1 2 30 1 2 3Ff (N).............................The force of friction Ff is directly proportional to the Normal force (weight)The .................of these graphs would be EQUAL to the coefficients of friction.FN (N)Friction Examples5kgFAA block is held against a vertical surface by a horizontal force.What is the coefficient of friction if the minimum applied force is 20 N?Friction ExamplesObject on an inclined plane.2 kgWhat is the maximum angle that the slope can have before the block begins to slide if the coefficient of friction is 0.8?2 kgAExam QuestionThe diagram shows a truck with a crate immediately behind the cab in position A.The truck suddenly accelerates forward and the crate slides towards the back of the truck and comes to rest in position (B)Draw a force diagram, indication and labeling the horizontal force(s) acting on the crate while the truck accelerates.(2)Briefly explain why the crate slides towards the back of the truck.(4)Name and state the law or principle that you have applied in order to reach an answer in (b).(4)WCED Nov 2001 TOTAL [10] ABExam QuestionThe diagram shows a truck with a crate immediately behind the cab in position A.The truck suddenly accelerates forward and the crate slides towards the back of the truck and comes to rest in position (B)Draw a force diagram, indication and labeling the horizontal force(s) acting on the crate while the truck accelerates.(2)

Friction between the crate and truckNewtons 3rd Law"For every action there is an and reaction.

The gasses experience a force out of the rocket, this has an equal but opposite . which drives the rocket forward!The force exerted by the hand on the head is the same . as the force exerted by the head on the hand! The head hits the hand just as hard as the hand hits the head!13An apple on a table. Non contact forces GravityExerted by the on the appleReaction by the .. on the earth.Contact forces: TableApple exerts a force on the table.Table exerts a . or . force on the apple.All these forces are . so there is no force.Forces on apple: gravity (down) & reaction (up)Walking & PushingDraw sketches to show and explain all the reaction pairs of forces that occur when a person is walkingA person is pushing a car.Person WalkingThe person pushes . on the ground.The ground pushes .. on the person.The person has an . force so moves in the direction of that force.(The earthmovementForces on personPushing a carThe person pushes backwards on the ground.The ground pushes forwards on the person.The person pushes forward on the car.The car pushes back on the person.The forces on the car are less than the forces on the ground.The person has an unbalanced force so moves in the direction of that force.MovementForces on personNewtons 3rd

BOB (who isn't very bright) is tired of pushing his van to the petrol station. He gets an idea -- "Hey! I go much faster when I am roller-blading than when I am walking. Why don't I wear my roller-blades!" So BOB gets his roller blades from the back of his van, and starts pushing. Will this work? Explain using Newton's laws what will happen. (Calculate the acceleration and displacement each will experience. BOB weighs 50 kg, and the van weighs 2,000 kg.)

Newtons 3rd

ANS: Since every action has an equal and opposite reaction, when BOB pushes the van, the van pushes BOB with the same size force but opposite direction. Assume he pushes for one second. BOB weighs 50 kg, and the van weighs 2,000 kg. Taking right as positive calculate the acceleration of both Bob and the van as well as the distance moved by both:Pairs of forces

2.Pairs of forcesAction-reaction pairsMan against ground - ground against man greater for . than for .Man on rope rope on man all .. but less than ... ground forces and greater than .. ground forces.A and B both experience a .1.ABInclined PlaneThe system shown is in equilibrium. What is the magnitude and direction of the friction force acting on the block?50kg25oPulley systemThe system shown is in equilibrium. What is the magnitude and direction of the friction force acting on the block?10 kg250 N30oFnett = F// + FA + Ff 0 = (-250 sin 30) + (98) + FfDown = ( - )Eg - Hanging WeightA weight of 30 N hangs on a rope fron a ceiling. A horizontal force pulls it to the side. The angle between the rope and ceiling is 600.Determine: the tension in the rope (T) and the horizontal force.30N60oForce Diagram30NTFgTF60oKeith Warnewww.TeachBomb.comNewtons - Connected objectsIn both these cases the tension gets as more blocks are connected I.e. T1 .. T2 . T3The systems can be treated as a .. :F = ..WhereF = .orF = ..So..T1T2T3m1m2m3fm3m2m1T3T2T1FgTo find the tension between blocks you must consider them . I.e. .25Newtons Laws - Pulley systemsThe accelerations of these systems can be easily found by considering them as a .: Fres = wheremtotal = And Fres = .. (always opposed)To find tension; .. Individual masses or tensions can be found by considering masses individually:...Tension in any one string is the same throughout!m2m1TTFg1Fg2m1m2m3T1T2Fg1Fg2Examples Newton 1 & 21. A 25kg container is acted upon by a horizontal force of 65N. The frictional force between the container and the floor is 15N. Calculate the acceleration of the container.(a = 2m.s-2)Examples Newton 1 & 2A ball with a mass of 18kg is suspended by a string.Calculate the tension in the string if the ball is stationery, accelerates upwards at 3m.s-2; accelerates downwards at 3m.s-2.T = 176.4N upwardsT = 230.4 upwardsT = 122.4N upwardsExamples Newton 1 & 2Two blocks rest on a frictionless horizontal surface connected by a string. A force of 80N is exerted on the 7kg block. Calculate:the acceleration of the blocksthe force exerted by the string on the 13 kg block.13 kg7kg80 NExamples Newton 1 & 24.A constant force of 120N is used to lift two masses of 4 kg and 6kg which are attached to each other with string. Ignore any mass of the string and calculate the acceleration of the system and the tension in the string.4kg6kg120 NExamples Newton 1 & 2 &35.The frictional force on the system is 10 N (6 N & 4 N). Calculate the force that the 4kg block exerts on the 6kg block.4kg6kg a = 2m.s-2(Ff=10N) (6N of 6kg block 4N on 4kg block)F