forces

Question 1

centre of mass

Answer 1

the point through which the weight of an object acts

Question 2

tension

Answer 2

the force experience by a cable, rope or string when pulled, hung, rotated or supported

Question 3

when does elastic deformation occur

Answer 3

when objects return to their original shape when the stretching force is removed

Question 4

inelastic deformation

Answer 4

when objects remain stretched and don’t return competely to their orginal shape even when the stretching force is removed

Question 5

Hookes’Law

Answer 5

the extension of an elastic object is directly proportional to the force applied, up to the limit of proportionality

Question 6

limit of proportionality

Answer 6

the point beyond which the relationship between force and extension is no longer directly proportional

Question 7

moment

Answer 7

the truning effect of a force about a pivot
M = Fd

Question 8

principle of moments

Answer 8

if an object is balanced, the total clockwise moment about a pivot equals the total anticlockwise moment about that pivot

Question 9

how does pressure change as altitude increases

Answer 9

at sea level: more air molecules colliding with a surface creating atmospheric pressure
the number of air molecules above a surface decreases as the height of the surface above ground increases
so as height increases there is always less air above a surface than there is at a lower height.
so atmospheric pressure decreases with an increase in height

Question 10

speed of sound in air

Answer 10

330 m/s

Question 11

terminal velocity

Answer 11

at first the upwards air resistance is very small because the sky diver isn’t falling very quickly
as they speed up, the air resistance increases, eventually growing large enough to balance the downwards weight force
once air resistance equals weight, there is no longer any resultant force, so the sky diver’s acceleration is 0 and they now travel at a constant speed, and have reached terminal velocity

Question 12

newton’s first law

Answer 12

objects will remain at rest, or move at constant velocity unless acted on by a resultant force

if resultant force acting on the object is zero and the object is stationary, the object remains stationary

if the object is moving, the object continues to move at the same speed and same direction, so same velocity

so when a vehicle travels at a steady speed, the resistive forces balance the driving force

Question 13

2nd law

Answer 13

the acceleration of an object is proportional to the resultant force acting on it and inversely proportional to the object’s mass
F = ma

Question 14

3rd law

Answer 14

whenever two objects interact, the forces they exert on each other are equal and opposite

Question 15

inertia

Answer 15

the tendency of an object to continue in its state of rest, or in uniform motion unless acted upon by an external force

basically, an object’s resistance to a change in motion

inertial mass is a measure of how difficult it is to change the velocity of an object
inertial mass is defined as the ration of force over acceleration

Question 16

stopping distance definition

Answer 16

the total distance travelled during the time it takes for a car to stop in response to some emergency
=thinking (the distance the vehicle travels during the driver’s reaction time)+braking distance

Question 17

reaction time

Answer 17

a measure of how much time passes between seeing something and reacting to it

Question 18

increase thinking distance/reation time

Answer 18

tiredness
distractions
intoxication

Question 19

factors affecting braking distance

Answer 19

friction between the brakes and the wheels
speed of the vehicle
weight of car
road—weather, wet/icy conditions

Question 20

how to estimate decelerating forces

Answer 20

braking force x braking distance = 0.5 x mass x velocity^2
same as:
work done by the brakes = kinetic energy of the car

Question 21

conservation of momentum

Answer 21

in a closed system, the total momentum before an event is equal to the total momentum after the event

the total momentum before a collision= the total momentum after a collision

Question 22

force and momentum

Answer 22

when a force acts on an object that is moveing, or able to move, a change in momentum occurs

use F = ma and a = (v-u)/t
force = change in momentum over time

Question 23

how to crashmats/seatbelts work

Answer 23

increase time of collision
so increase time for change in momentum
which decreases force of collision

Question 24

examples of contact forces

Answer 24

forces where the objects are physically touching

friction
air resistance
tension
normal contact force

Question 25

non contact forces

Answer 25

objects are physically separated gravitational force electrostatic force magnetic force

Question 26

how may a shape of an object be changed

Answer 26

stretching bending compressing

Question 27

why must more than one force be applied to change the shape of a stationary object

Answer 27

becuase a single force would simple cause the object to move in the direction in which the force is applied

Question 28

pressure in a fluid

Answer 28

fluid = liquid OR gas the pressure in fluids causes a force at right angles (normal) to any surface

Question 29

why in a liquid pressure at a point increases with the height of the column of liquid above that point and with the density of the liquid

Answer 29

increasing depth increases the height of the water column above which increases the weight of the water acting on the bottom

Question 30

upthrust

Answer 30

a partially or totally submerged object experiences a greater pressure on the bottom surface than on the top surface which creates a resultant force upwards this force is called upthrust

Question 31

what is the atmosphere

Answer 31

a thin layer of air round the earth the atmosphere gets less dense with increasing altitude

Question 32

typical values of speed for walking, running, cyclingq

Answer 32

walking = 1.5 m/s running = 3 m/s cycling = 6m/s

Question 33

how does an object fall through a fluid

Answer 33

initially accelerates due to the force of gravity eventually the resultant force will be zero and the object will move at its terminal velocity

Question 34

typical reaction time and how can it be affected

Answer 34

0.2 to 0.9 s tiredness drugs alcohol distractions

Question 35

factors affecting braking distance: car deceleration

Answer 35

when a force is applied to the brakes of a vehicle, work done by the friction force between the brakes and the wheel reduces the kinetic energy of the vehicle and the temp of the brakes increases the greater the speed of a vehicle the greater the braking force needed to stop the vehicle in a certain distance the greater the braking force the greater the deceleration of the vehicle large decelerations may lead to brakes overheating/loss of control