Mechanics

Question 1

given constant acceleration, we have 5 quantities of motion - suvat, which stands for

Answer 1

s = displacement
u = initial velocity
v - final velocity
a = acceleration
t = time

Question 2

modelling assumption

Answer 2

particle :
- mass of object concentrated at a single point
- rotational forces/air resistance can be ignored

smooth/light pulley
- no friction, means tension is the same on either side of the pulley
- pulley has no mass

rod:
- mass is concentrated along the line

peg/support
- dimensionless and fixed. can be rough or smooth

uniform body - mass is concentrated at a single point

rough surface - object in contact with the surface experience a frictional force

smooth surface - no friction

inextensible string - acceleration is the same

Question 3

earth gravity number

Answer 3

9.8

Question 4

units

Answer 4

mass - kg
length/displacement - m
speed / velocity - m s ^-1
acceleration - ms^-2
force/weight - N (kg m s^-2)

Question 5

how to convert to vector form from scalar form

Answer 5

use basic trig to find the x and y

Question 6

how to convert from vector to scalar

Answer 6

find the magnitude

Question 7

velocity and speed

Answer 7

velocity is vector
speed is scalar

Question 8

average velocity equation

Answer 8

displacement from starting point/time taken

Question 9

average speed equation

Answer 9

total distance travelled/time taken

Question 10

acceleration is

Answer 10

the rate of change of velocity

Question 11

the area under a velocity time graph is…

Answer 11

the distance travelled

Question 12

differentiating displacement gives…

Answer 12

velocity, and integrating velocity gives displacement

Question 13

deceleration =

Answer 13

gradient

Question 14

deceleration =

Answer 14

gradient

Question 15

deceleration =

Answer 15

gradient

Question 16

weight equation

Answer 16

W = mg (mass x gravity)

Question 17

what is the normal reaction

Answer 17

a force that acts perpendicular to the surface

Question 18

newtons 1st law of motion

Answer 18

states that an object at rest will stay at rest
- an object moving with constant velocity will remain at that velocity
- unless an unbalanced force acts on that object

Question 19

newtons second law

Answer 19

F=ma

Question 20

if a particle is at equilibrium…

Answer 20

• it’s at rest or moving at constant velocity
• there is no resultant force
• left = up and right=down

Question 21

State how you have used the fact that the object is modelled as a particle

Answer 21

The weight of the objectacts at point

Question 22

when do you use suvat and when do you use distance = speed x time

Answer 22

Use “distance = speed x time” when speed is constant, and “SUVAT equations” when acceleration is constant

Question 23

horizontal motion projectiles

Answer 23

s = vt as velocity is constant

• the horizontal component will also always remain constant

Question 24

What causes a string to go slack in mechanics problems?

Answer 24

When one object (e.g. B) stops or hits a surface, tension disappears, so the string becomes slack and no longer transmits force.

Question 25

What happens to tension when a string goes slack?

Answer 25

Tension = 0. The string no longer pulls on the other object.

Question 26

If a particle was being pulled by a string and the string goes slack, what happens to its motion?

Answer 26

It continues moving due to its momentum but slows down because there's no longer any force pulling it forward (only resistive forces like gravity on a slope).

Question 27

If a particle is on a slope and the string goes slack, what acceleration acts on it?

Answer 27

Only component of weight down the slope remains: a=gsinθ

Question 28

When a particle continues up a slope after the string goes slack, how do you find the extra distance it travels

Answer 28

Use SUVAT: Initial speed = speed at moment string goes slack Final speed = 0 Acceleration = −gsinθ Use v ^2 =u ^2 +2as

Question 29

How do you know when to split the motion into two phases?

Answer 29

When something stops moving suddenly (like B hitting the ground), the motion splits: Phase 1: Both objects move together with tension Phase 2: One object moves independently (string slack)

Question 30

How do you get the total distance travelled if the string goes slack?

Answer 30

Total distance = ➕ Distance travelled before string went slack ➕ Extra distance after string goes slack (using SUVAT)

Question 31

Common mistake when the string goes slack?

Answer 31

Still using the same acceleration from when the string was taut After slack: reassess forces and re-calculate acceleration

Question 32

when something is smoothly hinged

Answer 32

use x and y, not r and mew r

Question 33

When do I divide by g in mechanics?

Answer 33

Use F = (W/g) × a when you're given weight in newtons (N) instead of mass. Why? Because weight = mass × g, so: mass = weight ÷ g So if you're using weight directly, you must divide it by g to get the mass.