Physics-Forces and Motion

  • Created by: Jasmin3
  • Created on: 01-04-18 09:25
Why are forces a vector quantity?
They have magnitude and direction.
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What is the difference between scalar and vector?
Scalar has no direction, vector has direction.
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Name 2 vector and 2 scalar quantities.
Vector: Velocity, Acceleration, Force. Scalar: Speed, Distance, Temperature.
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What are the two different catergories of forces?
Contact and Non-Contact
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What is a contact force? Give 2 examples.
When two objects have to be touching for a force to act. A push or pull force.
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What is a non-contact force? Give 2 examples.
When two objects do not need to be touching for a force to act. Magnetic or Gravitational force.
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What is an interaction pair?
A pair of forces that are equal and opposite and act on two interacting objects. (Newton's Third Law)
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A tennis ball is dropped from a height. Name one contact and one non-contact force that act on the ball as it falls.
Contact: Air Resistance. Non-contact: Gravity
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What is gravtitatonal force?
A force of attraction between masses.
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What are gravitatonal forces main effects?
On the surface area of the planet it makes everything attract to the ground. It gives everything a mass.
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What is mass?
The amount of 'stuff' in an object. For any given object it will have the same mass anywhere in the world.
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What is weight?
Weight is a force which is acting on an object by the gravitational field.
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What can vary gravitational field strength? Why?
Location- It's stronger the closer you are to the mass causing it.
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Compare a 1kg sack of potatoes to whether it's on earth or on the moon, in terms of mass, weight and gravitational field strength.
Mass- Stays the same, 1kg, wherever. Weight- changes (less on moon) because gravitational feild strength is less on moon.
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What is weight measured in? What is mass measure in?
Weight- Newtons (N) Mass- Kilograms (Kg)
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What can you measure weight with?
A spring balance known as a Newtonmeter
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What is the centre of mass?
A force acting from a single point on the object, where the whole mass is concentrated.
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What is the relationship between Mass and Weight? What does this mean?
They are directional proportional. If you double mass, weight doubles too, etc..
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What is the equation to work out weight?
Weight (N) = Mass (Kg) x Gravitational Field Strength (N/Kg) W=mg
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Calculate the weight of a 5kg mass: a) on earth (g= 9.8 N/kg) b) On the moon (g= 1.6 N/Kg)
a) 49 N b) 8 N
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What do free body diagrams show?
Describes all forces acting on an object, in what direction and by how much force.
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What do the arrows show on a free body diagram?
The size shows the relative magnitude (how much force) and the direction shows the direction the force is acting.
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What is a resultant force?
The overall force acting on an object.
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How do you find a resultant force?
If all forces are in the same direction add them. If they are in opposite direction, take them away.
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Fill in the blanks. When a force moves an object through a distance ______ is transferred and ____ is ____ on the object.
1) Energy 2) Work 3) Done
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Why and when is 'work done'?
A force does 'work' to move an object. Doing this energy is transferred. Whether the energy is 'useful' or 'wasted' work is still done.
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What is the equation for work done?
Work Done (J) = Force (N) x Distance (M) W=Fs
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How many joules is a Newtonmeter?
1 J = 1 Nm
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A force of 20N pushes an object 20cm. Calculate work done on the object.
20 x 0.2 = 4. 4 Joules
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When is equilibrium reached?
What the resultant force of the object is 0.
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Draw and work out the following. A motor provides a 12N driving force to the North. The river's current causes of force of 5N West on the boat. Calculate the resultant force.
Draw 12cm line up. 5cm line left. Line between two end points in cm is answer. 13N.
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In terms of Elasticity, when do you need more than one force?
To stretch, compress or bend an object.
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What is the term when an object can go back to its original state after being stetched?
Elastically deformed - means it will be an elastic object.
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What is the term when an object can't go back to its original state after being stretched?
Inelastically deformed
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Work is done when strecthing a spring. What energy store is this transferrred to?
Elastic potential energy store.
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When talking about elasticity, what is extension directly proportional to?
Force
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What is the equation to do with a spring being stretched?
Force (N) = Spring Constant (N/m) x Extension (m) F=ke
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What happens to the spring constant if a spring is really stiff?
It has a greater spring constant.
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What is the limit of proportionality?
Th epoint where it is no longer proportional (extension to force).
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How is the limit off proportionality shown on a graph?
It will curve until the limit of proportionality and then start to plato (become a straight line) after this.
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A spring has a force of 1N. This causes it to stretch 2 cm. Calculate the spring constant.
1N/0.02 m = 50. 50 N/m
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What is a 'moment'? Give an exmaple.
The turning effect of a force. The force on a spanner causes a turning effect or moment.
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A larger force or a longer distance would mean a ______ moment.
Larger
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How would you get the maxiumum moment?
Push the turning effect at a right angle. Any other angle is a smaller moment.
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When won't the object turn? When is it balanced?
When th total anticlockwise moment equals the total clockwise moment.
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What is the equation for moment of a force?
Moment (Nm) = Force (N) x Distance (m) M=Fd
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How do levers have an effect on the moment?
Levers increase the distance from the force. This means less force is needed to get the same moment.
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What are gears used for?
To transmit the rotational effect of a force from one place to another.
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A larger gear will turn ______ thean a smaller gear.
Slower.
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Your brother weighs 330N and sits 2m from one end of a seesaw. You weight 600N, what distance from the end should you sit to balance it?
300N x 2m = 600 N/m. 600/600 =1. 1m
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What is pressure?
Force per unit area.
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In fluid pressure, what do fluid particles do to cause pressure?
They have a mass and exert a force on the objects they collide with.
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What is the equaton for fluid pressure?
Pressure (pascals(pa)) = Force normal to a surface (N) / Area of that surface (m2) P= F/A
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What two things depend on pressure in a liquid? Define them.
Density- a measure of compactness. Depth- How deep the liquid goes.
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How does density have an effect of pressure in liquid?
The more dense the liquid is the more particles it has in a given area. Meaning more particles collide. The pressure is higher.
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How does depth have an effect on pressure?
As the depth of the liquid increases, the number of particles above that point increases. The weight of these add to the pressure, so the pressure increases with depth.
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Calculate the force exerted on a 10 m squared area by a pressure of 200 kPa.
200,000 pa x 10 m = 2,000,000 N
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What happens to object completley submerged in water in terms of pressure and force?
Force is exerted on all sides but the pressure on the bottom of the object is more than on top because pressure increases with depth.
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What happens to the upthrust of an object completley submerged into water?
The upthrust is equal to the weight of the fluid that has been displaced. The resultant force is upthrust.
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When will an object float?
When its weight = to upthrust.
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When will an object sink?
If the objects weight is more thna the upthrust.
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If the ojects weight is _____ to upthrust it floats. If it sinks, it displaces a volume of water _____ to its weight.
1) Equal 2) Equal
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Explain why a wooden object (700 kg/m3) floats in water (1000 kg/m3)
Because the mass of the wood is less than the mass of the water so therefore it displaces enough water for an upthrust force to enable it to float.
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Whats the difference between distance and displacement?
Distance is scalar, its just how far something has moved. Displacement is Vector, it measure distance and direction.
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What is displacement?
Measures in a straight line, from start to finish.
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If you walk 5m North, then 5m south what is your dispalcement compared to your distance?
Displacement is 0m. Distance is 10m.
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What's the difference between speed and velocity?
Speed is just how fast you're going (scalar), velocity is speed in a given direction (vector).
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Compare an objects speed to velocity if it is moving in a circle?
The direction is always changing so it has a constantly changing velocity. However it has a constant speed.
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What is the equation for distance?
Distance travelled (m) = speed (m/s) x time (s) d=st
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What is the equation for distance and velocity?
Distance (m) = Final Velocity (m/s) x time (s) s=vt
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On average, name the typical speed of a) someone walking b) someone driving c) someone cycling
a) 1.5 m/s b) 25 m/s c) 6 m/s
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A sprinter runs 200m in 25s. Calculate his speed.
8 m/s
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Marie takes a route of 1500m that starts and returns to her house. What is her distance? What is her dislacement?
Distance = 1500m. Displacement = 0.
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What is acceleration?
Acceleration is the change in velocity of a certain amount of time.
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What is the equation for acceleration?
Acceleration (m/s) = Change in velocity (m/s) / time (s) a= v/t
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What is constant acceleration (uniform accelertaion)?
Acceleration due to gravity is uniform for objects to free fall.
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What is the average acceleration of free fall?
9.8 m/s
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What is the equation for constant acceleration (final velocity)?
Final velocity squared (m/s) - Intial Velocity squared (m/s) = (2 x acceleration (m/s2) x distance (m) ) v2- u2 = 2as
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A ball is dropped from a height. The speed of the ball just before the ground is 7m/s. Acceleration = gravity speed. Calculate the height.
u=0. v=7 a=g=9.8. 49-0 =49. (2x 9.8 =19.6) 49/ 19.6 =2.5 m
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What does the following show on distance/time graphs? 1) Steep Gradient 2) Flat Line 3) Straight uphill 4) Curves 5) Steepening/Levelling off
1) The steeper the faster its going 2) Stationary 3) Steady rate of speed 4) Acceleration or deceleration 5) Speeding up/Slowing down
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What do the following show on a velocity/time graph? 1) Gradient 2) Flat sections 3) Steep lines 4) Uphill/Downhill 5) Curves 6) Area under graph line
1) Acceleration 2) Steady Speed 3) greater acceleration/deceleration 4) Acceleration/ Deceleration 5) Changing acceleration 6) Equal to distance travelled in that time
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If an object has no force what will it do? Why?
Stop moving because friction acts in the pposite direction to movement.
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In terms of friction, what is needed for a steady speed?
Driving force must balance friction force.
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What is 'drag' force? Give an example.
Resistance you get in fluid. Air Resistance. Friction.
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Why reducing 'drag' force was does it improve?
More streamlined and allows fluid to flow much more easily.
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As speed increases, friction increases. What is this called?
Directly Proportional.
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What is terminal velocity?
When speed and friction have increased together so much they are now equal. Resultant force is now 0.
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Explain why a ball falling from the top of a tall building reaches terminal velocity?
As the ball falls, it goes to earth because of gravity (1). Air resistance acts on the ball in the opposite direction (1). As the speed of the ball increases so does air resistance until it cant speed up anymore (1). It has reached terminal velocity.
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Explain Newton's First Law.
If the resultant force is 0, the object will remain stationary. If the resultant force on a moving object is 0, it will keep moving at the same velocity.
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The velocity will only change if there is a non-zero reultant force. What is this? What is this like on a free-body diagram?
Something that will always produce acceleration. On afree body diagram the arrows will be unequal.
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In Newton's Second Law, what is acceleration proportional and inversely proportional to?
Proportional to Resultant Force. Inversely Proportional to Mass. More mass, less acceleration.
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What is the equation for resultant force?
Resultant Force (N) =Mass (kg) x Acceleration (m/s2) F=ma
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Find the force needed for an 80kg man on a 10kg bike to accelerate 0.25 m/s2.
90kg x 0.25 m/s2 = 22.5N
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What is Inertia?
The tendency for motion to stay the same. Either at rest or a constant velocity.
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What is inertia mass? How do you find it?
Intertia mass measures how hard it is to change velocity. It is the m in the F=ma formula.
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What is Newton's Third Law?
When two objects interact, the force they exert on each other is equal and opposite.
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Give an example of Newton's Third Law.
If you push a shoppong trolley, it will push back with the exact same force. Although it still moves because the mass is different. The shopping trolley weighs less than you do.
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Explain why you don't move when you lean on a wall. (Newton's Third Law).
Both the wall and man exert a force on each other (normal contact force) (1). You also exert a force on the ground and the ground on you (1). The resultant force is 0, you are stationary (1).
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What is stopping distance?
An emergency stop by a driver where maximum force is applied on the brakes to stop the car in the shortest possible distance.
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What is the equation for stopping distance?
Stopping Distance (m) = Thinking Distance + Braking Distance (m)
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What is thinking distance? What are 2 factors that affect this?
How far the car travels during the drivers reaction time. Factors are: Speed your travelling at and reaction times.
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What is braking distance? What are 4 factors of this?
Braking distance is distance to stop under braking force. Factors are: Speed, Weather or Road Surface, Tyre conditions, Brake conditions, etc...
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How does friction affect braking?
Friction is between the brake pad and the wheel which causes work to be done. Work done transfers the energy from the kinetic stores of the wheels to thermal energy of the brakes. The brakes increase temperature.
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How does speed affect braking distance compared to thinking distance?
Speed affects braking distance more because it increases at a quicker rate than thinking distance the more you speed up.
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His thinking and braking distance is 6m. Estimate his total stoppping distance if he was travelling 3x as quickly.
Thinking Distance = 6x3 =18 (increases linearly) (1). Braking distance increase 3 squared as much. (1) 3 squared x 6= 54m (1). 54+18=72m (1).
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What is the average reaction time? What factors can affect this?
0.2s-0.9s. Tiredness, alcohol, drugs, distractions etc...
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What is the equation for momentum?
Momentum (kg m/s) = mass (kg) x velocity (m/s) p=mv
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Is momentum a scalar or vector quantity?
Vector
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What is conservation of momentum? Why does this happen?
It means the total momentum before and event is the same as after. Its because it is a closed system.
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Give an example of onservation of momentum.
Snooker balls.
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If the momentum before is 0 what will the momentum be after?
0.
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Describe how momentum is conserved by a gun recoiling as it shoots bullets.
Before the gun fires, the total momentum is 0. (1) When the bullet leaves the gun, momentum is in one direction (1). The gun moves back so it has momentum in the opposite direction (1).This means total momentum after is also 0. (1)
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Calculate the momentum of a 60kg women running at 3m/s.
60 x 3 = 180 kg m/s
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Force causes a change in momentum. What is the equation for this?
Force (N) = Change in momentum (kg m/s)/ Change in time (s) F= m triangle v/triangle t
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In terms of momentum, the force causing the change is equal to ___________
the rate of change in momentum
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If somones momentum changes very quickly, the force will be equally as large. In a car crash, why is this a bad thing?
People are more likely to injury because there is more force on the body.
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