Dynamics

• The average speed of an object is the total distance travelled by an object divided by the time taken.
• The instantaneous speed of an object is its speed at one particular point during the journey
• $d=$$vt$Distance(m) = Speed(m/s) x Time(s)
• Measuring average speed

  • Measure distance (d) travelled using a measuring tape
  • Measure time (t) taken for the vehicle to travel the distance (d) using a stop watch
  • Use the equation v=d/t to calculate the average speed
• Measuring Instantaneous speed

  • Measure the length of the vehicle (or card attached to the vehicle) (d) using a measuring tape
  • Measure time taken (t) taken for the vehicle to pass a point using a light gate connected to an electronic timer
  • Use the equation $v=$$d/t$
• Vectors have both magnitude and direction and can be remembered using the pneumonic, MADFVW:

  Momentum

  Acceleration (a)

  Displacement (s)

  Force (F)

  Velocity (v)

  Weight (w)
• Scalars only have magnitude and include distance, speed, mass, and time.
• Vector Addition:

  • Ensure all vectors are drawn Tip2Tail
  • Draw a north line at the start point
  • Draw the resultant vector from the start to then end point and use Pythagoras' Theorem to calculate magnitude
  • All bearings are measured clockwise from the north line to the resultant vector and then use Tan to find out the angle at which the line goes and therefore the direction of the vector
• Acceleration is the change in speed every second and is measured in metres per second squared
• Acceleration Formula: $a=$$\frac{v-u}{t}$
• A velocity-time graph is a useful way of describing the motion of an object.
• VT Graph

  A) Constant Deceleration

  
• Forces are measured in Newtons (N) and can be measured with a newton balance
• Friction is a resistive force, which opposes the direction of motion of an object. This means that is acts in the opposite direction of the motion of the object.
• Friction acts between any two surfaces in contact
• Air frictions is called air resistance or drag, it depends on the shape and size of the object and the speed of the moving object
• Friction can be increased by choosing rougher surfaces which cause high friction or by increasing the surface area and/or choosing a shape to increase air friction
• Friction can be reduced by:

  • lubricating the surfaces with oil or grease
  • separating the surfaces with air
  • making the surfaces roll instead of slide
  • streamlining the shape of the object to reduce air friction
• Two forces which are equal in size but which act in opposite directions are called balanced forces.
• When the forces on an object are balanced, the object will remain at rest or travel at a constant velocity (or speed) in a straight line. This is Newtons first law.
• We can use a free body diagrams to analyse the forces on an object. This allows us to determine the motion of an object.
• When the forces acting on an object are not balanced, the velocity of the moving object can increase or decrease. This is Newtons second law and can be represented using the formula: $F =$$ma$
• Definition of Newton: The force which makes a 1kg mass accelerate at $1m/s^2$
• When several forces act on one object, they can be replaced by one force which has the same effect. This single force is called the resultant force.
• Free fall is the term used when an object is being acted upon only by the force of gravity (weight).
• As an object accelertaes downwards, air resistance will increase
• As the skydiver accelerates downwards, air resistance increases upwards until the value of air resistance = the skydiver’s weight. This results in the two forces having the same value acting in opposite directions. According to Newton’s 1st Law of Motion the skydiver will now travel at a constant velocity. This velocity is known as Terminal Velocity.
• Weight is a force caused by gravity acting on an object's mass.
• If an object A exerts a force (the action) on object B will exert an equal, but opposite force (the reaction) on object A. This is Newton's third law of motion
• Energy cannot be created or destroyed, but it can be changed from on form into another. All forms of energy are measured in the same unit: the joule (J).
• When energy is changed from one into another, energy is always conserved.
• Common energy transformations include:

  Light Bulb: electrical to light + (heat)
• When an object is moved from one place to another, energy has to be transformed.
• The amount of energy transferred (work done) will depend on the force applied to the box and the distance the box has been displaced from its original postion. This is calculated using the formula: Work done = Force x Distance
• Gravitational potential energy is the work done against gravity. The formula for gravitational potential energy is Ep=mgh
• Kinetic energy is the energy of an object has because it is moving. It is calculated using the formula: kinetic energy = 1/2 x mass x velocity squared.
• A projectile is an object which has been given a forward motion through the air, but which is also pulled downward by the force of gravity.
• A projectile has two separate motions at right angles to each other - horizontal and vertical speed
• The horizontal speed is at constant velocity since there are no forces acting horizontally if air resistance is ignored.
• The vertical motion is one of constant acceleration, equal to the value of g. For projectiles which are projected horizontally, the initial vertical velocity is zero. The vertical speed is calculated using $v=$$u+$$at$