Mechanics 1

Question 1

explain how you would measure the acceleration in a laboratoey

Answer 1

• (draw diagram of trolley
• arrange suitable weights to hang over the edge of the table so that the trolley is moving with an acceleration
• run the paper tape through a ticker tape timer
  u : s/(0.02 x dots)
  v: s/(0.02 x dots)
  a = (v-u)/t

Question 2

state the principle of conservation of momentum

Answer 2

when two or more bodies interact in a closed system the total momentum of the bodies before the interaction equals the total momentum of the bodies after the interaction

Question 3

which direction should the gas in an aircraft be expelled for it to be slowed down

Answer 3

the same direction as the motion

Question 4

explain how the principle of conservation of momentum is applied to changing the direction un which a spacecraft is travelling

Answer 4

if a gas is expelled in a direction perpendicular to the motion of the spacecraft, the gas has a momentum in this direction and the spacecraft has an additional momentum in the opposite direction.
the extra momentum us added to the original momentum of the spacecraft and will cause the spacecraft to change direction

Question 5

describe an experiment to find the resultant of 2 vectors

Answer 5

3 newton balances on a board (diagram), 1 and 2 are perpendicular and connected to board
pull balance 3 and then connect it to board
vector sum of forces 1 on 2 read on newton balances is force 3

Question 6

force to push a wheelchair up a ramp is equal to what?

Answer 6

component of weight of the person acting down the ramp

Question 7

Newton’s second law

Answer 7

when a resultant external force acts on a body the rate of change of momentum of the body is proportional to this force and takes place in the direction of the force

Question 8

why does a sky diver reach a constant speed?

Answer 8

the skydiver falls down due to gravity, the air resistance will build up and eventually will equal the force of gravity, total resultant force is zero - no acceleration

Question 9

describe an experiment to measure g due to free fall

Answer 9

draw diagram; (soft iron electromagnet, steel ball, trap door, millisecond timer)
values of distance measured from bottom of steel ball to trap door, electromagnet demagnetised, timer starts, trapdoor stops timer

Question 10

graph when measuring g x axis

Answer 10

2 x distance

Question 11

graph when measuring g y axis

Answer 11

time squared

Question 12

give 2 precautions for accuracy when measuring g

Answer 12

avoid the error of parallax when measuring s

use large values of distance, smaller % error

Question 13

define force

Answer 13

force i that which causes acceleration

Question 14

define momentum

Answer 14

mass x velocity

Question 15

establish the relationship force x mass = acceleration

Answer 15

force α rate of change of momentum
F α (mv - mu)/t
F α m((v-u)/t)
F α ma
F = kma, k=1, F=ma

Question 16

minimise the effect of friction when measuring PCM

Answer 16

polishing the table

Question 17

minimise the effect of gravity when measuring PCM

Answer 17

keep the table perfectly horizontal

Question 18

Principle of concentration of momentum

Answer 18

energy cannot be created nor destroyed but it may be changed from one form to another

Question 19

when a basketball loses kinetic energy where does it go

Answer 19

becomes sound or heat energy

Question 20

what is friction

Answer 20

a force that opposes the relative motion of 2 bodies in contact

Question 21

give 2 ways of minimising the effect of air resistance when measuring g

Answer 21

use a small dense ball

use a smooth ball

Question 22

Newton’s first law

Answer 22

a body continues in a state of rest or uniform motion in a straight line unless a resultant external force acts on it

Question 23

Newton’s third law

Answer 23

to every action there is an equal and opposite reaction

Question 24

when investigating f minimise the effects of gravity when measuring f α a, why would the graph not go through the origin? 2

Answer 24

friction between the trolley and the table could be responsible for the graph not passing through the origin - polish the table to resolve the problem
the table might have been sloped, levelling the table would resolve this problem

Question 25

3 ways to make a trolley move at a constant acceleration

Answer 25

adjust the slope of the table
lubricate the trolley wheels
make sure the table is polished to reduce friction

Question 26

constant velocity on TTT

Answer 26

the dots on the tape are equally spaced

Question 27

2 points of accuracy when verifying PCM

Answer 27

• avoid using trolleys of very small mass and avoid very small values of velocity, greater % error
• avoid error of parallax when measuring distance along the tape with meter stick

Question 28

derive v² = u² + 2as

Answer 28

v = u + at
v² = (u + at)² 
v²  = u²  + 2uat + at² 
v²  = u² + 2a(ut +1/2 at² )
v²  = u²  + 2as

Question 29

centripetal acceleration

Answer 29

v² /r