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Physics OCR A AS > Chapter 3.2 - Forces in Action > Flashcards

Chapter 3.2 - Forces in Action Flashcards

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1
Q

Equation for weight

A

W=mg

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2
Q

Tension

A

A force experienced by a string or wire that is being pulled

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3
Q

Normal contact force

A

A force that acts perpendicular to the point of contact between two surfaces

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4
Q

Upthrust

A

The upward force that a liquid or gas exerts on a body due to the liquid or gas being displaced

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5
Q

Friction

A

A force that occurs between two surfaces that opposes motion. It is a result of intermolecular forces.

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6
Q

What you show on a free body diagram (3)

A
  • The bodies
  • The directions of each force
  • The value of each force
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7
Q

Fluid

A

A liquid or a gas

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8
Q

Drag

A

The frictional force experienced by an object travelling through a fluid

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9
Q

Does friction depend on velocity

A

no sir

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10
Q

Does drag depend on velocity

A

o yes

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11
Q

How is drag related to velocity

A

D = kv^2

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12
Q

Factors affecting drag (4)

A
  • Density of fluid
  • Coefficient of drag
  • Cross sectional area of object
  • Velocity
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13
Q

The motion of an object falling in the presence of drag

A
  • Initially accelerates because force of gravity acts downwards and there is minimal drag
  • As speed increases the upwards force of drag increases
  • Eventually the force of weight is equal to the force of drag and the object no longer accelerates and is moving at its terminal velocity
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14
Q

What happens when a parachute is opened while falling at terminal velocity

A

The drag force increases due to the increased cross sectional area. This results in a net upwards force and therefore an upwards acceleration, which causes the skydiver to slow down. As they slow down the drag force decreases until a new terminal velocity is reached

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15
Q

Terminal velocity

A

The speed of an object at which the weight is equal to the drag

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16
Q

Experiment to determine terminal velocity of a body in a fluid

A

Set up a cylinder filled with fluid and wrap elastic bands around it at regular intervals. Drop a ball of known mass into the tube and time how long it takes to reach each elastic band. Towards the bottom the times should remain constant and this value is used to calculate terminal velocity

17
Q

Equilibrium

A

The resultant force acting on an object is zero

18
Q

How to find the resultant of two forces

A

Add them

19
Q

Moment

A

The turning effect of a force

20
Q

Equation for a moment

A

moment = force * perpendicular distance

21
Q

Principle of moments

A

For a body to be in equilibrium, the sum of the clockwise components must equal the sum of the anticlockwise components

22
Q

Couple

A

Pair of forces of equal size that act in parallel but opposite directions, resulting in a turning force (torque)

23
Q

Equation for torque of a couple

A

T = (magnitude of one force) * (perpendicular distance between the forces)

24
Q

Weight

A

Force experienced by a mass due to a gravitational field

25
Q

Centre of mass

A

A single point that you can consider the entire mass to exist at, regardless of orientation

26
Q

Centre of gravity

A

A single point through which the entire weight of an object can be thought to act

27
Q

When are the centre of mass and the centre of gravity the same

A

For a small body in a uniform gravitational field

28
Q

Experiment to find centre of gravity

A

Hang the object freely from a point and draw a vertical line down it. Repeat from another point. Where these two lines cross will be the centre of gravity

29
Q

Three forces act on an object in equilibrium. What must these forces be?

A

coplanar

30
Q

Density

A

Mass per unit volume

31
Q

Pressure

A

Force per unit area

32
Q

Unit of pressure

A

Pascal (Newtons per metre squared)

33
Q

Pressure in a fluid

A 
p=hρg
p - pressure
h - depth
ρ - density
g - acceleration due to gravity
34
Q

Upthrust

A

An upward force exerted by a fluid on an object that is fully or partially submerged

35
Q

Archimedes principle

A

The upthrust is equal to the weight of fluid displaced

36
Q

Equation for upthrust

A

F = (h2 - h1)ρgA
or
F = sρgA where s is the height of the object

37
Q

Remember

A

Pressure increases as you go deeper into a fluid but upthrust is the same wherever you are (as long as you are fully submerged)

Physics OCR A AS (14 decks)

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