Forces, movement, shape and momentum (continued)

1.20 know and use the relationship between momentum, mass and velocity: 

momentum (kg m/s) = mass (kg) x velocity (m/s)
             p                   =     m         x        v

1.21 use the idea of momentum to explain safety features 

force = change in momentum/time taken

This means that if the time taken for the momentum to change is increased, then the overall force will be smaller (divided by bigger value for time.) So, increasing the time taken for a change in momentum will reduce the force felt, which can be helpful in creating safety features for vehicles. For example, crumple zones increase the time it takes for a cars momentum to reach zero (to have stopped,) and therefore decrease the force felt by the car and the people in it. 

1.22 use the conservation of mass to calculate the mass, velocity or momentum of objects.

From formula triangle, it is evident that: 

• mass = momentum/velocity 
• velocity = momentum/mass 
• momentum = mass x velocity 

1.23 use the relationship between force, change in momentum, and time taken: 

force = change in momentum/time taken 

1.24 demonstrate an understanding of Newton's third law 

Newton's third law states that: For every action, there is an equal and opposite reaction. So, whenever a body exerts a force on another, that force is being exerted back, but in the opposite direction. One example would be a car on the road, which is exerting a downward force on the ground (due to gravity,) but the floor is exerting an upwards force, hence why the car is not sinking into the ground. Another example is shown in the picture below: one skater is exerting a force on the other, and as a result vice versa.

1.25 know and use the relationship between the moment of a force and its distance from the pivot: 

moment (Nm) = force (N) x perpendicular distance from pivot (m)

For example, if the force applied to a point is 10N and its perpendicular distance from the pivot is 0.8m, then the moment = force x perpendicular distance from pivot

 = 10N x 0.8m 

 = 8Nm.

1.26 recall that the weight of a body acts through its centre of gravity  

An objects's centre of gravity is where all of its weight acts through (weight is directly related to the objects centre of gravity, weight = mass x gravitational field strength.) 

1.27 know and use the principal of  moments for a simple system of parallel forces acting in one plane 

The principal of moments states that when the sum of the clockwise moments is equal to the sum of the anticlockwise moments, then the object will be balanced. 

These moments are equal (20N), so the object is balanced. Here, the sum of anticlockwise moments = sum of clockwise moments. 

The weight and distance are equal on both sides, so the moments are too. Because the sum of anticlockwise moments = sum of clockwise moments, the see-saw is said to be balanced. 

1.28 understand that the upward forces on a light beam, supported at its ends, vary with the position of a heavy object placed on the beam 

This means that, say you had a beam being held in balance by two springs, and you added a force, ie 10N to the beam, the upward force of the beam would have to exert more force (10N) so they equal the downward force (become balanced again.)

1.29 describe experiments to investigate how extension varies with applied force for helical springs, metal wires and rubber bands

1. Attach a spring to a newton metre and measure it's length 
2. Add a specific  weight (ie 50g) and measure again
3. Contiue to add more weight, preferably going up in the same units (50g each time)
4. Do this until you have measured 6-8 different weights, so up until you have added 300g-400g.
5. Plot a graph from your results, with the weights on one axis, and the lengths on the other. From the graph, it should be evident that as the force increases, so does the length of the spring (it s extension.) 

1.30 understand that the initial linear region of a force-extension graph is associated with Hooke's law. 

Hooke's law states that: the extension of an elastic object is directly proportional to the force applied to it. The initial linear region (straight, diagonal line) of a force-extension graph would show that the extension of said object increases as more force is applied, this is Hooke's law. 

The initial linear region of the graph shows that the extension of the elastic object is directly proportional to the force applied to it (Hooke's law.) The curve in the graph represents the object having reached its elastic potential (will not return to original shape after force is removed.)

1.31  describe elastic behaviour as the ability of a material to recover its original shape after the forces causing deformation have been removed. 

The initial linear part of the force extension graph above shows the correlation between the extension of an elastic object and the force applied to it. The straight line only continues up until the point where the object can no longer recover its original shape after having the force placed upon it, removed. This means the object has reached its elastic potential (what its elastic behaviour is) and can no longer recover its original shape, so no longer performs its elastic behaviour. 

d) Astronomy

d) Astronomy
1.32 understand gravitational field strength, g, and recall that it is different on
other planets and the moon from that on the Earth

Gravitational field strength (g) is the strength of the force that pulls everything on a planet towards its gravitational centre. The gravitational field strength of the Earth is 10N/kg, while the gravitational field strength of the moon is about one sixth of this. This is because the Earth is larger than the moon and the larger the planet, the larger its gravitational field strength. So, planets smaller than the Earth such as Jupiter and Neptune will have a smaller gravitational field strength than the Earth and those larger than the Earth such as Venus and Mars will have a larger gravitational field strength. 

1.33 explain that gravitational force

• causes moons to orbit planets

Planets are bigger and have a larger gravitational pull than their respective moons, so the moons orbit the planets because the pull is stronger and keeps them in their orbit. 

•  causes the planets to orbit the sun

The sun has is far larger than the planets in our solar system and therefore has a larger gravitational pull. This pulls on the planets and keeps them in its orbit. 

• causes artificial satellites to orbit the Earth

The Earth's gravitational pull keeps artificial satellites in its orbit.

• causes comets to orbit the sun

The Sun's strong gravitational pull causes comets to orbit it. A comet's orbit takes it very close to the sun and then far away again.

1.34 describe the differences in the orbits of comets, moons and planets

• Comets have elliptical orbits around the sun, which bring them close to the sun where they speed up due to the gravitational pull and then further out into the solar system.

• Moons orbit around planets because the planets are larger and therefore have a stronger gravitational pull.  They have elliptical (oval) orbits around their respective planets. 

• Planets complete an elliptical orbit around the sun, due to it's larger gravitational pull. 

1.35 use the relationship between orbital speed, orbital radius and time period:

orbital speed = (2 x π x orbital radius)/time period

Orbital radius = m (distance in metres)

Orbital speed = m/s (metres per second)

Time period = s (seconds) 

1.36 understand that:

• the universe is a large collection of billions of galaxies
• a galaxy is a large collection of billions of stars
• our solar system is in the Milky Way galaxy.

The universe contains billions of galaxies. These galaxies contain many stars, each of which have a solar system. Our solar system is a part of the Milky Way galaxy. 

c) Forces, movement, shape and momentum

c) Forces, movement, shape and momentum

1.9 describe the effects of forces between bodies such as changes in speed,
shape or direction

A force is a push or pull that can change the: speed, shape or direction of an object. (Force = mass x acceleration.)
Forces can affect bodies in different ways. 

• Speed - A force can increase or decrease the speed of an object, which will cause acceleration/deceleration. When accelerating, the forward force is larger than the backward force against the object. When decelerating, the backward force is larger than the forward force.  When an object is stationary it has an equal force pushing up and down and forwards and backwards.
• Shape - When a balanced force is acted upon an object, the object becomes stationary. However when a balanced force is acted upon a malleable object, the object changes shape to accommodate the force as it cannot move. 
• Direction - Which ever direction the force is greatest in will be the direction the object travels in.

1.10 identify different types of force such as gravitational or electrostatic

• Gravity or weight  - (Gravity gives everything a weight. It makes everything accelerate towards the ground. The weight of an object corresponds to the pull of its gravity.)

• Electrostatic force - (force between two charged objects, similar charges repel, opposite charges attract.) 

• Thrust - (or push or pull, occurs when a system expels mass in one direction and this mass will cause a force of equal magnitude but opposite direction on the system.)

• Friction (or drag) - (the force that resists movement between two surfaces that are in contact.)

• Tension - (Tension passes through strings, cables, ropes or wires when they are being pulled in opposite directions. The tension force is directed along the length of the wire and pulls equally on the object at the opposite ends of it.)

• Lift - (opposes the weight. Reaction to the surface, not allowing object to sink.)

• Reaction force (or contact force) - (when two objects are pushed together, come into 'contact,' they exert equal and opposite forces on each other. For example, the contact force from the ground pushes up when you stand and your weight pushes down.)

Weight/gravitational, electrostatic and magnetic force can act without touching an object.

                     

1.11 distinguish between vector and scalar quantities

Vectors have magnitude and a direction, whereas scalars just have a magnitude (ie.speed).

1.12 understand that force is a vector quantity

Force has a magnitude (measured in newtons) and acts in a direction. It is therefore a vector quantity.

1.13 find the resultant force of forces that act along a line 

The resultant force is the overall force acting in a direction on an object. This diagram shows how the forces should be added to reach the resultant force.

1.14 understand that friction is a force that opposes motion

Friction (or drag) resists movement between two surfaces that are in contact. It therefore opposes motion. 

1.15 know and use the relationship between unbalanced force, mass and acceleration:

Force = mass x acceleration 
Force (newtons, N) = mass (kg) x acceleration (m/s^2)

Example; What force must be applied to a 1kg bag of sugar to accelerate it by 15m/s^2.

Force = mass x acceleration, 

so force will be = 1 x 15 

 = 15N (newtons.)

What is the acceleration of a 1000 kg car when a force of 2500 N is applied to it?

Acceleration = force/mass 

So acceleration will be = 2500/1000

                                      = 2.5m/s^2.

1.16 know and use the relationship between weight, mass and g

Weight (N) = mass (kg) x gravitational field strength

W = m x g 

1.17 describe the forces acting on falling objects and explain why falling objects reach a terminal velocity. 

The two forces that act on falling objects are; gravity and drag (air resistance.)

When an object first starts to fall it is accelerating, because the downward force acting on it (gravity) is stronger than the opposing upwards force (drag or air resistance.) However, after some time the forces will become equal (balanced) and at this stage, the object will continue moving but will no longer accelerate. It is said to have reached its terminal velocity. 

1.18 describe experiments to investigate the forces acting on falling objects, such as sycamore seeds or parachutes. 

Parachutes

Dropping a parachute from a given height will immediately show gravity at work, as the parachute will begin to fall to the ground. However, if a person then jumped out of a plane with said parachute on, the force of gravity will be much quicker as it increases as the mass of the object does. Additionally, the area of the object will affect the drag force acting upon it. If the area is increased, say the parachute is opened, the object will face much more air resistance, and will therefore go at a slower speed. 


Sycamore seeds

• Collect a few sycamore seeds of different sizes (say 5.) 
• Measure them, and multiply their length by their width to find the surface area of each seed. 
• Hold one at the top of a ruler or another object you can measure length from, eg. a tape measure. 
• Drop each seed separately, but from the same height, and time how long it takes to reach the ground. 
• Repeat this process twice, to make sure you have taken note of the correct measurements. 
• Plot a scatter graph (surface area on y axis and time on x axis) to visually show your results. 
• If plotted correctly, the graph should display a straight line, going up as the graph travels right. This represents a positive correlation and shows you that the larger the surface area of the seed, the longer the seed took to fall (more surface area to experience air resistance, slowed down time it took for gravity to pull it to the ground.) 

1.19 describe the  factors affecting vehicle stopping distance including speed, mass, road condition and reaction time 

Vehicle stopping distance = thinking distance + braking distance 

Factors that effect the thinking and/or braking distance, and therefore the vehicle stopping distance: 

• Speed car is travelling at. The quicker the car is travelling, the larger the thinking/braking distance, therefore the larger the stopping distance. 

• Mass of vehicle (load,) the larger the mass, the longer the car will take to stop, increasing braking distance. 

• Road conditions - If the road is particularly wet, or bumpy or has sleet/snow, then these poorer conditions will increase the thinking/braking distance. 

• Reaction time - The time it takes for the person to see the obstacle and react to it. The longer this reaction time is, the larger the stopping distance will be as it will increase the thinking distance. (The driver's reaction time can be affected if they are drunk, under influence of drugs, have poor visibility etc.) 

Forces and Motion (Section 1)

Forces and Motion - a) Units

1.1 use the following units: kilogram (kg), metre (m), metre/second (m/s), metre/second2 (m/s2), newton (N), second (s), newton per kilogram (N/kg), kilogram metre/second (kg m/s).

kilogram (kg) = metric way of measuring mass.

metre (m) = metric measure for distance. 

metre/second (m/s) = this measures speed. (speed = distance/time)

metre/second^2 (m/s^2) = measure of acceleration.

newton (n) = measure of force. 

second (s) = measure of time. 

newton per kilogram (N/kg) = measure of acceleration. 

kilogram metre/second (kg m/ s) = measure of momentum. 

b) Movement and position

1.2 plot and interpret distance-time graphs 

Distance time graphs show the relationship between the distance travelled and the time taken to travel this distance. 

Plotting distance-time graphs

The y-axis refers to the distance travelled along the journey, and the x-axis refers to the time taken to travel this distance.  To plot, mark the distance travelled at each point and correspond with the time taken to get there. Continue horizontally along the line of your last point to represent a stop in the journey, while displaying the time that passed during this pause. 

Interpreting distance-time graphs 

• As mentioned before, the y-axis of a distance-time graph will refer to the distance travelled, and the x-axis will refer to the time taken to travel this distance. 
• Therefore, gradient of the line (change in y/change in x) will equal the speed of the journey (speed = distance/time).
• A straight horizontal line represents a stop in the journey, with the time obviously continuing but not the distance.
•  An upward line shows a journey away from the starting point, while a downward line shows a journey back to the starting point. 
• The steeper the line, the faster the object is travelling. 

1.3 know and use the relationship between average speed, distance moved and 

time: 

 average speed = distance moved/time taken. 

1.4 describe experiments to investigate the motion of everyday objects such as

toy cars or tennis balls

You can use the triangle above for this. Remembering that speed = distance/time, you can find the motion of an everyday object. For example, if an object travels 5m in 10seconds then motion(speed) = distance/time = 5/10 = 0.5m/s, 

1.5 know and use the relationship between acceleration, velocity and time:

Acceleration = change in velocity/time taken 

so, a = (v-u)/t 

where v = final velocity, 

          u = initial velocity 

          t = time 

1.6 plot and interpret velocity-time graphs

• Gradient = acceleration
• Flat sections represent steady speed. 
• The steeper the graph, the greater the acceleration or deceleration. 
• Uphill sections (/) = acceleration
• Downhill sections = deceleration.
• A curve represents a changing acceleration. 
• The area under any section of the graph = the distance travelled in that time interval. 
• Speed at any point = read off value from the velocity axis. 

1.7 determine acceleration from the gradient of a velocity-time graph

The acceleration of a velocity-time graph = the gradient. 

Gradient = vertical/horizontal 

              = velocity/time 

In the velocity-time graph displayed above, the gradient of the first line = velocity/time 

 = 20/20 

= 1m/s^2                    

     

1.8 determine the distance travelled from the area between a velocity-time

graph and the time axis.

The distance can be calculated by finding the area between the velocity axis and the time axis.

So, the area of triangle= 1/2 x 8 x 4 

                                     = 16

Area of rectangle = 8 x 6 

                             = 48

Total area = 16 + 48 

                  = 64m.