chapter 4 - forces in action

Question 1

uniform

Answer 1

weight is the same at all parts if it’s dimensions are the same

Question 2

free body diagrams

Answer 2

each force is labelled as a vector arrow scaled to the magnitude and direction of the force they represent

Question 3

newtons 1st law

Answer 3

all objects will continue to be stationary or move at constant velocity unless they are acted on by a resultant force

Question 4

newtons 2nd law

Answer 4

rate of change or momentum is directly proportional to the resultant force and occurs in the direction of resultant force
p = mv
F = ma
W = mg

Question 5

newtons 3rd law

Answer 5

if a body exerts force in an object the object exerts and = and opposite force on the body

Question 6

mass

Answer 6

measure of its resistance to acceleration or deceleration

Question 7

weight

Answer 7

the gravitational force which acts in the object

Question 8

g varies due to

Answer 8

• non uniformities in the shape and composition of the planet
• the effect of earths rotation about its axis

Question 9

drag

Answer 9

opposes motion
size depends on
- speed of object
- shape of object
- viscosity of fluid

Question 10

motion of an object through a fluid

Answer 10

• released from rest - only force is W
• speed increases so drag increases
• F = W - drag and it decreases so acceleration decreases
• drag = W so F = 0 and acceleration = 0 (terminal velocity)

Question 11

object falling in a vacuum

Answer 11

• acceleration = g - no drag
• straight line

Question 12

max speed of a vehicle

Answer 12

depends on
- max forward force of the engine
- how quickly drag increases to = engine force (shape of vehicle)
- F = engine force - drag force

Question 13

triangle of forces

Answer 13

if an object is in equilibrium under three forces they can form a closed triangle

Question 14

centre of gravity

Answer 14

point at which weight acts

Question 15

finding CofG of irregular objects

Answer 15

• suspend the object from a pin through a hole at the edge of it
• hook a plumb line from the pin and mark the line
• change the orientation and mark the line again (repeat 3 times)
• the CofG is the where they intercept

Question 16

moment of a force

Answer 16

turning effect produced by a force
force * perpendicular distance from the force

Question 17

principle of moments

Answer 17

• if an object is in equilibrium the sum of clockwise moments = sum of anti-clockwise moments

Question 18

couple

Answer 18

• consists of 2 = and oppositely directed parallel, coplanar forces, who’s lines of action dont intersect
• produce a rotation
• can’t produce a resultant force so can’t produce linear motion

Question 19

torque due to a couple

Answer 19

moment of a couple
torque = force * perpendicular distance between the forces

Question 20

conditions for equilibrium

Answer 20

• no changes to motion - stationary or constant v
• resultant force = 0 so linear acceleration is 0
• resultant torque must be zero - so angular acceleration = 0

Question 21

density

Answer 21

mass per unit volume
m/ v
- depends in mass of each atom and the way they are packed together

Question 22

pressure

Answer 22

normal force per unit area on a surface
F / A
unit is Pa = N/m^2

Question 23

why does a box move down a slope

Answer 23

• slope increases
• angle between W and direction of motion decreases
• component of W acting down the slope increases and becomes bigger than friction so box starts to move

Question 24

why do heavier objects have a larger terminal velocity

Answer 24

• have a larger mass but same size - so larger W (W= mg)
• will take longer for drag to increase enough (increases with speed) to = W
• will accelerate for longer and reach a higher terminal velocity later

Question 25

man standing in a lift equation

Answer 25

M(a+ g) = F

Question 26

density practical

Answer 26

regular
- measure dimensions with a ruler
- measure mass with balance
liquid
- same but convert mL> cm^3
irregular
- eureka can displacement
- water displaced = volume

Question 27

pressure in liquids

Answer 27

pgh
pressure of a fish = pressure due to water + atmospheric pressure = pgh + 101kPa

Question 28

atmospheric pressure

Answer 28

101kPa
1.01*10^5 Pa

Question 29

archenemies principle

Answer 29

the upthrust exerted on an object is = to the W of fluid displaced

Question 30

upthrust

Answer 30

= weight of th efluid the body displaces
= Axpg
= Vpg
= mg
= to the force when suspended in air - the force when submerged

Question 31

fraction of a block submerged

Answer 31

amount submerged/ total = density of object / density of water

Question 32

density of water

Answer 32

1* 10^3 kgm^-3

Question 33

forces acting on a ball moving in the water

Answer 33

up - upthrust
down - drag and weight

Question 34

if an object is stationary in water

Answer 34

U = W
p(W)Vg = p(B)Vg
therefore have the same density

Question 35

if an object is resting on the surface of a liquid (forces)

Answer 35

upthrust and weight are =

Question 36

average mass of an adult

Answer 36

50- 150 kg

Question 37

one newton

Answer 37

the force that will give a mass of 1kg an acceleration of 1m/s^2

Question 38

drag

Answer 38

opposes motion of an object in a fluid

Question 39

drag is prob to

Answer 39

speed squared

Question 40

terminal velocity experiment