Understanding Resultant Forces and Vector Diagrams
When multiple forces act on an object, they can be replaced by a single force called the resultant force, which has the same effect. While many examples involve forces acting parallel to each other, this guide focuses on calculating the resultant when forces act at an angle.
Step-by-Step Guide to Using Vector Diagrams
1. Define the Scale
Choose a scale to represent the forces, for example, 1 cm equals 1 Newton.
2. Draw the Forces
- Draw a free body diagram showing the object as a point.
- Use a ruler to draw the first force vector to scale (e.g., 10 cm for 10 N).
- Use a protractor to measure and draw the angle between the forces (e.g., 30°).
- Draw the second force vector to scale at the correct angle (e.g., 8 cm for 8 N).
3. Construct the Parallelogram
- Copy the second force vector and place it at the head of the first vector.
- Copy the first force vector and place it at the head of the second vector.
- Draw lines to complete the parallelogram.
4. Draw the Resultant Vector
- Draw a vector from the tail of the first force to the opposite corner of the parallelogram.
- Measure the length of this vector to find the magnitude of the resultant force.
5. Calculate the Resultant Force
- Convert the measured length back to Newtons using the scale (e.g., 17.5 cm = 17.5 N).
- Use a protractor to measure the angle of the resultant force if needed.
Example Problem
Two forces of 10 N and 8 N act at a 30° angle. Using the method above, the resultant force is found to be 17.5 N.
Practice Problem
Calculate the resultant force for two forces of 280 N and 320 N acting at a 20° angle.
- Choose a scale (e.g., 1 cm = 40 N).
- Draw vectors to scale and construct the parallelogram.
- Measure the resultant vector length (e.g., 14.8 cm).
- Calculate the resultant force (14.8 cm × 40 N/cm = 592 N).
Tips for Exams
- Draw vector diagrams as accurately as possible.
- Examiners allow a small range of answers for the resultant force.
- Use a ruler and protractor for precision.
Additional Resources
For more practice, refer to the Vis Vision workbook linked in the video description, which contains numerous questions on vector diagrams and resultant forces. Additionally, you can explore the following resources for a deeper understanding of related concepts:
- Mastering Vector Addition: A Comprehensive Guide to Physics
- Understanding Vectors: A Guide to Motion in Physics
- Understanding Vectors and Scalars in Physics: A Comprehensive Guide
- Understanding Vector Direction with North, South, East, and West
- Solving Varying Angle Problems Using Sine and Cosine Laws
[Music] hi and welcome back to free science lessons.
co.uk by the end of this video you should be able to use Vector diagrams to calculate the resultant of two forces
acting at an angle and this is for higher students only in the last video we started looking at resultant forces
remember that when several forces act on an object they could be replaced by a single Force which has the same effect
and scientists call this single Force the resultant Force we saw several examples where we calculated the
resultant Force now in all of these examples the different forces are acting parallel to each other but what if the
forces are acting at an angle I'm showing you typical question here two forces are acting on an object
one force has a magnitude of 10 Newtons and the other Force has a magnitude of 8 Newtons the angle between the two forces
is ° calculate the resultant Force now this looks much more difficult than it actually is we start by making a scale
drawing showing the forces involved scientists call this a vector diagram we're going to say that 1 cm equals 1
Newton so here's the object and remember that this is a free body diagram so we just show a point we now use a ruler to
draw a 10 cm long Arrow to represent the 10 Newton Force using a protractor we measure an angle of 30° and here it is
and now we use a ruler to draw an 8 cm Arrow to represent the 8 Newton Force now we need to create a
parallelogram to do that we copy the 8 cm line and position it at the head of the 10 cm Force Vector we should also
use a protractor to double check that the angle is still 30° now we copy the 10 cm line and
position it at the head of the 8 cm Force vector and then we draw a line from the tals of
the force vectors to the other side of the parallelogram finally we measure the
length of the vector and work out the resultant force that this represents in this case the vector is 17.5 cm long so
this means that the resultant force is 17.5 Newtons we can also use a protractor to measure the Angles and
here they are now in the exam you need to draw your vector diagram as accurately as you
can however the examiners will accept a small range of possible answers for the resultant Force here's one for you to
try a force of 280 Newtons and a force of 320 Newtons are acting on an object there's an angle of 20° between the
forces calculate the resultant Force now to do this you'll need to decide on a scale for example 1 cm for 40 Newtons
you will then need to draw a vector diagram and work out the resultant Force so pause the video now and try this
yourself okay using a ruler we draw the two vectors to scale and again I've used 1 cm equal 40 Newtons and using a
protractor we make the angle between them 20° now we complete the
parallelogram and now we draw in the vector for the resultant Force this Vector has a length of 14.8
CM remember that 1 cm equals 40 Newtons so the resultant force is 592 Newtons and once again we can use a protractor
to measure the Angles and here they are we remember find plenty of questions on using Vector diagrams in my Vis
Vision workbook and you can get that by clicking on the link above [Music]
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