Mechanics (Unit 2) Flashcards Preview

Physics > Mechanics (Unit 2) > Flashcards

Flashcards in Mechanics (Unit 2) Deck (38)
Loading flashcards...
1
Q

Difference between a scalar and a vector

A

vector has magnitude (size) and direction, whereas scalar only has magnitude (size).

2
Q

Examples of scalars

A

speed, mass, time, energy, power

3
Q

Examples of vectors

A

displacement, velocity, acceleration, force, weight

4
Q

Adding perpendicular vectors by calculation

A
  • draw vectors as a right angled triangle
  • use pythagoras’ theorem to find magnitude of resultant vector
  • use trigonometry to calculate the angle of resultant vector (sin=o/h; cos=a/h; tan=o/a)
5
Q

Adding vectors

by scale drawing

A
  • write down scale eg 1cm=2N
  • draw vectors to correct length and angle to each other “tip to tail”
  • add the resultant vector line (from original tail to free tip)
  • measure length and angle of resultant vector
  • convert length into appropriate quantity (using scale) to find magnitude of resultant vector
6
Q

Conditions for equilibrium of two or three coplanar forces acting at a point

A

total resultant force equals zero
or
if the vectors representing the forces are added together they will form a closed triangle.

7
Q

Two conditions for a body to remain in equilibrium

A
  1. resultant force acting on body is zero
  2. resultant moment about any point is zero
    object could be stationary OR travelling at constant velocity
8
Q

Definition of a moment

A

force multiplied by the perpendicular distance between the line of action of the force and the pivot.

9
Q

Units of moment

A

Nm

10
Q

Principle of Moments

A

in equilibrium, the sum of the clockwise moments about a point equals the sum of the anticlockwise moments.

11
Q

Definition of moment of a couple

A

(one) force multiplied by the perpendicular distance between the lines of actions of the two forces.

12
Q

Definition of centre of mass

A

point in a body through which weight appears to act
or
point in a body where the resultant moment is zero

13
Q

Stable equilibrium

A

When a body is displaced then released, it will return to its equilibrium position

14
Q

Unstable equilibrium

A

When a body is displaced then released, it will not return to its equilibrium position.

15
Q

Displacement

A

distance in a given direction

16
Q

Velocity

A

rate of change of displacement
or
change in displacement divided by the time taken

17
Q

Acceleration

A

rate of change of velocity
or
change in velocity divided by the time taken

18
Q

Gradient of displacement and velocity time graphs

A

Gradient of a displacement time graph = velocity

Gradient of a velocity time graph = acceleration

19
Q

Area under velocity and acceleration time graphs

A

Area under a velocity time graph = displacement

Area under an acceleration time graph = velocity

20
Q

Average velocity

A

total displacement divided by total time

21
Q

Instantaneous velocity at a point

A

rate of change of displacement at that point

gradient at a point on a displacement time graph (need to draw a tangent to calculate gradient)

22
Q

Conditions for an object falling at terminal velocity

A
  • resultant force on object is zero (weight and drag forces are balanced)
  • acceleration is zero (F=ma)
  • object travels at a constant velocity
23
Q

Factors affecting drag force on an object

A
  • the shape of the object
  • its speed
  • the viscosity of the fluid/gas (measure of how easily fluid/gas flows past a surface)
24
Q

Explain why an object reaches terminal velocity falling through air

A
  • initially only force acting is weight, so object accelerates at g.
  • drag force increases with increasing speed.
  • therefore resultant force decreases.
  • eventually drag force = weight, forces are balanced.
  • so resultant force is zero.
  • as F=ma, acceleration is zero so object falls at constant speed.
25
Q

Horizontal and Vertical motion of a projectile

A

In absence of resistive forces
Horizontal motion: no force horizontally, no acceleration so constant velocity.
Vertical motion: constant force due to weight, constant acceleration (equal to g).

26
Q

Newton’s Laws of motion

A

1st Law
An object will continue at rest or uniform velocity unless acted on by a resultant force
2nd Law
The acceleration of an object is proportional to resultant force acting on it, ie F=ma (providing mass is constant)
3rd Law
If object A exerts a force on a second object B, then object B will exert an equal and opposite force on object A.

27
Q

Principle of conservation of energy

A

Energy is neither created or destroyed, only converted from one form to another

28
Q

Energy conversions of an object falling in presence of resistive forces

A

loss in gpe = gain in ke + work done against resistance

work done typically appears as heat

29
Q

Definition of work done

A

force multiplied by distance moved in the direction of the force

30
Q

Units of work done

A

J

31
Q

Power

A

rate at which energy transferred
or
energy transferred (work done) divided by time taken

32
Q

Units of Power

A

W (Watts) or Js^-1

33
Q

Resolving vectors into two perpendicular components

A

See sheet

34
Q

Resolving components along, and perpendicular to, an inclined slope

A

See sheet

35
Q

Displacement and Velocity time graphs for uniform acceleration

A

See sheet

36
Q

Displacement and velocity time graphs for non-uniform acceleration

A

See sheet

37
Q

Sketch time graphs for an object falling under gravity then reaching terminal velocity

A

See sheet

38
Q

Sketch time graphs for a bouncing ball

A

See sheet