P5 Calculating Forces (page 204) Flashcards
Why are Scale drawing useful?
they can help you resolve forces or work out the resultant force.
How do you use scale drawing to find Resultant Forces?
1) Draw all the forces acting on an object, to scale, ‘tip-to-tail’.
2) Then draw a straight line from the start of the first force to the end of the last force - this is the resultant force.
3) Measure the length of the resultant force on the diagram to find the magnitude and the angle to find the direction of the force.
A man on an electric cicycle that has a driving force of 4 N north. However, the wind produces a force of 3 N east. Find the magnitude and direction of the resultant force?
1) Start by drawing a scale drawing of the forces acting
2) Make sure you choose a sensible scale (e.g. 1cm - 1 N).
3) Draw the resultant from the tail of the first arrow to the tip of the last arrow.
4) Measure the length of the resultant with a ruler and use the scale to find the force in N
5) Use a protractor to measure the direction as a bearing
(a bearing is an angle measured clockwise from north, given as a 3 digit number, e.g. 10° = 010°
See diagram 1 of drawing results on page 204.
An Object is in Equilibrium if the forces are what?
balanced. (See diagram 2 on page 204).
If all of the forces acting on an object combine to give a resultant force of zero, the object is in equilibrium.
On a scale diagram, this means that the tip of the last force you draw should end where the tail of the first force you drew begins. e.g. for three forces, the scale diagram will form a triangle) see diagram 3 on page 204
You might be given forces acting on an object and told to find a missing force, given that the object is in equilibrium, how would you do this?
you draw out the forces you do know (to scale and tip-to-tail), join the end of the last force to the start of the first force. This line is the missing force so you can measure its size and direction.
(make sure you draw the last force in the right direction. It’s in the opposite direction to how you’d draw a resultant force).
You can split a force into what?
components
not all forces act horizontally or vertically - some act at awkward angles.
How can you split awkward angles into components?
they can be split into two components at right angles to each other (usually horizontal and vertical).
Acting together, these componenets have the same effect as the single force.
How can you resolve a force?
split it into components, by drawing it on a scale grid. Draw the force to scale, and then add the horizontal and vertical components along the grid lines. Then you can just measure them.
An object in equilibrium is being acted on by three forces. The first force is 0.50 N acting south and the second force is 0.30 N acting on a bearing of 045°
Find the magnitude and bearing of the third force. (3 marks)
Draw the given forces to scale and tip-to-tail. The third force is found by joining the end of the second force to the start of the first force.
1 mark for the correct scale drawing with a sensible scale. 1 mark for a magnitude between 0.35 and 0.37 N, 1 mark for a bearing between 323 and 325°
see answer diagram on page 247, Q1 p.204
