CGP P5

Question 1

Vector quantities

Answer 1

. Vector quantities have a magnitude (size) and a direction
. Examples of vector quantities include:
. Force, velocity, displacement, acceleration, momentum

Question 2

Scalar quantities

Answer 2

. Physical quantities that only have magnitude (size) and no direction are known as scalar quantities
. Examples of scalar quantities include:
. Speed, distance, mass, temperature, time

Question 3

How are vectors represented

Answer 3

. Vectors are usually represented using an arrow, the length of the arrow shows the magnitude, and the direction represents the direction of the quantity

Question 4

Forces

Answer 4

. A force is a push or pull on an object that is caused by it interacting with something
. All forces are either contact or non-contact

. When two objects interact, there is a force produced on both objects

Question 5

What is an interaction pair

Answer 5

. An interaction is a pair of forces that are equal and opposite and act on two interacting objects
. This is basically Newton’s third law

Question 6

Sun and the Earth

Answer 6

The Sun and the Earth are attracted to each other by the gravitational force. This is a non-contact force. An equal but opposite force of attraction is felt by both the Sun and the Earth

Question 7

Chair and the Ground

Answer 7

. A chair exerts a force on the ground, whilst the ground pushes back at the chair, with the same force (the normal contact force). Equal but opposite forces are felt by both the chair and the ground

Question 8

Gravity

Answer 8

Gravity attracts all masses, but you only notice it when one of the masses is really big for example anything near a planet is attracted to it very strongly. This has two important effects:
1. On the surface of the planet, it makes all things fall towards the ground
2. It gives everything a weight

Question 9

Mass

Answer 9

Mass is just the amount of matter in an object. For any given object, this will have the same value anywhere in the universe

Question 10

Weight

Answer 10

. Weight is the force acting on an object due to gravity (the pull of the gravitational force on an object). Close to Earth, this force is caused by the gravitational field around the Earth.

. The weight of an object depends on the strength of of the gravitational field at the location of the object. This means that the weight of an object changes it’s location.

Question 11

Gravitational field strength

Answer 11

. Gravitational field strength is the pull of the gravitational force on an object
. Gravitational field strength varies on location. It’s stronger the closer you are to the mass causing the field, and stronger for larger masses

Question 12

Mass and weight example

Answer 12

. For example an object has the same mass whether it’s on the Earth or the Moon - but it’s weight will be different. A 1kg mass will weigh less on the moon than the earth because the gravitational field strength on the surface of the Moon is less

Question 13

Weight measurements and center of mass

Answer 13

. Weight is a force measured in newtons.
. You can think of force as acting from a single point on the object, called it’s center of mass (a point at which you assume the whole mass is concentrated)
. For a uniform object, this will be the center of the object

. Mass is not a force, it is measured with a mass balance in kilograms

Question 14

Mass and Weight calculation

Answer 14

. Mass and Weight are directly proportional
. You can calculate the weight of an object if you know its mass and the strength of the gravitational field it is in

Weight (N) = Mass (Kg) x Gravitational Field Strength (N/kg)

Question 15

Free body diagrams

Answer 15

. Free body diagrams describe all the forces acting on an isolated object or a system
. For example, a skydiver’s weight acts on him pulling him towards the ground and drag (air resistance) acts on him, pushing him in the opposite direction
. The sizes of the arrows show the relative magnitudes of the forces and the directions show the directions of the forces acting on the object

Question 16

Resultant force

Answer 16

. If you have a number of forces acting at a single point, you can replace them with a single force (so long as the single force has the same effect as the original forces together)
. This single force is called the resultant force
.

Question 17

Work

Answer 17

. If a resultant force moves an object, work is doe
. To make something move, force must be applied
. The thing applying the force needs a source of energy
. The force does work to move the object and energy is transferred from one store to another

Question 18

Equation to find work done

Answer 18

Work done (W) = force (N) x distance (s)

. One joule of work is done when a force of one newton causes an object to move a distance of one meter. You need to be able to convert joules to newton meter: 1J = 1Nm

Question 19

How do we use scale drawings to find resultant forces

Answer 19

1. Draw all the forces acting on an object, to scale ‘tip-to-tail’
2. Then draw a straight line from the start of the first force to the end of the last force - this is the resultant force
3. Measure the length of the resultant force on the diagram to find the magnitude and the angle to find the direction of the force

(remember example)

Question 20

Finding if an object is in equilibrium

Answer 20

. If all of the forces acting on an object combine to give a resultant force of zero, the object is in equilibrium

. On a scale diagram, this means that the tip of the last force you draw should end where the tail of the first force you drew begins.

. You might be given forces acting on an object and told to find a missing force, given that the object is in equilibrium. To do this, draw out the forces you do know (to scale and tip-to-tail), join the end of the last force to the start of the first force. This line is the missing force so you can measure its size and direction

Question 21

Splitting a force into components

Answer 21

. Not all forces act horizontally or vertically - some act at awkward angles
. To make these easier to deal with, they can be split into two components at right angles to each other
. Acting together, these components have the same effect as the single force
. You can resolve a force (split it into components) by drawing it on a scale grid. Draw the force to scale, and then add the horizontal and vertical components along the grid lines. Then you can just measure them.

Question 22

Stretching, Compressing or Bending

Answer 22

Stretching, compressing or bending transfers energy
. When you apply a force to an object, you may cause it to stretch, compress or bend
. To do this, you need more than one force acting on the object (otherwise the object would just move in the direction of the applied force)

Question 23

Elastically deformed

Answer 23

. An object has been elastically deformed if it can go back to its original shape and length after the force has been removed
. Objects that can be elastically deformed are called elastic objects (e.g a spring)

Question 24

Inelastically deformed

Answer 24

. An object has been Inelastically deformed if it doesn’t return to its original shape and length after the force has been removed

. Work is done when a force stretches or compresses an object and causes energy to be transferred to the elastic potential energy store of the object. If it is elastically deformed, all this energy is transferred to the objects elastic potential energy store

Question 25

Extension is Directly proportional to force

Answer 25

If a spring is supported at the top and then a weight is attached to the bottom, it stretches
. The extension of a stretched spring (or other elastic object) is directly proportional to the load or force applied

Question 26

Equation for force and extension

Answer 26

F = Ke
Force (N) = Spring constant (k) x Extension (e)

. The spring constant depends on the material that you are stretching - a stiffer spring has a greater spring constant

. The equation also works for compression

Question 27

Limit of proportionality

Answer 27

There’s a limit to the amount of force you can apply to an object for the extension to keep on increasing proportionally

. The graph shows force against extension for an elastic object
. There is a maximum force above which the graph curves, showing that extension is no longer proportional to force. This is known as the li it of proportionality and is shown on the graph at the point marked P p

Question 28

Investigating springs

Answer 28

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Question 29

What is a moment

Answer 29

. A force, or several forces ca cause an object to rotate. The turning effect of a force is called its moment

Question 30

Equation for finding the size of the moment of the force

Answer 30

M = Fd
Moment of a force (nm) = Force (N) x Distance (m)

Question 31

Turning force on a spanner example

Answer 31

. The force on the spanner causes a turning effect or moment on the nut (which acts as pivot). A larger force or a longer distance (spanner) would mean a larger moment
. To get the maximum moment (or turning effect) you need to push at right angles (perpendicular) to the spanner. Pushing at any other angle means a smaller distance and so a smaller moment

. If the total anticlockwise movement equals the total clockwise moment about a pivot, the object is balanced and won’t turn. You can use the equation above to find a missing force or distance in these situations

Question 32

Levers

Answer 32

Levers increase the distance from the pivot at which the force is applied. Since M=Fd this means less force is needed to get the same moment. This means levers make it easier to do work e.g. lift a load or turn a nut

Question 33

Gears

Answer 33

Gears transmit rotational effects
. Gears are circular discs with ‘teeth’ around their edges
. Their teeth interlock so that turning one causes another to turn, in the opposite direction
. They are used to transmit the rotational effect of a force from one place to another
. Different sized gears can be used to change the moment of the force. A force transmitted to a larger gear will cause a bigger moment, as the distance to the pivot is greater
. The large gear will turn slower than the smaller gear

Question 34

Fluid pressure

Answer 34

Pressure is the force per unit area
. Fluids are substances that can flow because their particles are able to move around
. As these particles move around, they collide with surfaces and other particles
. Particles are light, but they still have a mass and exert a force on the object they collide with. Pressure is force per unit area, so this means the particles exert a pressure
. The pressure of a fluid means a force is exerted normal (at right angles) to any surface in contact with the fluid

Question 35

How can we calculate the pressure at the surface of a fluid by using:

Answer 35

P = F / A
Pressure in Pascals (Pa) = Force Normal to a surface (N) / Area of that surface (m2)

Question 36

Pressure in a liquid

Answer 36

Pressure in a liquid depends on depth and density
. Density is a measure of the ‘compactness’ of a substance i.e. how close together the particles in a substance are
. The more dense a liquid is, the more particles it has in a certain space. This means there are more particles that are able to collide so the pressure is higher
. As the depth of the liquid increases, the number of particles above that point increases. The weight of these particles adds to the pressure felt at that point, so liquid pressure increases with depth

Question 37

How can we calculate the pressure at a certain depth due to the column of liquid above

Answer 37

p = h ρ g
Pressure (Pa) = Depth (m) x Density of liquid (kg/m3) x gravitational field strength (N/kg)

Question 38

Objects in fluids experience upthrust

Answer 38

. When an object is submerged in a fluid, the pressure of the fluid exerts a force on it from every direction
. Pressure increases with depth, so the force exerted on the bottom of the object is larger than the force acting on top of the object
. This causes a resultant force upwards, known as upthrust
. The upthrust is equal to the weight of fluid that has been displaced by the object

Question 39

If an objects floats

Answer 39

If the upthrust on an object is equal to the object’s weight, then the forces balance and the object floats
. If an object’s weight is more than the upthrust, the object sinks
. Whether or not an object will float depends on its density
. An object that is less dense than the fluid it is placed in weighs less than the equivalent volume of fluid. this means it displaces a volume of fluid that is equal to its weight before it is completely submerged
. At this point, the object’s weight is equal to the upthrust, so the object floats
. An object that is denser than the fluid it is placed in is unable to displace enough fluid to equal its weight. This means that its weight is always larger than the upthrust, so it sinks

Question 40

Atmospheric pressure

Answer 40

Atmospheric pressure decreases with height
. The atmosphere is a layer of air that surrounds Earth. It is thin compared to the size of the Earth
. Atmospheric pressure is created on a surface by air molecules colliding with the surface
. As the altitude increases, atmospheric pressure decreases
. This is because as the altitude increases, the atmosphere gets less dense, so there are fewer air molecules that are able to collide with the surface
. There are also fewer air molecules above a surface as the height increases. This means that the weight of the air above it, which contributes to atmospheric pressure, decreases with altitude

Question 41

Distance and displacement

Answer 41

. Distance is just how far an object has moved. it is a scalar quantity (doesn’t involve direction)
. Displacement is a vector quantity, it measures distance and direction in a straight line from an object’s starting to finishing point
. If you walk 5m north and 5m south, your displacement is 0m but distance travelled is 10m

Question 42

Speed and velocity

Answer 42

Speed and velocity both measure how fast you’re going, but speed is a scalar and velocity is a vector
. Speed is just how fast you’re going with no regard to direction
. Velocity is speed in a given direction

. This means you can have objects travelling at a constant speed with a changing velocity.
. This happens when the object is changing direction whilst staying at the same speed.
. An object moving in a circle at a constant speed has a constantly changing velocity, as the direction is always changing

Question 43

How do we measure the speed of an object

Answer 43

. If you want to measure the speed of an object that’s moving with a constant speed, you should time how long it takes the object to travel a certain distance. You can then calculate the object’s speed from your measurements using the formula:
s = vt
distance travelled (m) = speed (m/s) x time (s)