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Vectors and scalars
- Displacement is distance in a certain direction
- Vectors are physical quantities that have magnitude and direction, e.g. acceleration, force,
- Scalars are physical quantities that have magnitude but no direction, e.g. mass, temp
- The size of a quantity is its magnitude
Forces between objects
- Forces (N) can change the shape of an object, or change its motion or its state of rest
- A force is a push/pull that acts on an object because of its interaction w/ another object
- Contact forces are forces that must touch to interact, e.g friction, air resistance, tension. They occur when an object's supported by or strikes another object
- Non-contact forces are forces that don't need to touch to affect each other, e.g. magnetic force, electrostatic forces, force of gravity
- Newton's 3rd law states when 2 objects interact w/ each other, they exert equal and opposite forces on each other
Resultant forces
- The resultant force is a single force that has the same effect as all the forces acting on an object
- Newton's first law of motion states that an object at rest will remain at rest unless acted on by an unbalanced force and an object in motion will continue in motion w/ the same speed and direction, unless acted on by an unbalanced force.
- If the Rf isn't 0, the movement of the object depends on the size and direction of the Rf
- A free-body force diagram shows forces acting on an object, w/out any other objects drawn. The forces are represented by arrows pointing in the direction of the force
Moments at work
- The moment of a force is a measure of the turning effect of the force on an object.
- The turning effect can be increased by:
- increasing the size of the force
- using a spanner w/ a longer handle
Levers
- A crowbar is a lever used to raise one edge of a heavy object. The weight is the load, the force the person applies is the effort, the point the crowbar turns is the pivot. The crowbar enables only a fraction of the effort needed to life the object. The lever's a force multiplier, as the effort moves a bigger load
- moment, M (N/m) = force, F (N) x distance from pivot, d (m)
- T investigate the turning effect of a force, move a weight along a newton metre and record the distance, increasing the weight each time and use equation to calculate moment
- When a lever's used to increase a force, the force applied to the lever must act further from the pivot than the force it has to overcome, e.g. bottle opener
Gears
- Gears multiply the effect of a turning force. E.g. gears increase the turning effect of the engine, so a bigger force is exerted on the gears
- To increase the moment of a turning effect, a small gear wheel needs to drive a larger gear wheel
- Low gear gives low speed and a high turning effect - small gear wheel driven by engine shaft is used to turn a large gear wheel on the output shaft, so output shaft turns slower than engine shaft
- High gear gives high speed and a low turning effect - larger gear wheel driven by engine shaft is used to turn a small gear wheel on output shaft, so output shaft turns faster than engine shaft, so car moves at higher speed
Centre of mass
- The centre of mass of an object is the point at which its mass can be thought of as being concentrated
- After suspending and releasing an object, it will come to rest w/ its centre of mass directly below the point of suspension, the object's in equilibrium. Its weight doesn't exert a turning effect on the object as its centre of mass is directly below the point of suspension. An object is freely suspended if it returns to its equilibrium position after the turning force is taken away
- If an object has more that one axis of symmetry, its centre of mass is where the axes meet
- If object is irregular, put hole in one corner of card and suspend from rod. Use plumbline (string w/ small weight on end) to draw vertical line on card from rod. Repeat, hanging the card from different corners until you see centre of mass
These concepts may be difficult to understand, but it's vital in I/GCSE physics exam! Practice makes perfect!👨🏫
Moments and equilibrium
- If seesaw is balanced etc. , sum of all anticlockwise moments about any pivot= sum of clockwise moments about a pivot
- On a seesaw, the heavier person must sit nearer to the pivot and the lighter one must sit further from pivot, to balance it
The parallelogram of forces
- Use weights and pulleys to show parallelogram of forces. The parallelogram of forces is used to find the Rf of 2 forces that don't act along the same line. You'll need a protractor, ruler, sharp pencil and paper. The Rf is the diagonal of the parallelogram that starts at the origin of the 2 forces.
In I/GCSE Physics exam, it is best for you to draw the diagram, as it is very helpful in your calculations! 👩🏫
Resolution of forces
- Resolving a force means finding perpendicular components that have Rf that's equal to the force
- To resolve a force in 2 perpendicular directions, draw a rectangle w/ adjacent sides along the 2 directions so that the diagonal represents the force vector
- For an object to be in equilibrium:
- the Rf must be 0
- the forces acting on the object must have no overall turning effect
- To see if an object's in equilibrium:
- If the lines of action of the forces are parallel, the sum of the forces in 1 direction must be equal to the sum of forces in the opposite direction. So Rf = 0
- If the lines of action of the forces aren't parallel, the forces can be resolved into 2 components along the same perpendicular lines. Components must balance out if Rf = 0
Reference:
https://www.slideshare.net/transweb/resultant-force-mechanical-engineering
This is the end of the topic! Well done!
Drafted by Cherry (Chemistry)