P4, P5, P6

physics units 4, 5, 6

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P4 explaining motion - speed

·         Speed is the distance travelled in a certain time.

·         To find the speed of an object you need to know the distance travelled and the time it takes.

·         Speed is rarely constant, you accelerate and decelerate.

·         So speed is normally average speed, even instantaneous speed is average speed, but over a really short period of time.

                                                            Distance

                                                        Speed x Time

 

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P4 Speed and Velocity - Distance-time graphs

  • gradient = speed
  • flat sections = no speed
  • steeper graph = faster
  • ‘downhill’ means coming back to starting point
  • curves show acceleration or deceleration
  • steepening curve = speeding up
  • levelling out curve = slowing down

Speed is the gradient of a graph, so you divide the vertical by the horizontal (so travel 300m in 20s, 300/20 = 15 m/s)

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P4 - differences between speed and velocity

 Distances can be positive or negative; it just means that they are travelling in opposite directions.

 

·         Speed is just a number, velocity has a direction too.

·         velocity can be more useful than speed.

·         Scalar quantities – Mass, speed, time length ect

·         Vector quantities – Force, velocity, acceleration, momentum ect

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P4 Velocity

·         Velocity can be positive or negative which shows direction.

·         Velocity time graphs –

o    Flat = steady speed

o    Straight increasing = Acceleration or deceleration

o   A curve shows increasing or decreasing acceleration.

·         Tachographs plot speed against time so you can see how far someone travels. This is used in lorries so they don’t drive too far without a break. They are those circle things.

 

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P4 Forces and friction

·         Forces occur when two forces interact (interaction forces).

·         If you push against something it pushes equally hard back.

·         So you push down on a table with 10N of force, it pushed back with 10N or it breaks, and dies, and sets on fire.

·         Moving objects experience friction;

o   Friction between two gripping forces which stops them from moving.

o   Friction between two sliding surfaces.

Resistance from liquid or gas. You push the molecules out the way causing friction, which is why you get friction, think aerodynamics

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P4 Forces and motion

·         Arrows show the size and direction of force, a larger force = longer arrow.

·         If you have one force bigger than another the object moves, if they are equal then it doesn’t move.

·         A resultant force shows the speed and direction the object will go.

·         Acceleration – unbalanced forces.

·         If the thrust on a rocket is greater than the gravity and drag, it’ll fly up and speed up till they are equal, at which point it’ll go at a constant speed.

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P4 Forces and motion

·         In the triangle,

o   p = momentum

o   m = mass

o   v = velocity

 

Momentum (P)

Mass (m) x Velocity (V)

 

·         Momentum is how hard an object is to stop; heavier and faster objects have a larger momentum.

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P4 Forces and Motion

 

   Change in momentum = resultant force (N) x Time for which force acts (s)

  

When a resultant force acts on an object you get a change in momentum.

·         If a large force acts on you it hurts, so car safety features increase the time it takes to slow down, reducing forces.

o   Crumple zones crumple on impact increasing time to stop.

o   Helmet padding increases the time for your head to stop.

o   Air bags slow you down more gradually.

o   Seat belts stretch and slow you down slowly.

 

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P4 Work

 When a force moves an object, energy is transferred and work is done.

Work done = energy transferred

When you put ‘effort’ into moving something, you need an energy supply. The energy from your energy supply that you use up is work done.

 

·         Change in energy (J) = Work done (J)

·         In the triangle:

o   W = Work done (J)

o   F = Force (N)

o   Distance = (M)

·         Work done (J) = Force (N) x Distance (M)

 

 It only works in the force is in exactly the same direction as the movement.

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KInetic Energy

Kinetic Energy:

 

·         Kinetic energy is movement.

·         Anything moving has kinetic energy, the greater the mass and speed the bigger the kinetic energy (J).

 

    Kinetic energy = ½ x mass x velocity²

 

·         To increase something KE you need to increase its speed. The only way to do this is to apply a force.

·         Increase in KE = Work done (Just about)

·         Work done = energy transferred, but energy is wasted in heat, so like ‘the increase in an objects kinetic energy is normally a bit less than the amount of work done on it.

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Comments

Rababah Amjad

this is only P4 when it is labeld P4, P5 & P6

Rababah Amjad

"So you push down on a table with 10N of force, it pushed back with 10N or it breaks, and dies, and sets on fire." scientific fact?

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