Mechanics

Question 1

State all of the suvat equations

Answer 1

A = (v-u)/t
S = (v+u)t/2
S = ut + 1/2at^2
S = vt - 1/2at^2
V^2-u^2 = 2as

Question 2

What is value of g?

Answer 2

9.8

Question 3

When there is a box lying on the ground draw a Newton’s third law pair

Answer 3

Reaction from earth on box, and reaction from earth on box
NOT weight of box against reaction of earth from box!

Weight pair would be weight of box on earth, weight of earth on box (earth pulls box, box pulls earth, but because the earth is so much more massive than the box the earth doesn’t accelerate or does so barely)

Question 4

Weight is only ever between an object and __. Whereas reaction force is between ____

Answer 4

Earth
Objects

Question 5

When a lift is accelerating up, what force increases

Answer 5

Reaction force

Question 6

What is Newton’s first law

Answer 6

• An object will remain at rest unless acted on by a non-zero resultant force
  
  • An object travelling with a constant velocity will remain with that velocity unless acted on by a non zero resultant force

Question 7

Mass is a ___ whereas weight is not

Answer 7

Constant

Question 8

What is Newton’s third law

Answer 8

Newton’s third law:
If object A exerts a force on object B, object B exerts a force on object A:
- Equal in size, same type, opposite directions
- The pair of forces must be acting on two different objects
○ Push and pull forces acting on the same body are not equal and opposite

Question 9

What is Newton’s second law

Answer 9

Newton’s 2nd law = the resultant force on an object in a given direction is equal to the mass of the object multiplied by the acceleration of the object in that direction

Resultant force = mass x acceleration

Question 10

What does ‘equation of motion’ mean in an exam question

Answer 10

Write F = ma

Question 11

What is one modelling assumption to do with forces

Answer 11

They are constant

Question 12

What does a tow bar being inextensible mean for the model

Answer 12

The two objects remain the same distance apart
So the size of acceleration is equal for the 2 objects

Question 13

What does the tow bar being modelled as light mean for the model

Answer 13

• it has 0 mass
• so tension has same magnitude at both ends
  As F = ma (m =0) so T2-T1 = 0

Question 14

What does modelling a tow bar as rigid mean for modelling assumptions

Answer 14

It can be in compression

Question 15

What modelling assumption do we make about pulleys

Answer 15

It is smooth - otherwise there would be an extra tension from the mass on the pulley to the pulley
Smooth pulleys means teh acceleration of the two objects connected to the pulley have the same acceleration

Question 16

Friction tries to stop what kind fo motion - in what direction does it act?

Answer 16

Relative motion between 2 things in contact
- it acts parallel to the surface

Question 17

What is the equation for friction

Answer 17

Fmax = MU x R
MU = coefficient of friction
r = reaction force

Question 18

What happens to the friction force when you push a thing across a surface

Answer 18

Friction increases as you push it more up to a maximum value and then stays constant at that max value

Question 19

What does s stand for in suvat

Answer 19

Displacement ! NOT distance travelled

Question 20

In time t, a particle goes from point A back to the same point A
This means ut + 1/2at^2 = ?

Answer 20

0

Question 21

When sketching graphs of speed/time should you draw straight lines or curved lines?
What should you remember to add to the graph?

Answer 21

Straight lines
Label it with key points where the sketch changes

Question 22

At the turning point in a particle’s trajectory what is the velocity

Answer 22

0

Question 23

Give 3 modelling assumptions that we make in mechanics

Answer 23

• assuming things are particles
• assuming g = 9.8 (and not a more accurate number)
  Assuming only vertical components of force
• not accounting for wind effects
• not