AQA A Level Physics复习笔记4.1.1 Scalars & Vectors

• A scalar is a quantity which only has a magnitude (size)
• A vector is a quantity which has both a magnitude and a direction
• For example, if a person goes on a hike in the woods to a location which is a couple of miles from their starting point
  • As the crow flies, their displacement will only be a few miles but the distance they walked will be much longer

   

Displacement is a vector while distance is a scalar quantity

• Distance is a scalar quantity because it describes how an object has travelled overall, but not the direction it has travelled in
• Displacement is a vector quantity because it describes how far an object is from where it started and in what direction
• Some common scalar and vector quantities are shown in the table below:

Scalars and Vectors Table

• Vectors are represented by an arrow
  • The arrowhead indicates the direction of the vector
  • The length of the arrow represents the magnitude

   

• Vectors can be combined by adding them to produce the resultant vector
  • The resultant vector is sometimes known as the ‘net’ vector (eg. the net force)

   

• There are two methods that can be used to add vectors
  • Calculation – if the vectors are perpendicular
  • Scale drawing – if the vectors are not perpendicular

   

Vector Calculation

• Vector calculations will be limited to two vectors at right angles
• This means the combined vectors produce a right-angled triangle and the magnitude (length) of the resultant vector is found using Pythagoras’ theorem

The magnitude of the resultant vector is found by using Pythagoras’ Theorem

• The direction of the resultant vector is found from the angle it makes with the horizontal or vertical
  • The question should imply which angle it is referring to (ie. Calculate the angle from the x-axis)

   

• Calculating the angle of this resultant vector from the horizontal or vertical can be done using trigonometry
  • Either the sine, cosine or tangent formula can be used depending on which vector magnitudes are calculated

   

The direction of vectors is found by using trigonometry

Scale Drawing

• When two vectors are not at right angles, the resultant vector can be calculated using a scale drawing
  • Step 1: Link the vectors head-to-tail if they aren’t already
  • Step 2: Draw the resultant vector using the triangle or parallelogram method
  • Step 3: Measure the length of the resultant vector using a ruler
  • Step 4: Measure the angle of the resultant vector (from North if it is a bearing) using a protractor

   

A scale drawing of two vector additions. The magnitude of resultant vector R is found using a rule and its direction is found using a protractor

• Note that with scale drawings, a scale may be given for the diagram such as 1 cm = 1 km since only limited lengths can be measured using a ruler
• The final answer is always converted back to the units needed in the diagram
  • Eg. For a scale of 1 cm = 2 km, a resultant vector with a length of 5 cm measured on your ruler is actually 10 km in the scenario

   

• There are two methods that can be used to combine vectors: the triangle method and the parallelogram method
• To combine vectors using the triangle method:
  • Step 1: link the vectors head-to-tail
  • Step 2: the resultant vector is formed by connecting the tail of the first vector to the head of the second vector

   

• To combine vectors using the parallelogram method:
  • Step 1: link the vectors tail-to-tail
  • Step 2: complete the resulting parallelogram
  • Step 3: the resultant vector is the diagonal of the parallelogram

   

Vector Addition

Vector Subtraction