1. Physics
Science AS1.6

2. NSF North Mississippi GK-8
Topics to be covered
Energy
Work & Power
Heat
Motion
Forces
Momentum
Electricity
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3. Its about calculations
Don’t panic!!
Read the question CAREFULLY
WHY?
The examiners will often give you the two variables that you need to complete the calculation.
Check your answer – is it reasonable and realistic? If not, you’ve made a mistake!!
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4. Achieve, Merit, Excellence
Achieved questions – purely calculations given the variables.
Merit questions – calculations where you use the variable from the achieved part of the question.
Excellence questions – calculations up to a merit level with some discussion on the effects of different values of the variables.
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How it works
You get marks for the whole calculation –
State the formula
State the variables
Do the calculation
Get the answer
Remember the units – the answer is no good without them.
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Starter
Name three non-contact forces:
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7. Contact vs non-contact forces
Contact forces: Touch the object they are acting on
Pushing
Pulling
Twisting
Non-contact forces: move something with touching it
Gravity
Magnetic
Electrical
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Force
Forces act on us all the time.
We know a force is acting on an object if it deforms or changes the motion of the object.
Force is measured in Newtons (N).
The fractions are worked like the metric system.
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If two equal-sized forces act in opposite directions on an object then the net force is zero. The object is said to be in equilibrium.
Balanced forces on a body can be recognised when an object
is (a) stationary.
(b) moving at a constant speed.
The forces are balanced,
the box does not move
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When the action and reaction forces are equal in size
and opposite in direction they are said to be balanced.
If we increase the force from the right what happens?
The forces are now unbalanced, the box moves in the
direction of the imbalance and should do so indefinitely
in a frictionless environment.
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Forces are balanced so the object doesn’t move.
Some forces become unbalanced and the object moves
in the direction of the extra force.
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12. Movement and unbalanced forces
When the action force is different in size or direction from the reaction force, then the forces are said to be unbalanced.
The resultant unbalanced force always accelerates an object in the direction of the net force.
This means that an accelerating object is always acted on by unbalanced forces.
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13. Calculating the unbalanced force
For forces acting in opposite directions, the unbalanced force can be calculated by subtracting the smaller force from the larger one.
The resultant unbalanced force is in the direction of the larger force.
10 N
35 N
25 N
Two opposite forces Resulting unbalanced force
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More force rules
80 N + 25 N in the same direction = a single force of . . .
80 N + 25 N in opposite directions = a single force of . . .
55 N + 25 N in one direction and 80 N in the opposite directions =
NO MOVEMENT
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Forces
Gravity
Friction
Thrust
Support
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Weight vs Mass
Weight: is the defined as the force of gravity pulling downwards on an object and is measured in newtons
Mass: is measured in kilograms
Weight force = mass of object x acceleration due to gravity
Fgravity = m x g
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17. Acceleration due to gravity
On the surface of earth the potential acceleration of an object due to force of gravity is 10ms-2
If your mass on earth is 60kg, the the weight force acting on your body is given by 60kg x 10ms-2 = 600N
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Starter
The passengers in a car of mass 1500kg have the following masses; 125kg, 105kg, 95kg, and 75kg
Calculate the total mass of the car and passengers
What would the car and passengers weigh?
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Question
My puppy Oatis has a weight of 70N.
What is his mass?
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Starter
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23. Force, mass, acceleration
If the net force increases the acceleration will also increase. A motor bike can produce much more thrust than a scooter so its acceleration will be greater.
Acceleration is directly proportional to the net force
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24. Force, mass, acceleration
If the mass of an object increases, then its acceleration will decrease. A scooter with a pillion passenger will not be able to drag off a scooter with a single rider.
Acceleration is inversely proportional to mass
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25. The relationship between unbalanced forces and acceleration
This is given by the equation.
Force (F) = mass (m) x acceleration (a)
F = m.a
Force in newtons, mass in kg and acceleration is ms-2
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Example
A toy with a mass of 1.5 kg is pulled by a string and accelerates at 2 ms-1. What is the size of the force?
F=m.a
= 1.5 x 2
= 3 N
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Example
A force of 500 N is used to accelerate a 2 kg object. What is the acceleration?
F=m.a
a = F/m
= 500/2
= 250 ms-1
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Example
An object accelerates at 7.5 ms-2 when a force of N is applied to it. What is the mass of the object?
F=m.a
m=F/a
= 15000/7.5
= 2000 kg
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Tandem skydiving
Ariana is skydiving for the first time
Her and her jumpmaster have a combined mass of 150kg
Using Fg = mg find their weight
They jump out of the plane, one force acting on them is gravity
Name the other force and state the direction
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During the free fall the net force acting on adriana and the jumpmaster is 825N
Calculate their net acceleration.
Discuss how when released the parachute reduces their speed.
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32. Motion
This is about the observation, measurement and analysis of moving bodies.

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Acceleration – an increase in an object’s speed (or direction) caused by unbalanced forces
Deceleration – a decrease in an object’s speed (or direction) caused by unbalanced forces
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Distance – time graphs
It is possible to measure the distance of an object as it moves away from a starting position and plot this information on a graph.
This is called a distance-time graph.
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Stopped
Distance from
start (m)
Constant speed
Constant speed
decelerating
accelerating
Time (seconds)
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Stopped
Distance from
start (m)
Time (seconds)
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37. Draw the distance-time graphs for the below
A car traveling at top speed along the motorway
A Mother bird fetching food for its chick
A skateboarder going down a hill
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Distance, time, speed
Distance-time graphs give information about the speed of an object. The slope/gradient of a distance-time graph equals the speed.
Different slopes describe differences in speed
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Speed and velocity
An object has speed when it travels a distance in a given time.
The formula is:-
Speed (v) = distance travelled (d)/time (t)
v = d/t
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40. Remember to get the units right
Distance is measured in metres (m)
Time is measured in seconds (s)
Speed is metres/second (m/s or ms-1)
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41. VELOCITY
The word scientists use to describe how fast an object is moving
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42. Instantaneous and average speed
At any given instant an object has a speed. This is called instantaneous speed. In a car this is measured by the speedometer.
The average speed, is the speed of an object during a given period of time.
It takes into account the many different instantaneous speeds during the journey.
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Average speed
The distance becomes the total distance travelled.
The time becomes the total time taken to complete the journey.
An example
The bus to school travels at a number of different (instantaneous) speeds at it stops and starts, but at the end of the journey has traveled at an average speed for the whole journey.
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Constant speed
A body traveling at a constant speed is traveling equal distances during a similar time period.
This means that the average speed is the same as the instantaneous speed in this case.
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45. Velocity is measured as:
distance travelled over time taken
Vaverage = distance
time D
V T
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46. What are some examples of objects that move at high velocities?
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47. What are some examples of objects that move at low velocities?
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What units would you use to measure the velocity of an object moving in this classroom?
meters per second m
s or ms-1
Meters per second tells us how many meters something can move in one second.
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49. Graphing the Motion of a Particle?
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Problems
A traffic officer finds a car has travelled 600m in 20 seconds. What is the average speed of the car
Two snails are being raced in a school lab. Eric went 8cm in 24seconds, while splutter went 12cm in 48seconds. Which snail is faster?
A car is travelling at a constant speed of 30ms-1. How far does it travel in 40seconds
A shock wave from an earthquake travels at a speed of 8kms-1. How long does it take to reach a town 96 km away?
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51. What else can we know about an object’s motion besides velocity?
What if the object speeds up?
What if the object slows down?
What if the object changes direction
What if the object’s velocity changes?
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52. Scientists have a special way to measure a change in velocity:
Acceleration
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The Way Acceleration Works:
If you’re speeding up, you have positive acceleration.
If you’re slowing down, you have negative acceleration.
If you’re changing speed quickly, you have high acceleration.
If you’re changing speed slowly, you have low acceleration.
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Example:
You’re driving along in your car and suddenly have to slam on your brakes for a red light. Negative or positive acceleration? High or low acceleration?
High negative acceleration because you’re slowing down quickly.
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Example:
You’re an astronaut waiting in your shuttle for takeoff. Suddenly the rockets fire and you’re on your way to Mars. High or low positive or negative acceleration?
High positive acceleration because you’re speeding up quickly.
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Example:
You’re driving down the highway when your car runs out of gas and you slowly roll to a stop on the side of the road.
Low negative acceleration because you’re slowing down slowly.
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Challenge Question:
You throw a baseball straight up in the air. It rises to a certain height and then falls back to earth, where you catch it again in your glove.
First, high positive acceleration because it speeds up quickly as it leaves your hand.
Then, as it rises, low negative acceleration, because it begins to slow down slowly because of gravity.
As it begins to fall again, low positive acceleration, because it speeds up slowly.
Finally, when you catch it, high negative acceleration because it slows down very quickly in your hand.
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Acceleration
Acceleration = change in speed
change in time
a = v t v
a t
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Practice Problem
An eagle accelerates from 15 m/s to 22 m/s in 4 seconds. What is the eagle’s rate of acceleration?
Acceleration = change in speed
change in time
Acceleration = 22 m/s - 15 m/s s
Acceleration =
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Problems
A car is moving at 15 ms-1. It then speeds up to 35ms-1 in 40 seconds. What is the acceleration?
A toy car moves along a concrete path when its wound up and let go. The car accelerates from rest to a top speed of 6ms-1 in 0.5seconds. What is the acceleration?
Find the speed of an object when it accelerates at 4.5ms-2 for 20 seconds
Find the time taken for an object accelerating at 3 ms-2 to change speed by 12ms-1
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61. What causes an object’s velocity to change?
Quick Review
Velocity: speed of motion
Acceleration: change in velocity
What causes an object’s velocity to change?
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If you’re riding your bike and want to speed up, what do you do?
If you’re driving a car and want to slow down, what do you do?
If you’re playing soccer and want to move the ball, what do you do?
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Pushing pedals, pressing brakes, and kicking soccer balls are all examples of forces.
A force is anything that changes the velocity of an object.
But what’s a change in velocity?
A change in velocity is acceleration, so . . .
A force is anything that gives an object acceleration.
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64. Only a force can change the velocity of an object.
Nothing will change speed (acceleration) unless acted upon by a force. If it’s sitting still, it will keep sitting still. If it’s moving fast, it will keep moving fast.
Only a force can change the velocity of an object.
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65. Then why do things slow down?
Flying baseballs slow down and eventually stop because of the force of gravity.
Cars and bikes eventually slow down and stop because of the force of friction.
Spacecraft in outer space with no friction and no gravity will keep going at the same speed forever without a force to slow them down or speed them up.
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Starter
What is an object’s speed of motion?
What is a change of an object’s velocity?
What is the only thing that can change an object’s velocity and give it acceleration?
What affects fast a force can accelerate an object?
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Speed – time graphs
A speed-time graph gives information about acceleration.
Acceleration is the slope of a speed-time graph
The greater the slope of a speed-time graph, the greater the acceleration
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More graph – calculate the acceleration
at the points indicated. 30
a= v/t D 25
(25-20)/(12-8) C 20
(20-10)/(8-6) 15
Velocity (m/s) B 10
(10-5)/(6-2) A
(25-5)/(14-12) 5 E
(5-0)/(2-0) F
(5-2)/(16-14)
Time (s)
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Finding distance
Speed- time graphs can be used to find the distance it takes an object to reach a certain speed.
This distance is also the area underneath the slope of the graph.
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70. Area under graph = Distance
Area can be either a triangle or a square or a combination
Area of triangle = ½ base x height
Area of a square = length x width
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a= v/t
Velocity (m/s)
Time (s)
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