mechanics

Question 1

scalar quantities

Answer 1

• quantities that have magnitude (size), but not direction

- e.g. mass, energy, pressure, temp, power, speed, distance, time

Question 2

vector quantities

Answer 2

• quantities that have both magnitude (size) and direction

- e.g. displacement, acceleration, velocity, force, momentum

Question 3

an accelerating object can be moving at a constant speed if…

Answer 3

it is changing direction constantly

Question 4

displacement

Answer 4

the shortest distance from one point to another in a particular direction

Question 5

acceleration=

Answer 5

(final velocity - initial velocity) /time

• units : ms-2
• direction is the same as the direction of the change in velocity

Question 6

distance-time graphs

Answer 6

• can only be +ve, line cannot go down

- the gradient at a point = the speed at that point

Question 7

displacement-time graphs

Answer 7

• zero value (on x-axis): at starting point
• +ve value (above x-axis): at +ve side of starting point
• -ve value (below x-axis): at -ve side of starting point
• flat line: stationary
• ascending line: moving in +ve direction
• descending line: moving in -ve direction
• exponential increasing line: increasing velocity
• exponential decreasing line: decreasing velocity
• the gradient at a point = the velocity at that point

Question 8

velocity-time graphs

Answer 8

• zero value (on x-axis): stationary
• flat line: constant velocity
• ascending line: accelerating in +ve direction
• descending line: accelerating in -ve direction
• exponential increasing line: increasing acceleration
• exponential decreasing line: decreasing acceleration
• the gradient at a point = the acceleration at that point
• area under graph = change in displacement

Question 9

speed-time graphs

Answer 9

• can only be +ve

- area under graph = distance travelled

Question 10

equations of motion

Answer 10

• s=0.5(u+v)t
• 𝑠=ut + 0.5at^2
• v^2=𝑢^2+2𝑎𝑠
• average v = 0.5(u+v)

Question 11

forces

Answer 11

• the application of a push or a pull to an object by another object
• vector quantity,
• units: newtons (N)
• types: weight, normal contact, drag, friction, magnetic, electrostatic, upthrust, thrust, lift and tension

Question 12

force formulae

Answer 12

F=ma
F=momentum/time
P=F/a
moment=F x perpendicular distance
Ew = F x displacement

Question 13

weight formula

Answer 13

W=mg
W=weight in N
m=mass in kg
g=gravitational field strength in Nkg-1 (9.8 on Earth)

Question 14

normal contact force

Answer 14

-force as a result of 2 solid objects being in contact with each other

Question 15

tension

Answer 15

• when a spring/string/wire is pulled by equal and opposite external forces at each end as shown, it is said to be subjected to a tension force, 𝑇
• this tension force usually causes the length of the spring/string wire to increase slightly.
• the increase in length = the extension
• the greater the tension force, the greater the extension
• T=weight=mg

Question 16

force-extension graphs

Answer 16

• the steeper the graph, the greater the spring constant -> more rigid
• the shallower the graph, the greater the spring constant -> less rigid
• work done by this force = the area under the graph

Question 17

material types

Answer 17

• copper wire stretches uniformly initially, but then suddenly stretches much more just before reaching the breaking point
• glass is very rigid and deforms only very slightly before breaking
• rubber stretches non-uniformly

Question 18

deformation

Answer 18

• elastic deformation is reversed when the force is removed
• inelastic deformation is not fully reversed when the force is removed (permanent stretching)
• materials that undergo permanent stretching are ductile

Question 19

elastic limit

Answer 19

-the point on the force-extension graph where the extension goes from being elastic to inelastic (when the line on the graph starts to flatten)

Question 20

Hooke’s Law

Answer 20

F=kx

• F=force applied to the material/spring (N)
• x=extension of the material/spring (m)
• k=spring constant, a measure of the stiffness of a spring up to its elastic limit, high k->rigid (Nm-1)

Question 21

factors affecting spring constant

Answer 21

• the greater the cross-sectional area, the greater the spring constant
• the longer the wire, the smaller the spring constant

Question 22

combining springs

Answer 22

• if 2 springs are connected together in series, then it double the extension -> spring constant is = 0.5𝑘.
• if 2 springs are connected in parallel, then it will double the force -> spring constant = 2𝑘

Question 23

momentum formula

Answer 23

p = mv

• p=momentum (kg/ms-1 or Ns)
• m=mass (kg)
• v=velocity (ms-1)

Question 24

newton’s first law

Answer 24

an object will remain at rest or move with constant velocity in a straight line unless acted upon by an unbalanced force

Question 25

newton's second law

Answer 25

when a constant unbalanced force is applied to an object, the object will move with constant acceleration in the direction of the unbalanced force

Question 26

newton's third law

Answer 26

to every action there is an equal and opposite reaction

Question 27

gravitational field strengths

Answer 27

- the magnitude of the gravitational force acting per unit mass of an object in the field - Earth: 10Nkg-1 - Moon: 1.6Nkg-1 - Jupiter: 26Nkg-1

Question 28

acceleration of free-fall

Answer 28

an object falling freely due to gravity falls with an acceleration that is numerically equal to the gravitational field strength

Question 29

factors affecting air resistance

Answer 29

- increasing speed of motion increases air resistance - increasing cross-sectional area of the object, increases air resistance. - turbulent air flow gives greater air resistance force compared to streamlined air flow

Question 30

terminal velocity

Answer 30

- the speed at which air resistance = weight | - vertical forces acting upon an object are balanced

Question 31

power formulae

Answer 31

- P=E/t - P=work done/time - P=F x velocity

Question 32

kilowatt-hour

Answer 32

1kWh=3600kJ

Question 33

stored and active energy

Answer 33

active: - electrical - heat (thermal energy) - light - kinetic - sound stored: - chemical potential energy (stored in a battery/cell) - gravitational potential energy (stored due to height) - strain potential (energy stored in a stretched spring)

Question 34

energy formulae

Answer 34

- % efficiency = (useful energy/stored energy) x100 - Ek = 0.5mv^2 - Ep = mgh - total energy = Ep/Ek + work done against frictional force

Question 35

conservation of momentum (working out velocity of object B after a collision)

Answer 35

1. work out total momentum before the collision (momentum of object A + momentum of object B) 2. this will also be the total momentum after the collision 3. work out the total mass after the collision (mass of object A + mass of object B) 4. work out velocity of object B using v=p/m

Question 36

moment of a force about a point formula

Answer 36

T=Fd - T=moment of a force (Nm) - F=force (N) - d=perpendicular distance from pivot to the line of action of the force (m)