Scalars & Vectors (AQA A Level Physics)
Revision Note
Author
Katie MExpertise
Physics
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 far 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
Exam Tip
Do you have trouble figuring out if a quantity is a vector or a scalar? Just think - can this quantity have a minus sign? For example - can you have negative energy? No. Can you have negative displacement? Yes!
Combining Vectors
- 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
Worked example
A hiker walks a distance of 6 km due east and 10 km due north. Calculate the magnitude of their displacement and its direction from the horizontal
Exam Tip
Pythagoras' Theorem and trigonometry are consistently used in vector addition, so make sure you're fully confident with the maths here!
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