Knowledge-7 FEM2

• Newtons first law states that the velocity, speed and/or direction of an object only changes if a resultant force acts upon it.
• inertia of an object is its tendency to remain in a steady state at rest or moving in a straight line at a constant speed.
• if the resultant force is:

  • zero on a stationary object, it will remain stationary.
  • zero on a moving object, it will continue moving at the same velocity.
  • not zero on an object, its velocity will change.
• A change in velocity can mean:

  • starting to move.
  • stopping move
  • speeding up
  • slowing down
  • changing direction
  • if an object is doing any of these things, the forces on it will be unbalanced and a resulting force will be acting on it.
• Newtons second law states that the acceleration of an object is:

  • proportional to the resultant force on an object
  • inversely proportional to the mass of the object
• In situations where the mass of an object is not changing , the resultant force, mass and acceleration are linked by the equation:

  • resultant force = mass x acceleration
• acceleration is always in the same direction as the resultant force.
• F=ma can be used to show why all objects fall at the same rate if there is no air resistance.
• To show if there is no air resistance:

  • resultant force F acting on a falling object is its weight W, so W=F.
  • Since weight W =mg and resultant force F=ma , mg=ma.
  • m cancels out because gravitational mass and inertia are the same leaving g=a.
• Newtons third law states that whenever two objects interact , they exert equal and opposite forces on each other. this means forces always occur in pairs.
• A pair of forces will always:

  • act on separate objects.
  • be the same size
  • be in opposite directions
  • act along the same line
  • be the same type
• In an elastic collision , kinetic energy is conserved- the objects involved have the same kinetic energy after the collision than before.
• in an inelastic collision, kinetic energy is not conserved so the objects involved have less kinetic energy after the collision than before.
• Momentum is a property of all moving objects. It is a vector quantity with the unit kgms to the -1. Momentum depends on the mass and velocity of an object and is defined by the equation.
• Conservation of momentum:

  The principle of conservation of momentum states that, for a system of interacting objects, the total momentum remains constant if external forces act on a the system.
• if two moving objects collide, this can be written as:

  • m1 u1 + m2 u2 =m1 v1 + m2 v2
  • where:
  • m1 = mass of object 1
  • m2 = mass of object 2
  • u1 = initial velocity of object 1
  • u2 = initial velocity of object 2
  • V1 = final velocity of object 1
  • v2 = final velocity of object 2
• Since F=ma and a= change in velocity/ time you can write:

  • f = mass x change in velocity / time
• Force is equal to the rate of change of momentum of the object; this another way of stating newtons second law.
• The impulse of a force is the force multiplied by the time for which the force:

  • impulse = F x change in time = change in momentum
  • impulse = change of momentum
• For a ball rebounding from a wall:

  • change in momentum = (-mv)-(mu)
• The area of a force-time graph represents the impulse of the force.
• The greater the time taken for the change in momentum of an object:

  • the smaller its change in momentum
  • the smaller the force it experiences
• Ethical transport design: the increasing in time taken for the change in momentum in order to reduce the force of an impact. examples are:

  • air bags, seat belts and crumple zones in cars
  • cycle helmets
  • crash mats
• Work done and energy transferred mean the same thing , the unit of work done is joule which is equal to N m.
• The amount of energy transferred is:

  • work done = force applied x distance moved along the line of action
• The area under a force-displacement graph represents the total work done.
• Power is the rate of energy transfer or the rate of doing work:

  • power = energy transferred or work done/ time
  • p =e/t
  • power is also equal to force x velocity.
• efficiency is a measure how much energy is wasted in an energy transfer.
• the gravitational potential energy store of an object increases when it moves up and decreases when it moves down.
• the principle of conservation of energy states that energy cannot be created or destroyed , only transferred between different stores and objects.
• The total energy amount of energy in a system energy in a closed system is always the same.