p2 - motion & forces

• What is a scalar quantity?

  Has a magnitude (size)

  But no direction
• What is a vector quantity?

  Has a magnitude (size)

  And specific direction
• Difference between vector & scalar quantities?

  Vector quantities have a specific direction, scalar quantities don't
• Examples of vector quantities

  velocity , displacement , acceleration , force / weight , momentum
• Examples of scalar quantities

  distance , speed , mass , energy, temperature
• What is velocity?

  Velocity is speed in a stated direction
• average speed (m/s) = distance (m) / time (s)
• distance travelled (m) = average speed (m/s) x time (s)
• acceleration (m/s2) = change in velocity (m/s) / time taken (s)
• a = (v-u) / t

  v = final velocity

  u = initial velocity
• working out velocity formula

  $v^2\ -\ u^2\ =$$\ 2ax$

  v = final velocity

  u = initial velocity

  a = acceleration

  x = distance
• what is acceleration

  acceleration is the change in velocity per second
• Typical speed of walking

  1.5 m/s
• Typical speed of running


  3 m/s
• Typical speed of cycling

  6 m/s
• Typical speed of driving

  14 m/s
• Typical speed of aeroplane

  250 m/s
• Typical speed of train

  50 m/s
• Average speed in non-uniform motion

  average speed = total distance / total time
• Explain how motion of an object in circular path can occur at uniform speed but changing velocity

  Speed is a scalar quantity, the object travels at constant speed as it covers an equal distance per second.

  However the direction is constantly changing as it moves in a circular motion.

  So velocity must be changing as it is a vector quantity with both size and direction
• What is "centripetal force" ?

  The resultant perpendicular force towards centre of circle required to keep the object in constant circular motion

  (Always perpendicular to direction of travel)
• The velocity of an object which is accelerating it is increasing with time
• The velocity of an object which is decelerating is decreasing with time
• VT graphs

  -slope tells you acceleration of vehicle

  -horizontal line means constant velocity
• Calculating distance travelled by an object using VT graph

  Calculate the area under the graph

  (split into triangles / rectangles)
• Speed of sound in air

  330 m/s
• Describe how to determine speed of moving trolley

  Measure distance between two points that trolley moves past and divide it by time taken to travel between two points

  Or using light gates to measure time (known length of card breaks light beam)
• What is meant by a resultant force?

  Total amount of forces acting on an object
• Calculating resultant forces

  Forces in opposite directions are subtracted from each other

  Forces in the same direction are added together

  If forces are equal in size = no resultant force = balanced

  Which ever side the force is greater on is the side the force is directed to

  (eg 2N to the left)
• What is the difference between the mass (kg) and weight (N) of an object?

  Mass measures the amount of matter and is not affected by size of gravitational field. Scalar quantity

  Weight is the force experienced due to its mass and size of gravitational field. Vector quantity
• Factors which affect gravitational force exerted between two objects with mass

  Mass and distance
• Equation for weight of an object

  Weight (N) = mass (kg) x gravitational field strength (N/kg)

  W=MG
• Newton's first law

  An object continues in constant speed, or remains at rest, unless a resultant force acts on it

  (As long as the forces acting on it are balanced)
• Newtons first law example

  The forces on the car are balanced, the car continues to move at a constant speed unless an external force is applied
• Newton's second law

  F = M x A

  Force (N) = mass (kg) x acceleration (m/s2)

  Force acting on object is directly proportional to acceleration of object. So the greater the force, the greater the acceleration

  Mass of an object is inversely proportional to acceleration

  The greater the mass, the smaller the acceleration
• What is meant by 'inertial mass' of an object?

  Inertial mass is a measure of how difficult it is to change the velocity of a moving object.

  Defined as force over acceleration
• Force and acceleration CP

  Set up apparatus

  Set up light gates to take the velocity and time readings

  Record velocity and time for different values of mass on trolley

  Work out acceleration by dividing change in velocity by time taken for card to pass between gates

  Slope/ gradient need to remain the same
• Newton's third law

  When two objects interact , they produce equal but opposite forces

  (eg book exerts force onto table , table exerts equal but opposite force back onto book)
• What happens to an object that is falling through a fluid?

  Accelerates due to gravity
• Equation for momentum of object

  P = m x v

  Momentum (kg m/s) = mass (kg) x velocity (m/s)

  Momentum is a vector quantity