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Physics AQA AS Level > Newton's Laws of Motion > Flashcards

Newton's Laws of Motion Flashcards

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1
Q

State Newton’s First Law of Motion

A

An object remains at reest or in uniform motion unless acted by a force

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2
Q

When a resultant force is acting on an object, happens?

A

The object’s velocity changes, i.e. it accelerates

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3
Q

For an object that is at rest/is moving at a constant velocity, describe the force acting on it?

A

The resultant force on the object is zero (i.e. no forces acting on it (stationary) or the forces acting on it are balanced(moving at constant velocity))

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4
Q

What is Newton’s Second Law of Motion (include equation)

A
  • For constant mass
  • The acceleration of an object of constant mass m, is due to the resultant force acting on the object such that F is proportional to ma.
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5
Q

What is the equation associated with Newton’s Second Law of Motion?

A

F = ma
Resultant force = mass x acceleration

F = Resultant Force (N)
m = mass (kg)
a = acceleration (ms⁻²)
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6
Q

Which direction does an object accelerate compared to the direction of the resultant force?

A

The acceleration is always in the same direction as the resultant force.

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7
Q

What direction is the resultant force and acceleration for a freely moving projectile?

A

A freely moving projectile experiences a resultant force vertically downwards due to gravity. Therefore its acceleration is also vertically downwards, regardless of what its direction of motion is.

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8
Q

Why do two falling objects of different masses fall at the same speed?

A

Given that air resistance is negligible and the only force acting on the two objects is gravity i.e. its weight given by W =mg, the acceleration of both falling objects is 9.81ms⁻² (the acceleration of free fall, g). The mass of the object is independent of its motion, g.

a = F/m
acceleration= weight/mass = mg/m = g = 9.81ms⁻²

The acceleration due to gravity is the same for all objects near the surface of the Earth, regardless of mass.

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9
Q

How does a newtonmeter work?

A

When an object is measured using a newtonmeter, it is in equilibrium. Therefore, the support force acting on it is equal and opposite to the weight. Therefore an object placed on a newtonmeter exerts a force on it equal to the weight of the object. Therefore the newtonmeter measures the weight of the object.

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10
Q

What is g, also referred to as, a part from the acceleration of free fall?

A

The gravitational field strength at a given position, as it is the force of gravity per unit mass on a small object at that position.

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11
Q

What is inertia?

A

The resistance of any physical object, with a mass, to a change in its velocity.

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12
Q

How is inertia dependant on mass?

A

More force is needed to give an object a certain acceleration than to givee an object with less mass the same acceleration. Therefore the greater the mass of an object, the greater its inertia.

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13
Q

When an object is acted on by two unequal forces acting in opposite directions, which direction does the object accelerate in?

A

The object accelerates in the direction of the larger force.

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14
Q

If the forces on an object are F1 and F2 in the same place, where F1>F2, how do you calculate the mass or acceleration of the object?

A

F1-F2 = ma
Resultant force = mass x acceleration

F1-F2 = resultant force (N)
m = mass (kg)
a = acceleration (ms⁻²)
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15
Q

A car of Mass M is fitted with a trailer of mass m on a level road. When the car and the trailer accelerate, the car pulls the trailer forwards and the trailer holds the car back.
If air resistance, is negligible, what is the resultant force on the car and the fitted trailer to calculate both their mass and acceleration?

A 
Car:
Driving force (pushing forward) - Tension in the tow bar (holding it back), so F-T = Ma

Trailer:
Tension in the two bar (only force acting on it pulling it forward), so T = ma

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16
Q

What force pushes an object?

A

The force of friction pushes an object. If friction was negligible the object would not be able to move.
The frictional force between the road and tire is what allows the tire to “push” off the road, thus moving the car forward (Newton’s Third Law)

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17
Q

What does combinging the two equations for the car and trailer give?

A

F = Ma + ma = (M+m)a

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18
Q

If T is the thrust of the rocket engine when its mass is m and the rocket is moving forwards, its acceleration is given by?

A

T - mg = ma
T = mg + ma

T = Rocket Thrust (N)
mg = Weight (N)
m = mass
a = acceleration (ms⁻²)
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19
Q

What does the equation for the rocket tell us about the rocket thrust, required for take off?

A

It must be greater than the weight of the rocket.

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20
Q

If a lift is moving at constantly velocity, what is the acceleration and relationship between tension and weight?

Taking up as positive and down as negative

A 
a = 0
T = mg

(T = mg + ma, a = 0, so T =mg)

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21
Q

If the the lift is moving up and accelerating, what is the acceleration and relationship between tension and weight?

Taking up as positive and down as negative

A

a > 0
T > mg
mg + ma > mg

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22
Q

If the the lift is moving up and decelerating, what is the acceleration and relationship between tension and weight?

Taking up as positive and down as negative

A

a < 0
T < mg
mg + ma < ma

As acceleration is 0, mg + ma > mg becomes mg - ma > ma

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23
Q

If the the lift is moving down and accelerating, what is the acceleration and relationship between tension and weight?

Taking up as positive and down as negative

A

a > 0
T < mg
mg + ma < ma

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24
Q

If the the lift is moving down and decelerating, what is the acceleration and relationship between tension and weight?

Taking up as positive and down as negative

A

a < 0
T > mg
mg + ma > mg

As acceleration is 0, mg + ma > mg becomes mg - ma > ma

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25
Q

For a lift that is moving down and decelerating, what is the direction of velocity and acceleration?

A

Velocity acts downwards (negative), but acceleration acts upwards (positive).

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26
Q

For a lift that is moving down and accelerating, what is the direction of velocity and acceleration?

A

Velocity acts downwards (negative), but acceleration acts downwards (negative).

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27
Q

So when is the tension in the cable less than the weight?

A

The lift is moving up and decelerating ( velocity > 0 and acceleration < 0)
The lift is moving down and accelerating (velocity < 0 and acceleration < 0)

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28
Q

So when is the tension in the cable greater than the weight?

A

The lift is moving up and accelerating (velocity > 0 and acceleration > 0)
The lift is moving down and decelerating (velocity < 0 and acceleration > 0)

29
Q

Consider two masses, M and m (where M > m) attached to a thread hung over a frictionless pulley. When released Mass M accelerates downwards and mass m accelerates upwards. If a is acceleration and T is the tension in the thread, then what is the resultant force on both masses?

Draw a diagram and label this.

A

On mass M, Mg - T = Ma
On mass m, T - mg = ma

Diagram on pg143

30
Q

What does adding the two equations for mass M and m give?

A

Mg - mg = (M + m)a

31
Q

Label the forces on a block of mass sliding down a slope. This includes the weight, component of the weight down the slope, the force of friction.

A

Page143

32
Q

What is the equation to calculate the resultant force on a block sliding down the slope?

A

mgsinθ - F₀ = ma

33
Q

If there is an engine force Fₑ, the slope equation can be applied to a vehicle on a downhill slope of constant gradient. How does the equation to calculate ma, change?

A

Fₑ + mgsinθ - F₀ = ma

where F₀ is the sum of air resistance and braking force. What about friction?

34
Q

Any object moving through a fluid experiences….

A

…a drag force (e.g. air resistance, water resistance)

35
Q

What does the drag force acting on an object depend on?

A
  • The shape of the object
  • Its speed
  • The viscosity of the fluid, which is a measure of how easily the fluid flows past a surface.
36
Q

How does the drag force acting on an object travelling a fluid change with speed?

A

The faster an object travels in a fluid, the greater the drag force on it.

37
Q

Describe the motion of an object falling in a fluid.

A
  • Initially, the speed of an object in a fluid increase as it falls, so the drag force on it due to the fluid increases.
  • The resultant force on the object is the difference between the weight (the force due to gravity) and the drag force.
  • As the drag force increases, the resultant force decreases, so the acceleration becomes less as it falls.
  • If it continues fallinf, it attains terminal speed. This is when the drag force on it is equal and opposite to its weight. Its acceleration is then zero and its speed remains constant as it falls.
38
Q

For an obejct, falling through a fluid, what is the expresion to give resultant force at any point? From this what is the equation to give the acceleration of the object?

A

F = mg - D
Resultant force = weight - drag force

F = ma –> a = F/m –> a = mg-D/m –> a = g - D/m

a = g - (D/m)

39
Q

What is the initial acceleration of a fallin object?

A

The initial acceleration of a falling object is g, because the speed is 0, so the drag force is also zero. At this point, the only force acting on it, is weight (force due to gravity), giving it an acceleration of 9.81ms⁻².

40
Q

When an object is travellling at constant speed, what happens to its potenttial energy?

A

At the terminal speed, the potential energy of the object is transferred into the internal energy of the fluid by the drag force as it falls.

41
Q

Terminal velocity is not for objects that are falling only. They can be applied to a diver moving up to the surface of the water (upthrust and water resistance).

We can also consider a powered vehicle. What does the top speed/terminal velocity reached by a road vehicle depend on?

A
  • Its engine power

- Its shape

42
Q

There are two vehicles. Both vehicles have teh same engine power, however one of them is streamlined and the other is not. Which one reaches a higher top speed/terminal velocity?

A

The streamlined car.

43
Q

What are the two forces acting on a powered vehicle, when it is trying to reach terminal velocity? So how is the resultant force acting on it calculated?

A

Fₑ - Fᵣ = Resultant force

Driving force/Motive force - Resistive forces (fricition + drag forces) = Resultant force

44
Q

Now that you know how to calculate the resultant force of a powered vehicle, how to calculate its acceleration?

A

a = Fₑ - Fᵣ / m

45
Q

When is the maximum speed/terminal speed of the powered vehicle reached?

A

This is when the resistive forces (i.e. friction and drag forces) becomes equal and opposite to the engine force, so acceleration = 0.

46
Q

How to calculate the acceleration of an object reaching terminal velocity, plotted on a velocity-time graph?

A

Calculate gradient at that point.

47
Q

What is the thinking distance?

A

The distance travelled by a vehicle in the time it takes the driver to react.

48
Q

Equation to calculate thinking distance?

A

v = s/t –> s = vt –> s₁ = vt₀
thinking distance = speed x reaction time

s₁ = thinking distance (m)
v = speed (m/s)
t₀ = reaction time (s)
49
Q

What are some things that affect a driver’s reaction time?

A
  • Distractions
  • Drugs
  • Alcohol
50
Q

What is braking distance?

A

The distance travelled by the car in the time it takes to stop, from when the brakes are first applied.

51
Q

What is the equation to calculate breaking distance?

A

Assuming constant deceleration to zero speed from speed u:
s₂ = u²/2a

s₂ = braking distance
u = initial velocity
a = deceleration (ms⁻²)
52
Q

What is the stopping distance?

A

The sum of the thinking distance and braking distance.

53
Q

How to calculate stopping distance?

A

s₁ + s₂ = ut₀ + u²/2a

‘u’ here is the speed before the brakes are applied.

54
Q

How does the speed of a vehicle affect stopping distance?

A

The greater the speed of the vehicles, the greater the stopping distance.

55
Q

What affects the braking distance?

A

The road conditions

The condition of the vehicle tyres

56
Q

What is the limiting frictional force?

A

This is when the upper limit of friction between the road and tyres is reached, at which point the vehicle skids.

57
Q

How will an icy road affect braking distance?

A

The friciton between an icy road and the wheels is much less, so the limiting frictional force is much lower. This means the breaks must be applied with less force, as to prevent the friction between the tyres and road reaching the limiting frictional force. As a result, the braking distance increases.

58
Q

Ho does tires with treads of low depth affect breaking distance?

A

Low depth treads mean that the ability of the tyres to grip the road is reduced. Therefore there is less friction between the tyres and the road, so the breaking force increases.

Ask sir if this explanation is okay.`

59
Q

You can express deceleration or accceleration in terms of g (the acceleration due to gravtiy). What is -30ms⁻² in terms of g? What is 20ms⁻² in terms of g?

A 
-30/9.81 = -3.05 = -3g
20/9.81 = 2.03 = 2g
60
Q

When objects collide and bounce off each other, they are in contact with each other for a certain time.

The shorter the contact time….

A

…the greater the impact force for the same initial velocities.

61
Q

When two vehicles collide with each other, they may or may not separate from each other after the collision. If they remain entangled, how long do they exert forces on each other?

A

They exert forces on each other until they are moving at the same velocity.

*In the case of when two object collide and seperate form each other, impact time (duration of impact force) = contact time. However impact time is not equal to contact time, when the two objects stay together after collision.

62
Q

For a vehicle of mass m in time t: what is the impact time?

A

t = 2s / u+v

t = impact time (seconds)
s = distance moved by the vehicle during the impact (m)
u = (initial velocity)
v = (final velocity)
63
Q

For a vehicle of mass m in time t: what is the acceleration?

A

a = v - u/t

64
Q

For a vehicle of mass m in time t: what is the impact force?

A

F = ma

65
Q

What is impact time?

A

The duration of the impact force.

Not the same as contact time, because they could still be in contact when there is no impact force

66
Q

How else can impact force be calculated?

A

Work done = Force x distance
In this case the work done is the change of kinetic energy, so

Impact force = Change in kinetic energy/impact distance

67
Q

How can we reduce the impact force of a car?

A

By increasing the impact time, so deceleration occurs over a larger time period and thus the impact force is reduced.

68
Q

What features in cars increase the impact time?

A

Vehicle bumpers
Crumple zones
Seat belts - belt acts as a restraining force on the wearer that a hus much less impact force that if the wearer were to hit the vehicle frame.
Collapsible steering wheels - upon impact between steering wheel and driver, the steering wheel collapses.
Airbags - Acts as cushion to increases impact time + spreads over the contact area, so pressure on body is less.

69
Q

Draw an arrow on a car to demonstrate the ‘friction due to braking’. On a seperate car, draw the friction caused by the motive force.

A

pg147

Physics AQA AS Level (16 decks)

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