forces and motion and radioactivity

Question 1

balanced and unbalanced forces

Answer 1

• the forces acting on an object are balnced, there is no change in motion; resultant force = 0N
• the friction and drag on a car= 500N traction force = 500Nresultant force = 0N car moves at constant speed
• if the forces are unbalanced, the object accelerates or decelerates
  Drag = 700N
  Traction = 500 N
  resultant force= 200N

Question 2

acceleration

Answer 2

acceleration= change in velocity/ time taken
units = m/s squared
negative accelerations are decelerations
time taken = change in velocity / acceleration
change in speed = time taken * acceleration

Question 3

measuring distances from velocity - time graphs

Answer 3

• distance = area under graph

Question 4

Newton’s second law

Answer 4

A force accelerates a mass (m). what is the acceleration (a)?
bigger force means bigger acceleration so acceleration is proportional to force
bigger mass means less acceleration so acceleration is inversely proportional to mass
F= Ma
1 newton will accelerate 1kg at 1 m/s

Question 5

friction between surfaces

Answer 5

• caused when the jagged surfaces interlock- in order to move a book across a table you have to break the interlocking jaggedy bits- frictional forces always act in the opposite direction to your pulling force

Question 6

measuring the coefficient of friction surfaces

Answer 6

• measure the angle tan at which various objects just start to slide down a slope. then we can calculate the coefficient of friction.
  equipment: retort stand; boss; ramp; wooden block.
  record the angle at which the object starts to slide; repeat with larger weights.
  to work out the coefficient tyoe tan(angle) into your calculator correct to 2 decimal places

Question 7

air resistance and terminal velocity

Answer 7

terminal velocity is reached when air resistance= weight of falling object
larger the surface area and the faster the speed the more air resistance there is

Question 8

stopping veihcles

Answer 8

when you stop a vehicle the vehicle’s kinetic energy must be converted into heat and sound (usually by brakes)stopping distance is made up of two parts : 1) the thinking distance
2) the braking distance
thinking distance= distance over which you travel whilst thinking about braking
braking distance= distance over which you travel when the brakes are activated
stopping distance = braking distance + thinking distance

Question 9

scalars and vectors

Answer 9

scalars have magnitude only eg. a speed of 12 m/s
vectors have magnitude and direction eg. velocity of 15 m/s N a journey from hampton to ealing: the distance from H to E is 12 km
the displacement (vector) from H to E is only 8km (as the crow flies)
speed = distance / Time (scalar)velocity = displacement/ time (vector

Question 10

hooke’s law

Answer 10

• if a material obeys hooke’s law :
• the extension is proportional to the force
• if a material returns to its original shape or length when you remove the deforming forrce, the material is said to be elastic

Question 11

experiment : investigating whether springs and rubber bands obey hooke’s law

Answer 11

aim: we’ll measure force against extension for : 1) two springs in series 2) two springs in parallel 3) rubber band
apparatus: - damp, boss, springs, masses (upto 10n)

method:

• first measure the original length of the two springs or the rubber band and record this in a table
• add one 0.5 n weight to the end of the spring/s and measure it’s extension
• record this in a suitable table
• repeat this for all three experiments adding 0.5 n weights each rime upto 10n
• record your results in a line graph (extension against force)

Question 12

momentum

Answer 12

• momentum = MU
  M= mass
  U= velocity
• it can be positive or negative
• e.g. a 2000Kg car moves at 25m/s. what is its momentum?
  momentum = MU = 2000 * 25 = 50,000 kg m/s
  -

Question 13

collisions and conservation of energy

Answer 13

• whenever objects collide, the momentum after the collision is the same as the momentum before- always conserved
• law of conservation of momentum =
  momentum before = momentum after
  e.g. a 3000 kg lorry going at 20 m/s goes into a stationary mini (1000kg)
  the two stick together. what speed do they move off

momentum = MU = 3000 * 20
= 60,000
momentum after = 60,000 = MU = 4,000 * U
U= 60,000/ 4000 = 15 m/s

Question 14

recoil

Answer 14

• before you fire a gun the total momentum is 0. when you fire the bullet the total momentum must still be 0. this is only possible if the gun has momentum in the opposite direction to the bullet - gun recoils
• e.g. a 2kg gun fires a 0.001 kg bullet at 200 m/s . what is the gun’s recoil velocity?

gun momentum = bullet momentum
2kg*U = 0.001kg * 200 m/s
U= (0.001 * 2000) / 2
= 0.1 m/s

Question 15

newton’s second law

Answer 15

• to stop an object you need to change its momentum
• to change an object’s momentum you need to apply a force e.g. catching a ball
• the more momentum the ball has the bigger the force needed to stop it
• force = change in momentum / time taken
• e.g. a 1 kg custard pie travelling at 5 m/s hits a wall and stops in a time of 0.5 s. what force did the wall exert?

m= 1kg U= 5m/s V= 0 m/s (final velocity) t= 0.5 s

F= (mV - mU) / t = (1 * 0 ) - (1*5) /0.5
= 10n

Question 16

experiment: ‘weighing’ or massing people using the principle of moments

Answer 16

• principle of moments : mpdp = Mwdw
• mp = mass of person
• dp = distance of person from fulcrum
• Mw = mass of weight
• dw= distance of weight from the fulcrum
• mp= Mwdw / dp
  method :
• measure the mass of the weight and the distance the person is from the fulcrum
• measure the distance of the weight and record this in a suitable table
• add weights to one end of the plank until it’s balanced and record this in a table
• use the equation to calculate the mass of the person

3 sources of error:

• hard to tell where the plank balanced exactly
• planks not aligned perpendicular with the fulcrum
• hard to find the centre of gravity

Question 17

turning forces

Answer 17

• moment = force * distance from pivot point

Question 18

principle of moments

Answer 18

• clockwise moment = anti clockwise moment

- if an object is in equilibrium the anti clockwise moment = clockwise moment

Question 19

radioactivity

Answer 19

• isotopes are atoms of the same element with different numbers of neutrons
• radioactive atoms are unstable atoms
• they try to become stable by spitting out neutrons, protons and electrons

Question 20

particles of radiation

Answer 20

• alpha- slow and massive helium nuclei, so they’re easily stopped by only a thin layer of atoms
• beta particles: fast and tiny electrons
• gamma radiation- an electromagnetic wave which doesn’t collide with matter. you can never block 100 % of gamma radiation

Question 21

radioactive half life

Answer 21

• half life- the time it takes for half of all the atoms in a substance to decay
• the half life does not depend on how much substance you have
• you can’t change the half life of any nuclide by heating or any chemical reaction or any other means
• can be found by measuring the activity of a nuclide
• ACTIVITY = number of decays per second ( counts on a geiger counter)
• after n half lives have 1/2(n) of the original number of atoms or counts left

Question 22

experiment: measuring the half life of protactinium

Answer 22

• method: we’ll measure the activity in 10s intervals as a sample of protactinium - 234 decays
• background count in 10s : 4
• after every ten seconds take the count record it in a table and the take away the amount of background radiation
• plot these results onto a line graph and draw a curve of best fit
• to work out the half life - tae a specific count e.g. 50 and see how long it took to halve - the difference between the two numbers is the half life

Question 23

carbon dating

Answer 23

• measure amount of carbon 14 left in bone. compare with amount found in living material to estimate age

Question 24

nuclear equations

Answer 24

1) alpha decay = the unstable nucleus spits out a helium nucleus (He 4,2 )
eg. 238,92 U -> 234,90 Th + 4,2 He
the mass and atomic numbers on both sides must balance

2)beta decay: the unstable nucleus spits out an electron (0,-1 e)
eg. 14,6 C -> 14,7 N + 0,-1 e
you can also write the beta particles with the greek letter

3) gamma emission: gamma rays ave no mass or charge so the unstable nucleus is unchnaged

Question 25

nuclear fission

Answer 25

1) 235 U fissions 2) heat released 3) heat is transferred to a coolant H2O or CO2 4) the coolant is sent to a steam generator , steam turns turbines to make electricity 5) control rods: absorb neutrons to speed up or slow down the chain reaction 6) moderator: slows down neutrons so that they can fission the uranium nuclei more easily

Question 26

using radioactivity

Answer 26

- penetrating : - paper and foil thickness - looking for cracks in pipes or concrete - searching for leaks in oil pipelines tracer - you can follow the path of radiation - nutrient uptake in plants - following pollution - medicine- looking for blockages in kidneys half life - dating rocks with 40K - carbon dating ioniser - smoke detectors - sterilisingg - radiotherapy( killing tumours)

Question 27

the discovery of the nucleus

Answer 27

- early models of the atom had negative electrons embedded in positive charge like raisins, in a cake or plum in a pudding - in 1911 rutherford, geiger and marsden discovered the atomic nucleus by firing alpha particles at thin gold foil 1) the nucleus of the atom is much smaller than the size of the atom (because most alpha particles passed through) 2) the nucleus is positively charged (it deflects some of the positive alpha particles) 3) the nucleus contains most of the atom's mass ( a direct hit causes the alpha particle to bounce back) positively, massively, empty