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GCSE Physics > Topic 5 - Forces > Flashcards

Topic 5 - Forces Flashcards

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1
Q

What is a force?

A

A push or a pull. Its an interaction between at least 2 bodies.

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2
Q

What 3 things can forces change?

A
  • Change the speed of an object
  • Change the direction of an object
  • Change the shape of an object
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3
Q

How are forces measured and represented?

A

Measured in Newtons (N) and represented with arrows.

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4
Q

What is a contact force?

A

When the two bodies physically touch.

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5
Q

What are 9 examples of contact forces?

A
  • Normal force/ Reaction Force
  • Friction
  • Air resistance/ Drag
  • Water resistance
  • Tension
  • Spring force
  • Thrust
  • Upthrust/ Buoyancy
  • Lift
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6
Q

What is normal force/reaction force? (3)

A
  • Exists when 2 bodies are touching.
  • It is the force that stops objects passing through each other.
  • Acts at 90 degrees to the surface of contact.
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7
Q

What is friction? (4)

A
  • Exists when 2 bodies are touching and moving relative to each other.
  • It’s the force that resists the motion of the object.
  • It acts parallel to the surface of contact and opposes the direction of motion/intended direction of motion.
  • An object at rest on a slope must have friction acting up on it => friction acts up the slope.
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8
Q

What is air resistance/drag? (5)

A
  • Type of friction force where it only exists when objects are moving through air.
  • It is caused by the collision of air particles with the body in motion.
  • It opposes motion.
  • Higher speeds = higher air resistance
  • Larger cross sectional area = larger air resistance
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9
Q

What is water resistance? (2)

A
  • Similar to air resistance, but water molecules instead of air molecules.
  • An object travelling through water always experiences water resistance.
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10
Q

What is tension? (4)

A
  • Tension is the force acting when a material is stretched.
  • It is always there when a rope, wire, or spring is attached to a body.
  • Always a pull force, as ropes can never be pushed.
  • It acts away from the body it is attached to and along the rope.
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11
Q

What is spring force? (2)

A
  • A sprint always wants to restore to its original shape.
  • If a spring is being pulled right, then spring force is acting left and vice versa.
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12
Q

What is thrust?

A

Forward force of an object that has an engine, or one that is launched.

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13
Q

What is upthrust/buoyancy? (2)

A
  • All objects that float in a fluid, e.g, rubber duck, balloon, etc.
  • Acts upwards => away from fluid
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14
Q

What is lift?

A

The force that holds objects with wings up, e.g, planes, butterflies etc.

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15
Q

What is a non-contact force?

A

Force that acts at a distance.

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16
Q

What are 5 examples of non-contact forces?

A
  • Gravitational force/ Weight
  • Electrostatic force
  • Magnetic force
  • Strong force
  • Weak force
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17
Q

What is gravitational force/ weight/

A

Force of attraction between objects that have mass (acts towards centre of earth).

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18
Q

What is electrostatic force?

A

Force between objects that have a charge.

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19
Q

What is magnetic force?

A

Force between objects that have a magnetic field, like a magnet.

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20
Q

What is strong force?

A

Force that binds neutrons and protons together in the nucleus of an atom.

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21
Q

What is weak force?

A

Linked to radioactive decay of protons and neutrons.

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22
Q

What is a free body diagram?

A

A diagram representing all the forces acting on an object, with proportionate arrows and labels of newtons.

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23
Q

What is the resultant force and what does it allow us to do?

A
  • The sum of all forces acting on a body is called the resultant force.
  • It allows us to figure out the motion of a body.
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24
Q

What must you do when calculating resultant force?

A

We need to take the direction of any forces acting when we find the total.

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25
Q

What do forces in the same and opposite directions do in relation to resultant force?

A
  • Same direction, add up
  • Opposite direction, cancel out/ balance
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26
Q

When is the resultant force balanced and unbalanced?

A
  • When resultant force = 0 they are balanced
  • When it doesn’t = 0, they are unbalanced
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27
Q

What is mass?

A

Amount of matter in a substance, doesn’t change no matter where in the universe you are. Measured in kg.

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28
Q

What is gravitational field strength measured in?

A

N/Kg

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29
Q

How is weight measured?

A

Weight is measured using a calibrated spring-balance (a newtonmeter).

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30
Q

Where does the weight of an object act?

A

The weight of an object may be considered to act at a single point referred to as the object’s ‘centre of mass’.

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31
Q

How are weight and mass proportional?

A

The weight of an object and the mass of an object are directly proportional.

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32
Q

What is the equation to calculate weight?

A

W = mg

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33
Q

What does the value of gravitational field strength depend on?

A
  • Mass of the planet/satellite
  • Distance to the centre of mass of the planet/satellite
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34
Q

What is the gravitational field strength on earth?

A

9.8 N/Kg

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35
Q

What is the gravitational filed strength on the moon?

A

1.6 N/Kg

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36
Q

Look at flow diagram for balanced forces on phone.

A

Rate based on how well you know it.

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37
Q

Look at flow diagram for unbalanced forces on phone.

A

Rate based on how well you know it.

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38
Q

How can you find gravitational filed strength when given weight and mass?

A

g = W/m

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39
Q

How can you find gravitational filed strength when given 2 objects with the same radius but different mass, and the value of g for one of them?

A

gB = gA x number of times B is more massive than A

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40
Q

How can you find gravitational filed strength when given g at the surface, and need to find g at a distance away from the surface? Use e.g, of the surface having 24 N/Kg and needing to find 4 times away from this

A

g = 1/4(2) of 24 = 1.5 N/Kg

Inverse square law

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41
Q

How can you find gravitational filed strength when both mass and radius change compared to a known planet?

A

g = (gA x number of times B is more massive than A) / (radius of B compared to A) squared

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42
Q

Why do we feel heavy?

A

The feeling of being heavy/ having weight is due to the normal force => the push of the ground on you.

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43
Q

Why do you feel weightless in space?

A

The lack of normal force in space gives the sensation of being weightless.

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44
Q

What is weightlessness sometimes called and why is this wrong?

A

Objects that are experiencing weightlessness are said to be in zero-g, but g is not 0. Zero g is equivalent to no normal force.

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45
Q

What does it mean if something is in free fall?

A

When something is in orbit, the only force acting is the gravitational force. When this happens, we say it’s in free fall.

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46
Q

Do all objects fall at the same rate when there is no air resistance?

A

All objects fall at the same rate (same distance in same time) when there is no air resistance. How massive the object is has no influence on how quickly it falls.

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47
Q

How does air resistance affect how fast something falls?

A

In the presence of air resistance, the resultant force acting on objects changes, which affects how fas they fall.

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48
Q

What is static equilibrium?

A

When forces are balanced but the object is at rest, so it stays at rest.

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49
Q

What is dynamic equilibrium?

A

When forces are balanced but the object is already moving, so the object carries on moving in the same direction at the same speed.

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50
Q

How does a parachutist fall? (6 steps)

A
  1. Speed increasing rapidly downwards as there’s no air resistance.
  2. Speed increases still, but not as rapidly due to air resistance.
  3. Reached terminal velocity, so falling down at a constant speed => dynamic equilibrium.
  4. Parachute comes out, so speed iOS decreasing but they are still falling down.
  5. Reached terminal velocity again, but its slower than before.
  6. Falls to a complete stop, because normal force is bigger than weight when they hit the ground.
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51
Q

What is the rate of acceleration for a parachutist on earth?

A
  • g = 9.8 N/Kg so travelling at 9.8 m/s per second
  • means they’re gaining an extra 9.8 m/s every second
  • 1 second = 9.8 m/s
    2 seconds = 19.6 m/s
    3 second = 28.4 m/s
    etc.
  • air resistance can/will affect these numbers in some way
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52
Q

What is the resultant force acting on an object falling at terminal velocity?

A

0N

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53
Q

Why do objects falling through air eventually reach a terminal velocity?

A

The air resistance acting on a falling object increases as its speed increases. So as falling objects accelerate, the air resistance increases until it is equal to the objects weight. At this point the resultant force on the object is 0, so it is no longer accelerating and travels at a steady speed (its terminal velocity).

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54
Q

Why does a parachutists velocity decrease after opening the parachute if they had already reached their terminal velocity?

A

Before the parachute is opened, the weight of the skydiver acting downward equals the air resistance travelling upwards (because of terminal velocity). Opening the parachute increases the area of the skydiver, so the air resistance acting upwards on the skydiver increases whilst their weight remains the same. This causes the air resistance to be larger than the weight, so the velocity of the skydiver will decrease.

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55
Q

What are the variables of the required practical involving springs and extensions?

A
  • Independent variable = Force, F
  • Dependent variable = Extension, e
  • Control variable = Spring constant, k
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56
Q

What is the 8 step method for Investigating Force & Extension required practical?

A

1.Set up the apparatus as shown in the diagram, initially without any masses hanging from the spring.
2. Align the marker to a value on the ruler, record this initial length of the spring.
3. Add the 100 mass hanger onto the spring.
4. Record the mass (in kg) and position (in cm) from the ruler now that the spring has extended
5. Add another 100 g to the mass hanger.
6. Record the new mass and position from the ruler now that the spring has extended further.
7. Repeat this process until all masses have been added
8. The masses are then removed and the entire process repeated again, until it has been carried out a total of three times, and an average length is calculated

57
Q

Look at picture on phone of labelled diagram for Investigating Force & Extension required practical.

A

Rate based on how well you know it.

58
Q

What 2 conclusions can be made from the Hooke’s law experiment?

A
  1. The larger the force applied to the spring, the larger the extension of the spring is.
  2. The applied force is directly proportional to the extension (LBF is a straight line through the origin).
59
Q

What is Hooke’s Law?

A

The force applied to a spring is directly proportional to the extension of a spring.

60
Q

What equation can be derived from Hooke’s law?

A

F = Ke

F is applied force in N
K is spring constant in N/cm
e is extension in cm or m

61
Q

What is a spring constant?

A

a measure of the stiffness of a spring

62
Q

How do you set up a force extension graph?

A

Force on the y-axis, and extension on the x so the gradient represents the spring constant.
If the axes are warped, the gradient is no longer spring constant it is the reciprocal => 1/k.

63
Q

What is the limit of proportionality in relation to springs and graphs?

A

Beyond this value of force, the spring no longer follows Hooke’s law. However, when the load is removed, the spring still returns to its original length.

64
Q

What is the elastic limit in relation to springs and graphs?

A

The point beyond which if a larger force were applied, the spring no longer goes back to its original length. The spring becomes permanently deformed.

65
Q

What does it mean if an object has been elastically deformed?

A

It can go back to its original shape and length after the force has been removed => it is an elastic object.

66
Q

If you have a spring with an extension of 2 cm and a force applied of 1N, what happens to the spring constant when you put them in a series?

A
  • Original
  • F = Ke
  • K = F/e = 1/2 = 0.5 N/cm
  • Series
  • e doubles so becomes 4
  • K = F/e = 1/4 = 0.25 N/cm
  • K halves
66
Q

What does it mean if an object has been inelastically deformed?

A

It doesn’t return to its original shape and length after the force has been removed.

67
Q

If you have a spring with an extension of 2cm and a force applied of 40N, what happens to the spring constant when you put 2 springs in parallel?

A
  • Original
  • K = F/e = 40/2 = 20 N/cm
  • Parallel
  • e halves so becomes 1
  • K = F/e = 40/1 = 40 N/cm
  • Spring constant doubles
68
Q

Does a force that stretches or compresses a spring do work? How is energy stored?

A

A force that stretches (or compresses) a spring does work and elastic potential energy is stored in the spring. Provided the spring is not inelastically deformed, the work done on the spring and the elastic potential energy stored are equal.

69
Q

What is static friction?

A

The force that needs to be overcome to get an object moving, is called static friction.

70
Q

What is kinetic/dynamic friction?

A

The force that needs to be overcome to keep an object, that is already moving, in motion is called kinetic/dynamic friction.

71
Q

In what 2 ways is friction caused?

A
  • Rougher surfaces have ridges and bumps which catch on each other.
  • All materials are made up of tiny particles, called molecules, and these have a tendency to stick to each other when materials are pressed together.
72
Q

What does the size of the fictional force depend on? (2)

A
  • The type of surfaces in contact
  • How hard the surfaces are pressed together
73
Q

Which is usually larger static or kinetic/dynamic friction and why?

A

The size of the static friction between two surfaces is generally larger than the kinetic/dynamic friction between them. This is because it is easier to keep an object in motion once it’s already moving than to begin its movement in the first place.

74
Q

What are the factors affecting friction (class practical conclusions)? (3)

A
  • For the same 2 surfaces, the larger the mass of the object, the larger the static friction is.
    The larger the mass is, the larger the weight so the surface areas are pressed together even more, increasing the friction.
    Static friction is directly proportional to the weight of a body.
  • The type of surface the object is in contact with. In our experiment, out of the floor, the carpet and the bench => floor provided most static friction for the same mass.
  • The area of contact between surfaces is not a factor that affects friction.
75
Q

What is fluid friction with examples?

A

Gases and liquids are called fluids. There is fluid friction whenever an object moves through a fluid. Examples include; air resistance and water resistance.

76
Q

What are 2 ways of increasing the top speed of a vehicle?

A
  1. Reducing drag/air resistance. This can be done by altering the shape of a vehicle to make it more streamlined.
  2. Increasing the power of the vehicles engine. This way, the driving force becomes larger and so the drag force on the vehicle will equal the driving force at a higher speed.
77
Q

What are some typical speed values for someone walking, running and cycling?

A
  • walking - 1.5 m/s
  • running - 3 m/s
  • cycling - 6 m/s
78
Q

What are some typical speed values for a car, train and plane??

A
  • car - 25 m/s
  • train - 55 m/s
  • plane - 250 m/s
79
Q

How do you calculate speed?

A
  • speed = sdistance travelled / time
  • V = s/t
  • distance, s, in metres, m
    speed, v, in metres per second, m/s time, t, in seconds, s
80
Q

In what two ways can you calculate average speed?

A
  • V(avg) = total distance / total time
  • V(avg) = u + V / 2
    initial speed / velocity = u
    final speed / velocity = V
81
Q

How do you calculate acceleration?

A
  • acceleration = change in speed / time taken
  • a = V - u / t
  • a = change in V / change in t
  • acceleration, s, in metres per second squared, m/s(2)
    speed, v, in metres per second, m/s time, t, in seconds, s
    OR
  • (final velocity) squared - (initial velocity) squared = 2 x acceleration x distance
  • V(2) − u(2) = 2 x a x s
82
Q

How do you calculate distance for a constant speed?

A

s = V x t

83
Q

How do you calculate distance for a gradually changing speed?

A

s = V(avg) x t

84
Q

What does Newton’s first law state?

A

If the resultant force acting on an object is zero and:
* the object is stationary, the object remains stationary
* the object is moving, the object continues to move at the same speed and in the same direction. So the object continues to move at the same velocity

85
Q

What did Newton’s experiments prove in relation to his 2nd law (basically what is his 2nd law and how did he come to this conclusion)?

A
  1. Resultant force is directly proportional to acceleration.
  2. The acceleration is inversely proportional to the mass of the body for the same force.
    * This means F is directly proportional to ma , so if we choose the constant to be 1, the equation becomes;
    F = ma
    F is resultant force in N
    m is mass in Kg
    a is acceleration in m/s(2)
86
Q

What are the variables for Newton’s 2nd law required practical?

A
  • Independent variable = force, F
  • Dependent variable = acceleration, a
  • Control variable = Mass, m
87
Q

What is the 5 step method for Newton’s 2nd law required practical?

A
  1. Connect the ticket timer tape to the cart and put it through ticket timer.
  2. Place the cart at the starting line, and place the necessary masses on the cart and at the end of the strong.
  3. Let go of the cart after starting the ticket timer.
  4. Turn the ticket timer off once the masses hit the floor.
  5. Repeat the steps for ‘repeat’ masses, and the different required masses.
88
Q

How do you use the ticket timer in Newton’s 2nd law required practical to calculate acceleration?

A
  • Produces 50 dots in 1 second so the separation between 2 dots in 0.02s. That means 10 dots = 0.2s
  • Measure the length of the first 10 dots and the last 10 dots to calculate the u and v values.
  • With this data you can calculate acceleration
89
Q

How can Newton’s 2nd law required practical method be improved?

A
  • An air track can be used to reduce friction.
  • To improve time measurements, a light gate can be used.
90
Q

When is work done?

A

When a force causes an object to move through a distance work is done on the object. So a force does work on an object when the force causes a displacement of the object.

91
Q

What is the equation for work done?

A
  • Work done = Force x (distance travelled in the direction of the force)
  • W = Fd
  • W = work done in joules, J
    F = Force in newtons, N
    d = distance in metres, m
92
Q

When is one joule of work done?

A

One joule of work is done when a force of one newton causes a displacement of one metre.
1 joule = 1 newton-metre

93
Q

Where is work done applied?

A

Only in the direction of motion

94
Q

How can you calculate work done in the situation of for example a car, if there is both friction and thrust forces?

A

You can subtract the work done by friction, from the work done by the thrust if necessary.

95
Q

What does work done by friction/against friction convert into?

A

It is converted into heat/thermal energy. Any remaining work done is turned into kinetic energy.

96
Q

What happens to the body if all the work done on that body, is equal to the work done by friction?

A

There is no net gain in energy, and the body moves at a constant speed.

97
Q

What happens to a body if the work done by friction is larger than the work done by applied force?

A

The body will slow down, and come to rest if it was already in motion.

98
Q

What happens to the work done on an object, if there is no friction and no change in height (relative to some point)?

A

All the work done would be converted into kinetic energy, so the object would gain speed.

99
Q

What happens when work is done against the gravitational force/weight?

A

The energy is converted into gravitational potential energy. This involves an object changing/gaining height. Lifting an object involves working against the pull of gravity.

100
Q

What happens if there is work done against fictional forces - when there is already gravitational potential energy?

A

Some of the GPE is converted into heat energy (caused by the friction) and any left over energy is converted into kinetic energy.

101
Q

What do Newton’s laws allow us to do?

A

Newton’s laws allow us to work out the subsequent motion of a body if we know the forces acting on it.

102
Q

What does Newton’s first law state?

A

When the resultant force acting on a body is zero (forces are balanced), then:
- a body at rest remains at rest
- a body in motion will carry on moving in the same direction at the same speed.

The reverse is also true; objects at rest/moving the same must have a resultant force of 0.

103
Q

What is Newton’s first law also referred to as?

A

The law of inertia.

104
Q

What is inertia?

A

The unwillingness of a body to change its state of motion.

105
Q

What is an example of inertia? (4)

A
  • When a person in a speeding car feels pushed into their seat.
  • Although they have the sensation of being pushed, there is no force acting on them in that direction.
  • The reason for this is the passenger has inertia.
  • Alternatively, when a car slows down, passengers feel like they are pushed out/ forwards.
106
Q

How does ‘a passenger in a speeding car feeling pushed into their seat’ show inertia?

A

As the car speeds up, so does the seat which is attached to the car, but the person is carrying on at the speed the car was travelling before it sped up.

107
Q

What is Newton’s second law? (2)

A
  • When there is a resultant force acting on an object, then the object’s speed will change (non-zero acceleration).
  • The acceleration is directly proportional to the resultant force and inversely proportional to its mass.
108
Q

What is the derived equation from Newton’s second law?

A

F = ma

m => mass in kg
a => acceleration in m/s squared
F => resultant force in N

109
Q

What is Newton’s third law? (3)

A
  • The third law states that for every force there is an equal and opposite force.
  • It is often referred to as the “action - reaction law” => for every action there is an equal and opposite reaction.
  • It states that forces always act in pairs.
110
Q

How would you explain the pairs of forces in Newton’s third law?

A

When two bodies, A and B, are interacting with each other, the force of A on B is the same magnitude of the force of B on A but in the opposite direction.

111
Q

What is important about the pairs of forces in Newton’s third law? (2)

A
  • These action-reaction pairs have the same nature (i.e. they are both pulls, or they are both linked to friction, or they are both to do with gravitational force etc.).
  • They act on different bodies.
112
Q

Why would the Normal force and Weight not be 3rd law pairs for a person standing at rest on the Earth’s surface?

A

Despite being equal and opposite in directions, they both act on the same body (the person) therefore they can’t be 3rd law pairs. They are like this due to the 1st law.

113
Q

What would the 3rd law pairs be for a person standing at rest on the Earth’s surface? (2)

A
  • The normal & the push of the person on the ground.
  • Weight & gravitational pull of person on earth.
114
Q

What is the turning moment of a force?

A

Turning moment of a force = Force x Perpendicular distance from the pivot to the line of action of the force

115
Q

What kind of quantity is moment?

A

Vector quantity => only Clockwise and Anticlockwise

116
Q

What is the unit for moments?

A

Nm

117
Q

What must happen for an object to be in equilibrium (regarding moments)? (2)

A
  • Sum of the clockwise moments = Sum of the anti-clockwise moments
  • Forces are balanced.
118
Q

What would a small push do to a stable object?

A

It can create a moment, which is balanced by the moment of the weight => stabilises block.

119
Q

How would you topple a relatively stable object? (3)

A
  • The line of action of the weight, needs to create a moment in the same direction as the push.
  • The critical point is where the line of action of the weight passes over the pivot.
  • At this point the object is at an unstable equilibrium.
120
Q

What 2 things must objects have for stability?

A
  • A wide base
  • A low centre of mass
121
Q

How do levers work?

A

They increase the distance from the pivot at which the force is applied => means less force is needed to get the same moment.

122
Q

Why are levers known as force multipliers?

A

As they reduce the amount of force that’s needed to get the same moment, by increasing the distance.

123
Q

What are gears?

A

Gears are circular discs with ‘teeth’ round the edge. The teeth interlock, so turning one gear causes another to turn in the opposite direction.

124
Q

What are gears used for?

A

they’re used to transmit the rotational effects of force from one place to another. E.g, three linked gears will all turn together when one gear is turned.

125
Q

How does size affect gears? (3)

A
  • Different sized gears can be used to change the moment of a force.
  • A force transmitted to a larger gear will cause a bigger moment, as distance from edge to pivot is greater.
  • A larger gear will turn slower than a smaller gear.
126
Q

How do bicycles use gears?

A

Bicycles use gears to transmit turning effect of pedals to the back wheel => you can change gear to alter the ratio between how fast you pedal and how fast the wheels turn.

127
Q

What is a lever? (definition)

A

A simple machine.

128
Q

What is a fulcrum? (definition)

A

The point a lever turns around OR the point at which it pivots.

129
Q

What is the effort? (definition)

A

The force you put in.

130
Q

What is the load? (definition)

A

The force that you overcome.

131
Q
A
132
Q

What are the 4 SUVAT equations?

A
  • v = u + at
  • s = ( (u + v) / 2 ) t
  • v squared = u squared + 2as
  • s = ut + 1/2 a(t) squared
133
Q

When do you use the SUVAT equations? (3)

A
  • When acceleration is constant.
  • Object dropped from a height.
  • Objects thrown upwards or forwards.
134
Q

How do you use the SUVAT equations? (4)

A
  • Read the question and list the quantities given.
  • In order for a problem to be solvable we need to have at least 3 of those quantities.
  • Do not forget that displacement, velocity and acceleration are vectors, so directions are importantly ( + and - ).
  • Choose the equation that gives the required or missing quantities.
135
Q

What does SUVAT stand for?

A

s = displacement
u = initial speed/velocity
v = final speed/velocity
a = acceleration
t = time

Include units.

136
Q

What should you ignore when calculation 2d motion?

A

Air resistance

137
Q

How do you work out 2d motion => steps? (6)

A
  1. Analyse the horizontal and vertical motions separately.
  2. The acceleration in the horizontal distance = 0 (assume there is no air resistance).
  3. The initial and final horizontal velocities are the same.
  4. The acceleration in the vertical = 9.8 m/s squared.
  5. The initial vertical velocity is zero.
  6. Time is the same for both.
138
Q

How do 2 of the SUVAT equations change for 2d motion?

A
  • s = ut for horizontal distance as there is no acceleration
  • s = 1/2 a(t) squared for vertical distance as there is no initial speed

GCSE Physics (8 decks)

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