Mechanics

Question 1

Newtons 1st Law

Answer 1

A body will remain at rest or continue at uniform velocity, unless acted upon by a resultant force

Question 2

Newtons 2nd Law

Answer 2

The acceleration experienced by a body is parallel and directly proportional to the resultant force acting upon it

F=ma

F = (mu-mv)/t

Question 3

Newtons 3rd Law

Answer 3

When two bodies interact, they exert forces upon each other which are equal in magnitude, but opposite in direction

Question 4

Resultant force

Answer 4

Only required for acceleration

Any body moving at a constant velocity does not have a resultant force acting

Question 5

What is Mass

Answer 5

A measure of a body’s inertia - its resistance to a change in velocity

Question 6

What do bodies with more mass have

Answer 6

Greater inertia

Require more force to change their velocity

Question 7

What is Weight

Answer 7

Force of attraction that acts on a body - owing to the body having mass and being in a gravitational field

W=mg

Question 8

What is the Centre of Mass and where does it lie

Answer 8

A single point at which the body’s mass can be thought to be concentrated

Lays where all lines of symmetry cross

Question 9

CoM practical

Answer 9

Suspend the body from a position near its edge

Allow it to come to rest

Draw a line vertically down - using plumb line to guide

Suspend the body from another point

Allow it to come to rest - vertical line straight down

CoM lays where lines cross

When body is in rest - it’s in equilibrium - weight must act through CoM - gives rise to a zero moment

Question 10

Free fall

Answer 10

Motion of a body through a drag producing medium without a drag producing device

Resultant force accelerating body (the weight) stays the same - accelerates uniformly - use dynamic equations

Question 11

Forces acting when a body falls through the air on earth

Calculate resultant force

Answer 11

Weight and drag

F=ma

W-D=ma

a=(W-D)/m = mg/m - D/m

a = g - D/m

Question 12

Free fall assumption

Answer 12

Drag can be ignored as drag force up isn’t same order of magnitude as weight force

Times it can’t be ignored:

Body’s mass is very small in relation to surface area

Body is falling at a considerable velocity

Question 13

Free fall ignoring drag working out

Answer 13

a = (W-R) / m
a = W/m

W = mg

a = mg/m

a=g

Question 14

Driving force acting on a body of a certain shape in a given fluid

Answer 14

Body accelerates
Velocity increases
Drag force increases
Resultant force decreases
Acceleration decreases

Eventually drag force = driving force

Zero resultant force - zero acceleration - constant maximum speed attained - Terminal speed

Question 15

Terminal speed

Answer 15

Maximum speed attained by a body during free fall, when it’s weight is balanced by the drag force acting upon it

Question 16

Principle of moments

Answer 16

For a system in equilibrium

Components in every plane are balanced

Sum of clockwise moments = sum of anti-clockwise moments

Question 17

Support force

Answer 17

Acts where a body is in contact with a surface

Bodies balanced horizontally on a pivot - support force always acts upwards

Bodies in equilibrium - support force up must equal all forces acting down

Supports force acts at the pivot in a single support - has no moment

Two pivots = two support forces - moments about one pivot means you must take into account the moment of the support force from the second pivot - value of support force at each pivot is determined by where CoM lies

Question 18

Only way to get system in rotation without translational motion

Answer 18

Apply a pair of forces about the pivot point - acting in the same plane but opposing directions

Question 19

How to get system in rotation with translational motion

Answer 19

Single force about a pivot point - unbalanced force in horizontal plane

Question 20

Couple

Answer 20

Pair of forces which give rise to a resultant moment but no resultant force

Moment of a couple = torque

Question 21

Triangle of forces

Answer 21

Triangle that represents magnitudes and directions of three forces whose resultant is zero

Question 22

Density

Answer 22

Mass per unit volume

Question 23

Measure mass

Answer 23

Scales

Question 24

Measure volume

Answer 24

Liquid - measuring cylinder

Regular shaped solids - sides using ruler/vernier callipers/micrometer - calculate volume

Irregular shapes solids - displacement can

Question 25

Pressure

Answer 25

Normal force exerted per unit area F / A

Question 26

Pressure in a liquid

Answer 26

At any given depth below the surface of a liquid - pressure is determined by weight of the water above that must be supported Greater depth - more weight of water - greater pressure Same depth - greater density - greater weight of water - greater pressure

Question 27

Pressure in a liquid at a given depth formula

Answer 27

Pressure - density x g x depth

Question 28

Archimedes Principle

Answer 28

Upthrust exerted on a body is equal to the weight of the liquid displaced by the body Upthrust = density x g x submerged depth x surface area

Question 29

Upthrust

Answer 29

Occurs whenever a body is immersed within a fluid Results from the pressure difference that exists between the top and bottom of the fluid

Question 30

What is instantaneous speed

Answer 30

Speed of an object over a very short interval of time

Question 31

How to find instantaneous speed at a specific time

Answer 31

Draw a tangent at the gradient

Question 32

What is acceleration when the gradient of a velocity of a time graph is constant and positive

Answer 32

Constant Acceleration

Question 33

What is acceleration when the gradient of a velocity of a time graph is constant and negative

Answer 33

Constant deceleration

Question 34

What is acceleration when the gradient of a velocity of a time graph is zero

Answer 34

Zero acceleration

Question 35

What is acceleration when the gradient of a velocity of a time graph is a curve

Answer 35

Acceleration is changing

Question 36

What is the area under a velocity-time graph

Answer 36

Displacement

Question 37

What is the area under a velocity-time graph

Answer 37

Displacement

Question 38

What is thinking distance

Answer 38

Distance travelled between the moment when you first see a reason to stop, to the moment when you use the brake

Question 39

What is braking distance

Answer 39

The distance travelled from the time the brake is applied until the vehicle stops

Question 40

What is stopping distance

Answer 40

Total distance travelled from when the driver first sees a reason to stop, to when the vehicle stops

Question 41

Thinking distance formula

Answer 41

Speed x reaction time

Question 42

When is an object said to be in free fall

Answer 42

When an object is accelerating under gravity, with no other force acting on it

Question 43

What is the value for free fall

Answer 43

g

Question 44

Equation for force parallel to the slope

Answer 44

W x sin theta mg x sin theta

Question 45

Equation for force perpendicular to the slope

Answer 45

W x cos theta mg x cos theta

Question 46

What is normal contact force equal to

Answer 46

mg x cos theta

Question 47

Define Drag

Answer 47

A frictional force that opposes the motion of the object

Question 48

What affects drag

Answer 48

Speed Shape Roughness Texture

Question 49

What is pressure at the base equal to

Answer 49

Weight of the column divided by A

Question 50

What is pressure directly proportional to

Answer 50

height

Question 51

What does pressure not depend on

Answer 51

Cross sectional area

Question 52

Upthrust at the top surface of an object

Answer 52

height x density x gravity x area h is the submerged depth

Question 53

Force at the bottom surface of an object

Answer 53

(h + x) x density x gravity x Area x is height of object h is the submerged depth

Question 54

When will an object sink

Answer 54

If upthrust is less than the weight of the object

Question 55

When will an object float

Answer 55

upthrust = weight

Question 56

What is momentum

Answer 56

Mass x velocity

Question 57

What is the principle of conservation of momentum

Answer 57

For a system of interacting objects, the total momentum in a specified direction remains constant, as long as no external forces act on the system

Question 58

Describe perfectly elastic collisions

Answer 58

Momentum is conserved Total energy is conserved Total kinetic energy is conserved

Question 59

Describe inelastic collisions

Answer 59

Momentum is conserved Total energy is conserved Total kinetic energy is conserved

Question 60

What is net force directly proportional to

Answer 60

Rate of change of momentum Newtons second law

Question 61

Newtons second law

Answer 61

F = (k (constant) x change in momentum) / change in time

Question 62

When do we use F=ma

Answer 62

When the mass of the object remains constant during the period of acceleration

Question 63

What happens to momentum because of Newtons 3rd Law

Answer 63

Net change is 0 therefore it always conserved

Question 64

What is the impulse of a force

Answer 64

The change in momentum

Question 65

Impulse equation

Answer 65

F x change in time = change in momentum

Question 66

What is the area under a force time graph

Answer 66

Impulse

Question 67

Resolving momentum in the horizontal plane

Answer 67

total initial momentum = total final momentum m1v0 = m1v1cos(theta) = m2v2cos(theta)

Question 68

Resolving momentum in the vertical plane

Answer 68

total initial momentum = total final momentum m1v0 = m1v1sin (theta) = m2v2sin(theta)