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Physics > Topic 5-Forces > Flashcards

Topic 5-Forces Flashcards

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1
Q

What is a scalar quantity?

A

A quantity that only has a magnitude

A quantity that isn’t direction dependent

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2
Q

What is a vector quantity?

A

A quantity that has both a magnitude and an associated direction

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3
Q

What are 5 examples of scalar quantities?

A

Speed

Distance

Time

Mass

Energy

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4
Q

What are 5 examples of vector quantity?

A

Velocity

Displacement

Acceleration

Force

Momentum

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5
Q

How can a vector quantity be drawn and what does it show?

A

It is drawn as an arrow

The length of the arrow represents the magnitude

The arrow points in the associated direction

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6
Q

What is a force?

A

A push or pull acting on an object due to an interaction with another object

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7
Q

What are the two categories that all forces can be split into?

A

Contact forces (objects touching)

Non-contact forces (objects separated)

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8
Q

Give three examples of contact forces

A

Friction

Air resistance

Tension

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9
Q

Give three examples of non-contact forces

A

Gravitational forces

Electrostatic forces

Magnetic forces

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10
Q

Is force a vector or a scalar quantity?

A

Vector

It has both a magnitude and an associated direction

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11
Q

What is weight?

A

The force that acts on an object due to gravity and the object’s mass

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12
Q

What quantities does weight depend on?

A

The objects mass

The gravitational field strength at the given position in the field

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13
Q

What is the unit used for weight?

A

The Newton (N)

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14
Q

What is the unit used for gravitational field strength?

A

N/kg

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15
Q

What is meant by an object’s centre of mass?

A

The single point where an object’s weight can be considered to act through

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16
Q

What piece of equipment can be used to measure an object’s weight?

A

A calibrated spring-balance or newton-meter

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17
Q

What is the name given to the single force that is equivalent to all the other forces acting on a given object?

A

The resultant force

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18
Q

Describe the forces acting upon a skydiver from when they initially jump to when they reach terminal velocity

A

As they fall, they accelerate, increasing their speed

As air resistance increases, the resultant force from weight decreases

So acceleration decreases, so they are not speeding up as quickly

Eventually they are equal and balance, so there is no resultant force.

So there is no acceleration when the resultant force is 0. They travel at terminal velocity

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19
Q

What does it mean if a force is said to do ‘work’?

A

The force causes an object to be displaced through a distance

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20
Q

Units for work done, force and distance

A

Work done (Joules)

Force (Newtons)

Distance (metres)

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21
Q

What distance must be used when calculating work done?

A

It must be the distance that is moved along the line of action of the force

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22
Q

Under what circumstance is 1 joule of work done?

A

When a force of 1 Newton causes a displacement of 1 metre

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23
Q

How many Newton-metres are equal to 1 joule of energy?

A

1Nm = 1J

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24
Q

How many Joules in a kilojoule?

A

1Kj=1000J

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25
Q

What occurs when work is done against frictional forces?

A

A rise in temperature of the object occurs because kinetic energy is converted to heat

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26
Q

Why does air resistance slow down a projectile?

A

The object does work against the air resistance

Kinetic energy is converted in to heat, slowing down the object

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27
Q

Explain the relationship between the force applied and the extension of an elastic object

A

The extension is directly proportional to the force applied, provided that the limit of proportionality is not exceeded (Hooke’s Law)

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28
Q

What is meant by an inelastic deformation?

A

A deformation which results in the object being permanently stretched

The object doesn’t return to its original shape when the force is removed

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29
Q

Units for force, spring constant and extension

A

Force (Newtons)
Spring Constant (N/m)
Extension (m)

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30
Q

What type of energy is stored in a spring when it is stretched

A

Elastic potential energy

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31
Q

What can extension be replaced with in the equation for spring force?

A

Compression

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32
Q

What is meant by the term ‘fluid’?

A

A liquid of a gas

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33
Q

In any fluid, at what angle do the forces due to pressure act on a given surface?

A

At right angles (normal to) the surface

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34
Q

Units for pressure, force and area

A

Pressure (Pascals)
Force (Newtons)
Area (Metres squared)

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35
Q

What happens to the density of the atmosphere with increasing altitude?

A

The atmosphere becomes less dense as altitude increases

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36
Q

Explain why atmospheric pressure decreases with an increase in height

A

Pressure is created by collisions of air molecules

The quantity of molecules (and so weight) decreases as the height increases

This means atmospheric pressure decreases with an increase of height

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37
Q

What is the Earth’s atmosphere?

A

A thin (relative to the magnitude of the Earth) layer of gas surrounding the Earth

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38
Q

How does pressure in fluids increase with depth?

A

As the depth increases, the mass of liquid above that depth also increases

This means that force due to the mass increases

Since the force has increases whilst the area has remained constant, the pressure will increase

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39
Q

Why does pressure in fluids increase with density?

A

As the density of a fluid increases, the number of particles in a given volume increases

Consequently the weight of the fluid is greater

This means that the force from the fluid above a certain point is larger

Since the force has increased, the pressure also increases

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40
Q

Units for pressure due to a column of liquid, column height, density and gravitational field strength

A

Pressure (Pa)

Column height (m)

Density (kg/metres cubed)

GFS (N/kg)

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41
Q

Why are objects in a fluid subject to upthrust?

A

When an object is submerged in fluid, it has a higher pressure below it than it does above, as there is more weight above it at the bottom than there is at the top.

This leads to an upwards force called upthrust. You float or sink depending on whether the upthrust is more or less than your weight

42
Q

What is upthrust always equal to?

A

The weight of the fluid that the object displaces

43
Q

What factors influence whether an object will sink or float?

A

Upthrust

Weight

Density of fluid

44
Q

Explain why an object with a density greater than that of water can never float

A

Upthrust is equal to the weight of the displaced fluid

If the density of the object is high, there would not be enough volume displaced to produce an upthrust larger than the object’s weight. This means that it will sink

45
Q

If an object moves 3 metres to the left and then 3 metres back to its initial position, what is the object’s total displacement?

A

The object has zero displacement

Displacement is a vector quantity so it also involves direction

The object starts and ends at the same point

46
Q

Typical value for human walking speed?

A

1.5m/s

47
Q

Typical value for the speed of sound

A

330 m/s

48
Q

Typical value for human running speed?

A

3 m/s

49
Q

Typical human cycling speed?

A

6 m/s

50
Q

Why can an object travelling at a constant speed in a circle not have a constant velocity?

A

Speed is a scalar quantity

Velocity is a vector quantity which means it can only be constant if the direction is constant

In circular motion, the direction is continuously changing

51
Q

How can speed be calculated from a distance-time graph?

A

The speed is equal to the gradient of the graph

52
Q

What must be done to calculate speed at a given time from a distance-time graph for an accelerating object?

A

Drawing a tangent to the curve at the required time

Calculating the gradient of the tangent

53
Q

State the units for acceleration, velocity and time

A

Acceleration (m/s squared)

Velocity (m/s)

Time (s)

54
Q

How can the distance travelled by an object be calculated from velocity-time graph?

A

It is equal to the area under the graph

55
Q

Give an approximate value for the acceleration of an object in free fall under gravity near the Earth’s surface

A

9.8 m/s squared

56
Q

What can be said about the resultant force acting on an object when it is falling at terminal velocity?

A

The resultant force is zero

When at terminal velocity, the object is moving at a constant speed and so isn’t accelerating

57
Q

State Newton’s first law for a stationary object

A

If the resultant force on a stationary object is zero, the object will remain at rest

58
Q

State Newton’s first law for a moving object

A

If the resultant force on a moving object zero, the object will remain at constant velocity (same speed in same direction)

59
Q

What can be said about the braking forces and driving forces when a car is travelling at constant velocity?

A

The braking forces are equal to the driving forces

60
Q

If an object changes direction but remains at a constant speed, is there a resultant force?

A

Since there is a change in direction, there is a change in velocity and so there must be resultant force

61
Q

What is inertia?

A

The tendency of an object to continue in its state of rest or uniform motion

62
Q

State the defining equation for Newton’s Second Law

A

Resultant force = Mass x Acceleration

63
Q

State Newton’s Second Law in words

A

An object’s acceleration is directly proportional to the resultant force acting on it and inversely proportional to its mass

64
Q

What is inertial mass?

A

A measure of how difficult it is to change a given object’s velocity

The ratio of force over acceleration

65
Q

What is the symbol used to represent an approximate value?

A

~

66
Q

State Newton’s Third Law

A

Whenever two objects interact, the force that they exert on each other are always equal and opposite

67
Q

What is the stopping distance of a vehicle equal to?

A

The sum of thinking distance and braking distance

68
Q

For a given braking distance, if the vehicle’s speed is increased, what can be said about its stopping distance?

A

The stopping distance is increased with an increase in speed

69
Q

Give a typical range of values for human reaction time

A

0.2 seconds - 0.9 seconds

70
Q

Give three factors which can affect a driver’s reaction time

A

Tiredness

Drugs

Alcohol

71
Q

Give two factors which may affect braking distance

A

Adverse (wet/icy) road conditions

Poor tyre/brake conditions

72
Q

Describe the energy transfers that take place when a car applies its brakes

A

Work is done by the friction force between the brakes and wheel

Kinetic energy of the wheel is converted to heat and is dissipated to the surroundings through the brake discs

73
Q

To stop a car in a given, if its velocity is increased, what must happen to the braking force applied?

A

The braking force must also be increased

74
Q

State two consequences of a vehicle undergoing very large decelerations

A

Kinetic energy converted to heat is very high causing brakes to overheat

Loss of control of the vehicle

75
Q

What is the unit used for momentum?

A

kg m/s

Kilogram metres per seconds

76
Q

In a closed system, what can be said about the momentum before and after a collision?

A

The total momentum before is equal to the total momentum afterwards

77
Q

What quantity is equal to the force experienced in a collision?

A

The rate of change of momentum

78
Q

If an object’s change of momentum is fixed, what is the only way to reduce the force that the object experiences?

A

Increase the length of time over which the change of momentum occurs

79
Q

Explain how a seatbelt improves a passenger’s safety during a collision

A

Passenger must decelerate from the vehicle’s velocity at impact to zero, meaning the undergo a fixed change of momentum

The force they experience is equal to the rate of change of momentum

Seatbelts increase the time over which the force is applied, reducing the rate of change of momentum and therefore reducing the force experienced

80
Q

The weightlifter has lifted a weight of 2250 newtons above his head. The weight is held still

What force is used by the weightlifter to hold the weight still

Give a reason for your answer

A

2250N

Forces must be balanced

81
Q

At the end of the lift, the powerlifter hold the bar stationary, above his head, for two seconds

How much work does the powerlifter do on the bar during these two seconds

Give a reason for your answer

A

0

Work is only done when a force makes an object move

82
Q

A slide is made of plastic

The child becomes electrically charged when he goes down the slide

Explain why

A

There is friction between the child and slide

This causes electrons to transfer between the child and the slide

83
Q

Going down the plastic slide causes the child’s hair to stand on end

What conclusion about the electrical charge on the child’s hair can be made from this observation? Give a reason for your answer

A

All the charges on the hair are the same

Charges/hairs are repelling

84
Q

Why would the child not become electrically charged if the slide was made from metal?

A

The charge would pass through the metal to earth

85
Q

Ice hockey players wear protective pads filled with foam

Explain how the protective pads help to reduce injury when the players collide

A

Protective pads increases the time taken to stop during the collision

So the rate of change of momentum decreases

Reducing the force (on the ice hockey player)

86
Q

As the aircraft moves along the runway to take off, its acceleration decreases even though the force from the engines is constant. Explain why.

A

As speed increases, air resistance increases

This reduces the resultant force

87
Q

What happens to the kinetic energy of the bus as it is braking?

A

Kinetic energy decreases and is transferred by heating to the breaks

88
Q

A fielder, as he catches a cricket ball, pulls his hands backwards.

Explain why this action reduces the force on his hands.

A

It increases the time to change/reduce momentum

89
Q

Applying the brakes of the car causes the temperature of the brakes to increase.

Explain why.

A

Work done by friction between brakes and wheel

Causes a decrease in KE and increase in thermal energy

90
Q

Hybrid cars have an electric engine and a petrol engine. This type of car is often fitted with a regenerative braking system.

A regenerative braking system not only slows a car down but at the same time causes a generator to charge the car’s battery.

State and explain the benefit of a hybrid car being fitted with a regenerative braking system.

A

The battery needs recharging less often and increases the range of the car as the efficiency of the car is increased

The decrease in kinetic energy charges the battery

91
Q

The data needed to draw the bar chart was obtained using a robotic athlete fitted with electronic sensors

Why is this data likely to be more reliable than data obtained using human athletes

A

Conditions can be repeated

With humans, the conditions needed to repeat tests may not be constant

92
Q

Once a skateboard starts to recoil, it slows down and its kinetic energy decreases

Explain why

A

Work is done

Against friction

Transforming kinetic energy into heat

93
Q

In a collision, momentum is not always conserved

Why?

A

An external force acts on the colliding objects

94
Q

The skateboard moves backwards as the skateboarder jumps forwards.

Explain, using the idea of momentum, why the skateboard moves backwards.

A

Momentum before jumping=momentum after jumping

Before jumping, momentum and skateboarder is zero

After jumping skateboarder has momentum forwards so skateboard must have equal momentum backwards

95
Q

The student used the spring, a set of weights and a ruler to investigate how the extension of the spring depended on the weight hanging from the spring.

Before taking any measurements, the student adjusted the ruler to make it vertical.

Explain why adjusting the ruler was important.

A

To reduce the error in measuring the extension of the spring

As the ruler at an angle would make the measured extensions shorter

96
Q

The student used the spring, a set of weights and a ruler to investigate how the extension of the spring depended on the weight hanging from the spring.

Describe one technique that you could have used to improve the accuracy of the measurements taken by the student.

A

Attach a horizontal pointer to the bottom of the spring

So that the pointer goes across the ruler scale

97
Q

A student investigated the behaviour of springs. She had a box of identical springs.

The student suspended a spring from a rod by one of its loops. A force was applied to the spring by suspending a mass from it.

No other masses were provided.

Explain how the student could test if the spring was behaving elastically.

A

Remove the mass

Observe if the spring returns to its original length

98
Q

Explain why atmospheric pressure decreases with increasing altitude

A

Air molecules colliding with a surface create pressure

At increasing altitude distance between molecules increases

So number of collisions with a surface decreases

99
Q

The diagram shows the cross-section of one type of aircraft window.

Explain why the window has been designed to have this shape.

A

Force from air pressure acting from inside to outside bigger than force acting inwards

So keeps the window in position

100
Q

What property of a liquid enables a hydraulic brake system to work?

A

Liquids are incompressible

101
Q

Musicians often use loudspeakers

The loudspeaker cone vibrates when an alternating current flows through the coil.

Explain why.

A

The current flowing through the coil creates a magnetic field around the coil

This magnetic field interacts with the permanent magnetic field

This produces a resultant force and the cone moves

When the direction of the current changes, the direction of the force changes to the opposite direction

Physics (10 decks)

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