Topic 9: Forces and their effects

Question 1

9.1how can objects interact without contact using example?

Answer 1

usually caused by interacting fields - e.g. gravitational attraction between objects (Earth & Sun) is caused by their gravitational fields interacting
interacting magnetic fields cause attraction or repulsion between magnetic objects
electrostatic force causing attraction and repulsion between electrical charges = due to interactions between electric fields

Question 2

9.1how can objects interact by contact

Answer 2

sometimes objects need touching for a force to act (push or pull)
e.g. normal contact force that acts between all touching objects
or
friction between a car tyre and the road

Question 3

9.1 how can objects interact, producing vectors

Answer 3

whenever 2 objects interact, both objects feel an equal but opposite force
= interaction pair
- represent using vectors (arrows)

Question 4

9.2 difference between vector and scalar quantities

Answer 4

vector = both size and direction
e.g. force, velocity, displacement
scalar = only size
e.g. speed, distance, mass

Question 5

9.3 equilibrium vector diagram

Answer 5

• if all forces acting on an object combine to give a resultant force of zero = object in equilibrium
• must be given forces acting on an object and told to find a MISSING force - given that the object is in equilibrium

Question 6

9.4 free body force diagrams

Answer 6

= shows an isolated body and all the forces acting on it
- includes every force acting on it but none of the forces it exerts on rest of world
- sizes of arrows show the relative magnitudes of the forces
- the directions show the directions of the forces

Question 7

what is a resultant force

Answer 7

the overall force on a point or object

Question 8

9.5 explain how multiple forces acting on an object can lead to a resultant force on an object

Answer 8

if you have a number of forces acting at a single point - can replace them with a single force = resultant force (or net force)
- if forces all act along same line - overall effect = by adding those in same direction and subtracting those in opposite direction

Question 9

objects in equilibrium

Answer 9

resultant force = 0
either stationary or moving at steady speed

Question 10

9.6 how can forces cause rotations

Answer 10

a larger force/longer distance = larger moment
- to get max moment, push at right angles to spanner
- other angle = smaller distance = smaller moment = ‘normal to the direction of the force’

Question 11

9.7 Moment of a force equation

Answer 11

Moment of a force (Nm) = force (N) x distance (m)

Question 12

9.8 what is the principle of moments for an object in equilibrium

Answer 12

sum of clockwise moments = sum of anticlockwise moments

Question 13

9.9 how do leavers transmit rotational effects of forces

Answer 13

transfer the turning effect of a force
- push one end of a lever down and the rotation around the pivot causes the other end to rise
- make it easier to do work - increase distance from pivot at which a force is applied : longer lever, smaller force needed to give same moment

Question 14

9.9 how do gears transmit rotational effects of forces

Answer 14

teeth on the gears interlock so that turning one causes another to turn in the opposite direction
- the larger the gear the slower it spins

• used to transmit a rotational effect of a force from one place to another

Question 15

9.10 explain ways of reducing unwanted energy transfer through lubrication

Answer 15

lubrication reduces friction and unwanted energy transfers

gears are often lubricated to improve the efficiency of machines