Topic 9 - forces and their effects

examples of interactions objects at a distance without contact

• gravitational attraction

  • there is an attraction between two objects with mass, the larger mass gives greater attraction

• electrostatic attraction/repulsion

  • a large charge gives greater force

  • like charges repel, opposite charges attract

• magnetic attraction/repulsion

  • a stronger magnet gives stronger field, having a greater force

  • like poles repel, opposite poles attract

examples of interaction of objects with contact

• normal contact force

  • the force is perpendicular to the place of contact

• friction

  • surfaces that are rough cause friction when moved

what is a vector

• a vector has size and direction - eg. a force of 10N directed downwards

  • weight, velocity, force, displacement etc

what is scalar

• scalar has just size - so direction plays no part in describing the value

  • mass, distance, speed etc

vector diagrams

free body diagrams

• shows the direction of forces that are present in a situation

points to note:

• the reaction force always acts at the normal to the line of contact, from the point of contace

• friction acts in the opposite direction to movement, along line of contact

• weight always acts downwards, acting from Centre of Mass

scale drawings

• the length of each arrow represents its size (in relation to the other forces acting on the object)

  • so direction with larger arrows shows the resultant force

• if arrows are in opposite directions with equal length

  • (equal in magnitude but opposite in direction)

  • the forces cancel out

  • so the object is in equilibrium

  • so travels at a constant velocity

diagram:

• at B, the drag is a lot less than the weight, as shown by the arrows, so the resultant force causes him to accelerate

• at C, the difference in arrow lengths is less, so the resultant force is smaller, so smaller acceleration

what is an isolated solid system

isolated solid systems means no forces are present that come from a source outside the system

• eg. a magnetic ball just rolling down a hill, an external force would be a magnet at the top of the hill

explain forces when skydiving

• forces that act are air resistance and weight

• initially, the skydiver has no air resistance and the only force acting on him is weight

• as he falls, he accelerates, increasing his speed

• this makes air resistance increase

• therefore, the resultant force decreases

• therefore, acceleration decreases as F=ma, so he is not speeding up as quickly

• eventually weight and air resistance are equal and balanced, so there is no resultant force

• so there is no acceleration and terminal velocity is reached

explain forces on a vehicle

• initially, low air resistance and thrust is only hindered by friction

• air resistance increases, decreasing resultant force

• eventually the car is travelling at terminal velocity, where the thrust is balanced by drag and friction, so no resultant force acts

  • (this is immediately changed when more thrust is added, as it now becomes the resultant force until the drag increases to balance it again)

describe situations where forces can cause rotation

rotation occurs….

• if an object is attached to a pivot point

  • a point which it can rotate about, but it cannot move away from

• and a force is applied not towards the point (see diagram)

  • the object will not rotate, and will just be held still, as there is no resultant force

• if the force is applied perpendicular to the object

  • it will move about the pivot in this direction

• if the force applied not perpendicular to object

  • need to find perpendicular distance from pivot to line of force

  • see which direction it will turn

moment of a force (newton metre, N m) = force (newton, N) x distance perpendicular to the direction of the force (metre, m)

bike riding - pressing your foot down on the pedal, causes a moment about the pivot, turning the pedal arm

equilibrium occurs when: sum of anticlockwise moments = sum of clockwise moments

explain how levers and gears transmit the rotational effects of forces

• gears can change speed force or direction by rotation

for an example when the first gear is supplying the force

• if connected to a gear with fewer teeth (ie. smaller gear)

  • the second gear will turn faster

  • but with less force

  • in an opposite direction to the first gear

• if connected to a gear with more teeth (ie. larger gear)

  • turns slower

  • more force

  • in opposite direction

the second gear will always turn in the opposite direction

• the blue gear is supplying the power

• to increase the power, a larger gear is used for the secondary (red)

  • as the force on the red gear is a further distance from its pivot, the momentum of the larger gear is greater

way to reduce unwanted energy transfer

lubrication - reduces friction, so reduces unwanted energy transfer (less heat loss etc) and increases efficiency