P4 Explaining Motion_1

P4 Explaining Motion

FORCES
1. Forces occur when two objects interact
  1. An interaction pair of partner forces are when an object exerts a force on another object, it always has a force in return: If you push against a wall, the wall will push back with exactly the same force.
  2. Forces can still cause things to move even if they are balanced, because they act of different objects.. The forces on the gas and the forces on the rocket are an interaction pair
  3. An object resting on a surface has a reaction force. If you put a book on a table, the book pushes down on the table with a force equal to it's weight and the table exerts an equal and opposite force upwards on the book. The upward force is a reaction force.
2. Friction
  1. When an object is moving against another, both experience friction, which is a reaction force. It happens because of an applied force.
  2. Friction between solid surfaces which are gripping. This is a kind of friction that lets you walk around. The friction between your shoes and the ground allow you to push against it and move forwards. If there was no friction, you would slip
    1. Friction between solid surfaces which a sliding past each other. For example, the moving stuff in a car engine. You can reduce sliding and gripping friction through the use of lubricants like oil between the surfaces.
      1. Resistance or drag from fluids (liquids or gases). Drag is an object moving through a fluid that has to force its way past all the molecules in that fluid which causes friction. For example, a van driving would be an object moving through the air. There is no drag in space because in space, there is no fluid.
3. Arrows show the size and direction of forces
  1. The length of the arrow shows the size of a force and the direction fo the arrow shows the direction of the force.
  2. The reaction of a surface - BALANCED FORCES. Steady speed - BALANCED FORCES
  3. Resultant force - overall force acting on an object. Add up all the individual forces and their directions. This forces decides the motion of the object (accelerate, decelerate or stay at a steady speed). If there is a resulting force acting upon an object, its speed or direction (or both) changes.
    1. Accelerate just means change velocity. Velocity has both speed and direction. If a car is turning a corner, it is changing velocity (accelerating), not necessarily speeding up.
MOMENTUM (kg/s)
1. Acceleration - UNBALANCED FORCES. If a car's engine exerts a bigger driving force than the drag counter force, the car will accelerate. The forward arrow will therefore be bigger.
  1. A rocket taking off accelerates away from the ground, so the upward force (thrust) must be greater than the downward forces that are slowing it down. There are two downward forces: gravity and drag. If the thrust stopped, that the downward forces would be greater than the upward forces and the rocket would slow down until it stopped and then accelerate downward.
    1. The same thing happens when an object is tossed in the air then it comes crashing down again. Their motion is all about the relative sizes of the upward and downward forces.
2. MOMENTUM = MASS X VELOCITY
  1. EXAMPLE: A 90kg donkey is moving in a straight line at 10 m/s. Calculate it's momentum
    1. Momentum = mass x velocity
      1. 900 kg m/s = 90 x 10
3. Momentum is how hard it'd be to stop an object from moving. The heavier the object is, and the faster it is moving, the harder it is to stop.
4. It is a vector quantity because it has size and direction like velocity, but not speed. A resultant force of zero means that a stationary object will remain still. If the object was moving, it stays at constant velocity (same speed in the same direction) and constant momentum. If the resultant force on an object is not zero, its momentum changes in the direction of force
5. When a resultant force acts on an object, it causes a change in momentum in the direction of the force..
  1. EXAMPLE: A rock with mass 1kg is travelling through space at 15 m/s. A coment hits giving it a resultant force of 2000 N for 0.7 seconds. Calculate the rocks inital momentum and then the change in its momentum resulting from the impact with the comet.
    1. Initial momentum: mass x velocity = 1 x 15 = 15 kg m/s
      1. Change of momentum: force x time = 2500 x 0.7 = 1750 kg m/s
  2. The greater the time for a change in momentum, the smaller the force. If your momentum changes slowly, like in a controlled braking in a car, the forces acting on your body are small and your unlikely to be hurt
    1. The average force on an object can be lowered by slowing the object down over a longer time. You can't affect the change in momentum in a car collision so safety features in a car increase the collision time
      1. CRUMPLE ZONES crumple on impact, increasing the time taken for the car to stop
      2. AIR BAGS also slow you down more gradually
      3. SEAT BELTS stretch slightly, increasing the time taken for the wearer to stop. This reduces the forces acting on the chest
      4. CYCLE AND MOTORCYCLE HELMETS provide padding that increases the time taken for your head to come to a stop if it hits something hard
WORK
1. When a force moves an object it does work and energy is transferred to the object.
2. Whenever something moves, something else is providing some sort of effort to move it. The thing putting the effort in, needs a supply of energy. It then does work by moving the object - and one way or another, it transfers the energy it receives into other forms. If energy is transferred then the object doing the work loses energy.
3. Amount of energy transferred (J) = Work Done (J)
4. Work done by a force (J) = Force (N) x Distance moved in direction of force (m)
  1. This formula only works if the force is in exactly the same direction as the movement
  2. EXAMPLE: Kids drag a tire 5m over flat ground. They pull with a total force of 340 N. Find the work done.
    1. W = F x D = 340 x 5 = 1700 J
ENERGY
1. Kinetic energy
  1. Kinetic energy is energy of movement (K.E.). The only way to increase kinetic energy is to apply a force to it (work). If you do work on an object but it doesn't accelerate, then you haven't increaed its kinetic energy.
    1. Increase in K.E = Work done
  2. Kinetic energy = 1/2 x mass x velocity²
    1. EXAMPLE: A car of mass 1450 kg is travelling at 28 m/s. Calculate its kinetic enrgy.
      1. 1/2 x 1450 x 28² = 568 400 J
  3. When work is done on an object, energy is transferred to that object, which is probably going to make it start moving or move faster. You can't create or destroy energy, it just gets transformed from one type to another. You would expect that kinetic energy = work done but when it gets transferred, some of the energy gets wasted so the increase in an objects K.E. is normally less than the amount of work done on it because some energy is wasted as heat.
    1. But if there is no air resistance or friction acting on the object (vacuum) or you were told to ignore it, then the increase in an objects kinetic energy is equal to the amount of work done on it.
2. Gravitational Potential Energy
  1. Gravitational Potential energy is height energy (G.P.E). It is the energy stored in an object when you raise it to a height against the force of gravity. When you lift an object, its G.P.E increases as it's raised and vise versa. You increase G.P.E by doing work.
    1. Increase in G.P.E = work done
  2. Change in G.P.E. (J) = Weight (N) x Vertical height difference (m)
    1. EXAMPLE: A 6000 N lamb is tossed up 10 m. Calculate its change in G.P.E
      1. 6000 x 10 = 60 000 J (or 60 kJ)
  3. Falling objects convert G.P.E into K.E. meaning the further is falls, the faster it goes. K.E. gained = G.P.E lost
SPEED AND DISTANCE
1. Speed (m/s) = Distance travelled (m) / Time taken (s)
  1. EXAMPLE: A cat walks 20m in 40 seconds. Work out it's speed and how long it'll take it to travel an extra 75 m.
    1. Speed: 20/40 = 0.5 m/s
      1. T = d/s = 75/0.5 = 150 s = 2 minutes 30 seconds
2. One direction is positive displacement whereas the opposite direction is negative. If an object moves in one direction and then back again, the distance travelled will be greater than the displacement.
  1. EXAMPLE: A girl is playing with a yoyo, it drops from her hand 0.8 cm down, She tries to pull it back up but it ends up dangling 0.2 below her hand. What distance did the yoyo travel and what is its displacement
    1. Distance: 0.8 + 0.6 = 1.4 m
      1. Displacement: +0.2m. It is positive as the yoyo started off moving downwards and has ended up lower from where it started
3. Distance Graphs
  1. Horizontal = STOPPED Shallow = SLOW Steep = FAST
    1. Getting steeper = Increasing Speed Getting shallower = Decreasing Speed Straight line = CONSTANT SPEED
4. Displacement Graphs
  1. Sloping down = BACK TOWARDS starting point Sloping up = AWAY FROM start point
    1. If the line is sloping down and it is getting steeper, the speed is increasing. Getting shallower means the speed is decreasing.
      1. Displacement graphs show velocity (speed with direction). If two objects are heading in opposite directions, you can say one had positive velocity and the other had negative velocity. Acceleration is how quickly your speeding up. Deceleration is negative acceleration.
5. Acceleration (m/s²) = Change in Velocity (m/s) / Time taken (s)
  1. EXAMPLE: A boy accelerates steadily from 2 m/s to 6 m/s in 5.6 s. Find it's accleration
    1. (6 - 2) / 5.6 = 0.7 m/s²
6. Velocity Graphs
  1. Gradient = Acceleration Flat lines = Constant Speed
    1. Sloping down = BACK TOWARDS start point (Negative velocity) Sloping up = AWAY FROM start point (Positive velocity)
7. Working out from graphs
  1. Distance Time graphs: To calculate the speed, use the graph to find the numbers for the numbers for the speed formula.
    1. Displacement Time graph: To calculate the velocity, use the graph to find the numbers for the velocity formula

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