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Physics P2 > 2 Forces > Flashcards

2 Forces Flashcards

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1
Q

What can a force do?

A

change the shape of an object or change its state of rest or its motion

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2
Q

What is the unit of force?

A

newtons (N)

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3
Q

What happens whenever two objects push or pull on each other?

A

they exert equal and opposite forces on one another

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4
Q

What is the motive force?

A

the driving force on a car/vehicle is the force which makes it move
sometimes called the engine force

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5
Q

What is the motive force due to ?

A

friction between the ground and tyre of each drive wheel

acts where the tyres touch the ground

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6
Q

What happens, in terms of friction, when a car/vehicle moves forwards?

A
  • the force of friction on the ground of the tyre is in the forward direction
  • the force of friction of the tyre on the ground is in the reverse direction
    (the two forces are equal and opposite to each other)
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7
Q

What is the resultant force?

A

a single force that has the same effect as all the forces acting on an object

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8
Q

When the resultant force of an object is 0, what happens?

A
  • the object remains stationary if it was at rest
    or
  • the object continues to move at the same speed and in the same direction if it was already moving
    (if 2 forces only act on the object, they must be equal to each other and act in opposite directions)
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9
Q

What is the resultant force of a glider on a linear air track floating on a cushion of air?

A

provided the track is level, the glider moves at a constant velocity because friction is absent
this means the resultant force on the glider is 0

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10
Q

What is the resultant force of a heavy crate pushed across a floor at a constant velocity?

A

the resultant force is 0
the push force on the crate is equal in size but acts in the opposite direction to the force of friction acting of the floor on the crate

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11
Q

What happens when the resultant force doesn’t equal 0?

A

the movement of the object depends on the size and direction of the resultant force

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12
Q

What happens, in terms of resultant force, when a jet plane takes off?

A

the thrust of its engines is greater than the force of air resistance on it
the resultant force is the difference between the thrust force and the force of air resistance on it which is therefore not 0
the greater the resultant force, the quicker the take-off is

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13
Q

What happens, in terms of resultant force, when a car driver applies the brakes?

A

the braking force is greater than the force of the engine
the resultant force is the difference between the braking force and the engine force
it acts in the opposite direction to the car’s direction so it slows down

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14
Q

What is the resultant force if two forces act on an object along the same line?

A
  • their sum if the forces act in the same direction

- their difference if the forces act in opposite directions

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15
Q

Complete the sentence:

The bigger the resultant force on an object is…

A

the greater its acceleration is

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16
Q

Complete the sentence:

The greater the mass of an object is…

A

the smaller its acceleration is for a given force

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17
Q

How does resultant force increase the velocity of an object?

A

if it’s in the same direction

positive because it is in the same direction as velocity

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18
Q

How does resultant force decrease the velocity of an object?

A

if it’s in the opposite direction to its velocity

negative because it is in the opposite direction of velocity

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19
Q

How do we calculate resultant force?

A

resultant force (N) = mass (kg) x acceleration (m/s2)

20
Q

What are the 2 forces which oppose the driving force of a vehicle?

A
  • friction

- air resistance

21
Q

What does the braking force needed to stop a vehicle depend on?

A
  • the speed of the vehicle when the brakes are first applied

- the mass of the vehicle

22
Q

What is thinking distance?

A

the distance travelled by the vehicle in the time taken for the driver to react (driver reaction time)

23
Q

What is braking distance?

A

the distance travelled by a vehicle during the time the braking force acts

24
Q

What is the stopping distance?

A

the shortest distance a vehicle can safely stop in

= thinking distance + braking distance

25
Q

What are the factors which affect stopping distances?

A
  • tiredness, alcohol and drugs increase reaction times: also distractions such as mobile phones, all increase the thinking distance (speed x reaction time) which increase the stopping distance
  • the faster the vehicle is travelling: the further it travels before it stops due to increased speed increasing thinking distance and braking distance
  • in adverse road conditions: e.g. wet or icy roads, drivers have to brake with less force to avoid skidding therefore stopping distances are greater in poor road conditions
  • poorly maintained vehicles: e.g. worn tyres or brakes, take longer to stop because they are less effective
26
Q

What is the weight of an object?

A

the force of gravity on it, measured in newtons

27
Q

What is the mass of an object?

A

the quantity of matter in it, measured in kilograms

28
Q

What can we use to measure the weight of an object?

A

a newtonmeter

29
Q

How do we calculate weight?

A

mass x gravitational field strength

30
Q

What is ‘acceleration due to gravity’?

A

if an object falls with no other forces acting on it, the resultant force on it is its weight
it accelerates at a constant acceleration of 10m/s2

31
Q

What happens if an object falls in a fluid?

A

the fluid drags on the object
the drag force increases with speed
the resultant force of the object is its weight - drag force

32
Q

In a fluid, why does the acceleration of the object decrease as it falls?

A

drag force increases as it speeds up

therefore resultant force decreases and acceleration decreases

33
Q

What is terminal velocity (of a falling object)?

A

the velocity it reaches when it is falling in a fluid

the weight is then equal to the drag force on the object

34
Q

What is the name of drag force when an object moves through air?

A

air resistance

35
Q

How does an elastic object regain its original shape?

A

when the forces deforming it are removed

36
Q

What extension?

A

the difference between the length of ‘the spring’ and its original length

37
Q

What is the extension of a spring directly proportional to?

A

the force applied to it. provided the limit of proportionality isn’t reached

38
Q

What happens when an elastic object is stretched?

A

it stores elastic potential energy
because work is done on the object by the stretching force
when the stretching force is removed, the elastic energy is released- some may be transferred to kinetic energy or thermal energy where atoms vibrate more

39
Q

What is Hooke’s law?

A

the extension of a spring is directly proportional to the force applied, provided its limit of proportionality is not exceeded

40
Q

What is the limit of proportionality?

A

if the force is too large, the spring stretches more than expected beyond its limit

41
Q

What is the equation of Hooke’s law?

A

force applied (N) = spring constant (N/m) x extension (m)

42
Q

What is the spring constant of a spring?

A

the force per unit of extension needed to stretch it

43
Q

How can the fuel economy of road vehicles be improved?

A
  • reducing the speed to lower air resistance
  • fit a wind deflector which reduces the air resistance on a vehicle which means less engine force and less power are needed to maintain a certain speed - fuel costs are reduced because less fuel is needed
44
Q

What do speed cameras do?

A

linked in pairs which measure the average speed of a vehicle over a certain distance which encourages drivers to follow the speed limit which improves road safety

45
Q

What do anti-skid surfaces do?

A

increase the friction between a car tyre and the road surface
this reduces skids or even prevents them altogether

Physics P2 (7 decks)

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