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OCR-A A Level Physics > 3.2 Forces in Action > Flashcards

3.2 Forces in Action Flashcards

1
Q

If you apply a net force on an object, what will happen?

A

The object will accelerate in the direction of the net force applied.

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2
Q

What equation relates force, mass and acceleration?

A

Force (N) = Mass (kg) x Acceleration (ms-2)

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3
Q

What does 1 Newton give?

A

1 Newton gives 1kg of mass an acceleration of 1ms-2 in the direction of the force.

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4
Q

What is the difference between weight and mass?

A

Weight is a gravitational force acting on an object - varies with location - and mass is the amount of matter of an object - constant.

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5
Q

How is the weight of an object calculated?

A

Weight (N) = Mass (kg) x Gravitational force

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6
Q

What device can be used to determine the weight of an object?

A

A newtonmetre.

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7
Q

What is friction?

A

The force that arises when two surfaces rub against each other.

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8
Q

What is drag?

A

The resistive force on an object travelling through fluid (liquid/gas).

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9
Q

What is tension?

A

The force within a stretched cable or rope.

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10
Q

What is upthrust?

A

An upward buoyancy force acting on an object when it is in fluid (liquid/gas).

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11
Q

What is a normal contact force?

A

A force arising when one object rests against another object.

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12
Q

How are forces represented in a free body diagram?

A

Each force is represented by an arrow labelled with the force, they’re drawn to scale (longer arrow = greater force).

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13
Q

How do you find the vertical component of a force?

A

mgsinθ

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14
Q

How do you find the horizontal component of a force?

A

mgcosθ

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15
Q

What is a fluid?

A

A fluid is a liquid or gas substance.

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16
Q

What is a drag force?

A

The frictional force that opposes motion of an object.

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17
Q

What is the magnitude of drag dependent on?

A
  • Speed of an object
  • Object’s cross-sectional area
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18
Q

How is drag related to speed?

A

Drag is directly proportional to speed².

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19
Q

Describe how forces of weight and drag change when an object undergoes a vertical fall.

A
  • As the object starts to fall, there is no drag force and acceleration is g.
  • As it falls, the speed increases and the drag force increases, resultant force decreases and the acceleration becomes less than g.
  • The object will reach terminal velocity, so drag and weight are equal and opposite, speed is constant and the acceleration is 0.
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20
Q

What is the magnitude of drag at the instant an object begins to fall?

A

Zero.

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21
Q

What is terminal velocity?

A

Terminal velocity is the constant speed reached when the drag force and weight are equal and opposite on an object.

22
Q

What is an experiment conducted to investigate the motion of an object falling through a fluid?

A
  • A falling object is attached to a pulley and is dropped through a cylinder of liquid, pulling a light polystyrene ball upwards on the other side of the pulley.
  • The motion of this ball is identical to that of the object falling through the fluid.
  • The exact values of data can be collected using data logger software. and a motion sensor.
23
Q

What is the experiment to determine terminal velocity of a metal ball falling through fluid?

A
  • Have two ball bearings made with the same material, but with two different radii.
  • Draw marks on the cylinder that are a known distance apart.
  • Start a stop watch and slow motion camera, and drop one of the bearings into a cylinder of glycerine.
  • Do this multiple times and repeat with the second bearing, dropping it from the same height.
  • When the time taken between markings becomes equal, it has reached terminal velocity.
24
Q

What is centre of gravity?

A

An imaginary point at which the entire weight of an object appears to act.

25
Q

What is centre of mass?

A

A point through which any externally applied forces produce straight line motion but no rotation.

26
Q

What is the experiment used to determine the centre of gravity of a 2D object?

A
  • Make holes along the edges of card (2D object) and insert pins firmly in them.
  • Allow the object to swing freely and it will come to rest with the centre of gravity vertically below the pin.
  • Use a plumb line to hang from the pin and draw a line across the string.
  • Repeat for the other holes, and the point of intersection of the lines is the centre of gravity.
27
Q

What is meant by equilibrium?

A

A body is in equilibrium when the net force and net movement acting on it are 0.

28
Q

If an object is in equilibrium, what does this mean in terms of the horizontal and vertical forces acting on the object?

A

They must be equal so resultant force is 0.

29
Q

What is meant by the moment of a force?

A

The moment of a force is the turning effect of a force about an axis or a point.

30
Q

How is the moment of a force determined?

A

Moment (Nm) = Force (N) x Perpendicular Distance of the Line of Action of Force from the Point of Rotation (m)

31
Q

What is the principle of moments?

A

For a body in rotational equilibrium, the sum of anticlockwise moments about any point is equal to the sum of the clockwise moments about that same point.

32
Q

How can you make an object rotate without any translational motion?

A

Apply a pair of equal and opposite, parallel forces to the object, that are applied along different lines.

33
Q

What is a couple?

A

A pair of equal and opposite forces acting on a body but not in the same straight line.

34
Q

What is torque?

A

Torque is the moment of a couple.

35
Q

How is torque calculated?

A

Torque (Nm) = Perpendicular Separation Between Forces (m) x One of the Forces (N)

36
Q

How is density defined?

A

Density is the mass per unit volume of a substance.

37
Q

How is density calculated?

A

Density (kgm-3) = Mass (kg)/Volume (m3)

38
Q

How do you find density of a regular object?

A
  • Mass = weigh using scales/balance.
  • Volume = Measure with rule/micrometre/digital calliper.
  • Density = m/v
39
Q

How do you find the density of an irregular object?

A
  • Mass = weigh using scales/balance.
  • Volume = Displacement (putting object in water and measuring the volume before/after it is put in).
  • Density = m/v
40
Q

How do you find the density of a liquid?

A

Find the volume using a measuring cylinder, and weigh using scales or a balance for mass.
Density = m/v

41
Q

How is pressure defined?

A

Pressure is the normal force exerted per unit cross-sectional area.

42
Q

How is pressure calculated?

A

Pressure (Pa or Nm-2) = Force (N)/Area (m2)

43
Q

Why do gases and liquids exert pressure on surfaces?

A

Fluids exert pressure on surfaces because of constant bombardment by their molecules.

44
Q

What is the equation used to calculate the pressure exerted by a vertical column of liquid?

A

Pressure (Pa or Nm-2) = Height (m) x Density (kgm-3) x Acceleration by Free Fall (ms-2)

45
Q

What does pressure of a fluid depend on

A

Pressure is directly proportional to height, so water pressure increases with depth and denser liquids exert greater pressure. Pressure of a fluid at a particular depth is the same in all directions.

46
Q

Why do objects submerged in water experience an upwards force?

A

It happens due to pressure differences at its upper and lower surfaces that result in an upwards resultant force.

47
Q

What is an expression for calculating upthrust?

A

Upthrust (N) = Volume of Liquid Displaced (m3) x Density of Liquid (kgm-3) x Acceleration due to Gravity (ms-2)

48
Q

What is Archimedes’ principle?

A

The upthrust exerted on a body immersed in a fluid, whether fully or partially, is equal to the weight of the fluid the body displaces.

49
Q

What condition is required for a body to sink?

A

The upthrust is less than the weight of the object.

50
Q

What condition is required for a body to float.

A

The upthrust is equal to the weight of an object, and the weight must be equal to the weight of fluid displaced.

OCR-A A Level Physics (9 decks)

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