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iGCSE Section 1 Force and Motion

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Jim Moyes

on 24 November 2013

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Transcript of iGCSE Section 1 Force and Motion

Force and Motion
Stopping Cars
Objective:
List the factors that effect how far a car travels before stopping.
Factors that effect how far you travel while thinking:
Alcohol
Tiredness
drugs
distractions
mobile phones
speed of car
Factors that effect how far a car travels while you are braking
Speed
mass
tyres
brakes
weather
road surface
Stopping distance = Thinking distance + braking distance
Stopping distance: total distance traveled while coming to a stop.
Thinking distance: distance traveled while thinking
Braking distance: distance traveled while braking
Exam hint
If asked what increases the braking distance you need to talk about worn tyres, poor brakes, wet weather. YOU CAN NOT JUST SAY BRAKES AND TYRES
Energy transfers in braking:
The Kinetic energy of the car is turned into thermal energy by the brakes.
All of the kinetic energy is turned into thermal energy, unless there is some sound.
Braking Forces
If the braking force is greater than the grip with the road then the vehicle skids
To be able to explain the factors that determine how far a car travels before coming to a stop
Falling objects
Objectives
To be able to explain the forces involved when objects fall and why objects reach terminal velocity
Label the forces acting on the parachutist as they fall. The parachute has been open for 10 seconds (Remember the size of the arrow represents the size of the force)
Drag
Weight
What is drag force
When an object moves through a fluid it collides with particles.
These exert a force on the object.
The more collisions the greater the force.
The drag can be increased by:
Increasing the surface area
increasing the speed
Moving through a denser fluid (eg water rather than air)
Terminal Velocity
This is the maximum speed an object can fall at.
Write 5 bullet points to explain why falling objects reach terminal velocity in terms of the forces on them, and why they have a lower terminal velocity when the parachute is open
At first weight > drag so object accelerates
As speed increases drag increases
Eventually drag=weight and the object falls at a steady speed.
Experiment
You are going to drop a piece of A4 paper and time how long it takes to fall.
Then fold it in half and repeat, do this until you can not fold in half any more.
Think about:
What your dependent, independent and control variables are
draw a results table
explain your results in terms of forces
Momentum
Recap
Close your books

On your white board write 3 things you
have learnt this lesson.
Safety!
In order to reduce the impact force we:

Increase the time taken to stop
This reduces the rate of change of momentum
This reduces the force.


This applies to all safety /padding situations.
When you want to write it absorbs the force
DON’T! write this instead!
Momentum
Momentum is a measure of how hard an
object is to stop.
You need to know:
How we calculate it
What it is measured in


You can ask me 10 yes or no questions to find out
this information.
Hint: if you know how to calculate it you can work
out the unit.
Objectives
To be able to calculate the momentum of an object.

To be able to explain how to reduce the impact force during a collision, and its implications for car safety.
Momentum
Collisions
Impact force

Impact Force = Change in momentum / time taken

Eg a ball of mass 250g is going at 30m/s.
It is caught and the persons hands take 0.15
seconds to stop the ball. What is the impact force?
Force = change in mom / time
= ((0.25 x 30)-0) / 0.15
= 7.5/0.15 = 50N
Collisions
List the safety features of a
car that try and protect the
driver after impact
How would you catch an
egg if thrown at you.

How do you catch a cricket
ball to stop your hands hurting.
Momentum



Momentum = mass x velocity
Starter
List as many things as you can that an apple
and a haddock have in common.

You have 1 minute
Calculations
Find the momentum in the following cases.
Velocity = 20m/s mass = 6kg
Velocity = 15m/s mass = 10kg
Velocity = 27m/s mass = 73 kg
Velocity = 320m/s mass = 50g
Velocity = 2m/s mass = 250kg
Find the velocity in the following cases
Momentum = 20kgm/s mass = 4kg
Momentum = 72kgm/s mass = 8kg
Mom = 50000kgm/s mass = 25kg
Mom = 2000kgm/s mass = 25g
Mom = 650000kgm/s mass = 4000kg
Crumple zones

Air bags

Cars
Questions
Weight of paper remains constant
drag reduces as folded
Resultant downward force (weight -drag) is greater.
Accelerates more quickly
Takes less time to fall
Hooke's Law
describe experiments to investigate how extension varies with applied force for helical springs, metal wires and rubber bands

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

describe elastic behaviour as the ability of a material to recover its original shape after the forces causing deformation have been removed.
Objectives
Write down the units the following quantities are measured in

Speed
Velocity
Mass
Energy
Momentum
Force
Time
Starter
Write down the units the following quantities are measured in

Speed m/s
Velocity m/s
Mass kg
Energy J
Momentum Kgm/s
Force N
Time s
Experiment
You need to write a 5 bullet point plan explaining the experiment you would do to show how extension varies with force applied to a spring
suspend spring from a clamp stand that is secured in place
Hang a know mass from the spring and record the force it exerts and the extension
increase the mass and record extension
Repeat for 10 different masses and then slowly unload the spring recording Force and extension.
Plot Force Vs extension
Plot a graph showing how the extension of the spring ( y axis) varies with the Force applied (x axis)
Graph
I can explain what a resultant force is
I can describe what happens when the resultant force is zero for a stationary or moving object
I understand the term ‘resultant force’ and can determine the result of opposite or parallel forces acting in a straight line
A number of forces acting at a point may be replaced by a single force that has the same effect on the motion as the original forces all acting together. This single force is called the resultant force.
A number of forces acting at a point may be replaced by a single force that has the same effect on the motion as the original forces all acting together. This single force is called the resultant force.
So what is a resultant force?
If it was moving there would be a drag force.
No Drag
Force
No Drive
Force
Decelerating
Accelerating
Moving with
constant speed
Stationary

Is the car…
Reaction (push back from ground)
Weight
Resultant force = zero (how do you know?) and the car is stationary.
Situation 1:
Situation 2:
Decelerating
Accelerating
Moving with
constant speed
Stationary

Is the car…
The car remains at the same speed, in the same direction
Resultant force = zero
Situation 4:
Decelerating
Accelerating
Moving with
constant speed
Stationary

Is the car…
The car accelerates in the direction of the resultant force (forwards)
Resultant force is NOT zero, so…
Situation 4:
Decelerating
Accelerating
Moving with
constant speed
Stationary

Is the car…
The car slows down (decelerates).
Resultant force is not zero,
1. Draw four diagrams of a car (same car in each case – nothing too elaborate).

Car one is at rest.
Car two is moving at constant speed forwards.
Car three is accelerating.
Car four is decelerating.

Each diagram should show all of the labelled forces acting on the cars.

2. Explain why all cars have a top speed.


A car which was moving at constant speed drives off a cliff.

3. Explain – in detail - what happens next in terms of the motion of the car and
all of the forces involved. Include diagrams.
I can do this, this is kind of easy!
I might need help, stay close by!
Help! I am stuck!
Use your traffic light cards to show me how
you feel about the questions.
Take worksheet P2.9 3b and complete.
Complete worksheet P2 1.1 due

Wednesday :Period 1 the 4th of July 2012
Homework
Mr. P. R. Collins
If the resultant force acting on a stationary body (object) is zero the body (object) will remain stationary
Plenary: What are the resultant forces
Mr. P. R. Collins
OR
If the resultant force acting on a stationary body (object) is not zero the body (object) will accelerate in the direction of the resultant force.
Forces
Mr. P. R. Collins
If the resultant force acting on a moving body (object) is zero the body (object) will continue to move at the same speed and in the same direction
Forces
Mr. P. R. Collins
If the resultant force acting on a moving body (object) is not zero the body (object) will accelerate in the direction of the resultant force
Forces
Mr. P. R. Collins
When a vehicle travels at a steady speed the frictional forces balance the driving force





The greater the speed of a vehicle the greater the breaking force needed to stop it in a certain direction
Forces
Force and Accleration
Objectives: To be able to recall and use the relationship between force and acceleration.
To be able to apply this relationship to falling objects
Practical Results:
Use the graph to help you come up with an equation that links force and acceleration.
Hints:
What is the equation of a straight line?
What would happen to the acceleration at a given force if the gradient was greater?
What could you decrease in the experiment that would increase the acceleration?
Starter:
If you could have any superpower, what would it be and why?
Newton's 2nd Law:
Force = mass x acceleration
How to solve problems:
1. Write down the equation
2. Put the numbers by the corresponding letter
3. Do the sum.
Assessment questions:
1. A bag of rice of mass 5kg accelerates at 10m/s/s. What is the force on it?
2. A person exerts a force of 240N on a box of mass 15kgs what is its acceleration?
3. A force of 6N makes an object accelerate at a rate of 0.3m/s/s. What is the objects mass?
4. A Rocket of mass 300,000kg has a weight of 500,000N, its engines exert an upward force of 2,000,000N. What is its acceleration?
5. A plane has 4 engines each producing 15kN of force, if the plane accelerates at 0.3m/s/s what is its mass?
Answers:
1 F=ma F= 5 x 10 F=50N

2 F=ma 240 = 15 x a 240/15 = a =16m/s/s

3 F=ma 6 = m x 0.3 6/0.3 = a = 20m/s/s

4 Resultant force = 2,000,000 - 500,000 = 1,500,000N
F=ma 1,500,000 = 300,000 x a
a = 1,500,000/300,000 - 5m/s/s

5 Total Forward Thrust = 4 x 15kN = 60kN
60kN = 60,000N
F=ma 60,000 = mx 0.3
m=60,000/0.3 = 200,000kg
Starters
Moments
Objectives
Know and use the relationship between a the moment of a force and its distance from the pivot.
Recall that the weight of a body acts through its centre of gravity.
Know and use the principle of moments for a simple system of parallel forces acting in one plane.
Understand that the upward force on a light beam, supported at the ends, vary with the position of the heavy object on the beam.
Astrophysics
Objectives
Understand gravitational field strength, g
Explain the orbits caused by gravitational forces
Be able to use a given relationship to find the orbital speed.
Understand the scale of the universe.
Gravity
What does gravity do?
What is the difference between mass and weight?
How do you calculate weight?
Gravity causes objects with mass to be attracted to each other.

Mass is the amount of stuff something is made of, weight is the force of gravity acting down on it.

Weight = mass x gravitational fields strength
W=mg
Gravitational Field Strength
The value of gravitational field strength changes from planet to planet.

On Jupiter g = 26N/kg

On Earth g = 10n/kg

On the Moon g = 1.6N/kg
Orbits
The earth orbits ...... in a ...

The moon orbits the .... in a ....

Comets orbit the ....... in a .......
Orbital speed
Speed = distance / time

For an object moving in a circle the distance travelled = 2 pi r

The time taken to go this distance is the period of the orbit (time for 1 orbit)

Orbital speed = 2 pi r / time period
Questions
A ball on a string moves in a circle of radius 1.2m, it takes 0.5s for 1 turn, how fast is it going?
The London eye has a radius of 15m, and takes 30 minutes to make 1 revolution, what is its orbital speed? (remember time needs to be in seconds)
Scale of Space
Solar System: star and planets
Galaxy: billions of stars
Universe: billions of galaxies
Moments
In any collision momentum is conserved. This means that the momentum before the collision is the same as the momentum after the collision
Momentum in collisions

When 2 objects collide momentum is conserved,
providing no external forces act.


This means that:
Momentum before collision = momentum after collision


When answering questions on this, take care and write
things out fully.


A train of mass 5000kg is moving at 10m/s, it
collides with a stationary carriage of mass 3000kg.
What is the initial momentum of the train?
mom = m x v
mom = 5000kg x 10m/s = 50000kgm/s

Example

2. What is the initial momentum of the carriage?
mom = m x v
mom = 3000 x 0 = 0kgm/s

3. What is the total momentum before the
Collision.
total momentum = 50000 + 0
= 50000kgm/s

What is the total momentum after the collision.

Mom before = mom after
50000kgm/s = mom after

What is the velocity of the carriage and train after
They collide and stick together
Mom = m x v
50000 = (5000+3000)v
50000 = 8000v
50000/8000 = v
V = 6.25m/s

Momentum conservation

Collisions

1. A gun of mass 15kg fires a bullet of mass 0.05Kg at
250m/s . What is the momentum of the gun before fired?
What is the momentum of the bullet before fired?
What is total momentum before fired?
What is total momentum after fired?
What is the momentum of the bullet after fired?
what is the recoil velocity of the gun?

2. A ball of mass 0.350kg hits your hands at 50m/s, What is its momentum?
If your hands have a mass of 1.4kg how fast do they move backwards
when they have caught the ball

3 A railway engine of mass 6000 kg travelling at a speed of 4.0 m/s collided
with and coupled to a truck of mass 2000 kg that was initially not moving.
Calculate the momentum of the engine before the collision.
Show that the engine and the truck moved at a velocity of 3.0 m/s after the collision.


Use your diary pages to show whether you understand

Your turn

Your turn

A moment is a turning force.
It depends on the force, and the distance from the pivot.
Moment = force x distance from the pivot
.
Moments are measured in Newton metres (Nm)
Examples

A spanner of length 0.3m has a force 700N exerted on it. What is the turning force?
moment = force x distance from pivot
= 700 x 0.3
=210Nm
Balanced Objects
For an object to be balanced the clockwise moments have to be equal to the anti clockwise moments.
We say it is in equilibrium.
Example
Dave has a weight of 800N, he is sat 2.5m from the pivot of the see saw. Mary is on the other side of the see saw and has a weight of 500N. How far from the pivot does she need to be sat?
If balanced: Clockwise moment = anti clockwise moment
500 x Dist = 800 x 2.5
500 x Dist = 2000Nm
Distance = 2000/500
Distance = 4m
Page 16 of Homework book Q 1-3
Principle of moments
For an object to be balanced the clockwise and anticlockwise moments must be balanced.
Practical:
Place known masses on either side of a balanced ruler.
Record the distance of the masses from the pivot when it is balanced
Force
Force
dist
dist
moment
moment
Clockwise
anticlockwise
0.1
0.3
0.4
15cm
3cm
2cm
0.3
0.2
0.1
Forces on a Beam
An object supported at either end (a bridge or a beam) will have a downward force on it, equal to the upward force from the edges.
F1
F2
Weight
Also the clockwise and anticlockwise moments will be equal
Weight = F1 + F2
d1
d2
Also the moments are equal so:
Clockwise moments = anticlockwise moments
Weight x d1 = F2 x d2
Your turn
A car of mass 2400kg is on a bridge, that is 30m long. It is 6m from one end.

What is the weight of the car?
Draw a diagram to show the forces on car and bridge
Find values for the forces at either end of the bridge.
the extension is proportional to the force .
When the force is removed it returns to its original length.
this is elastic behaviour
This is called Hooke's Law
Force = spring constant x extension
F=kx
A straight line graph with a positive correlation starting at the origin means that the two variables are proportional to each other.
As you double one, the other doubles
Weight = mass x gravitational field strength
24000N
F1
F2
Weight = F1 + F2 = 24000N
Taking moments about F1
24 x 24000 = 30 x F2
24 x 800 = F2
F2 = 19200N
weight = F1 + F2
24000 = F1 + 19200
F1 = 4800N
A lorry of weight 300 000N is on a bridge 64m long. It is 10m from one edge, what are sizes of the two the upward forces on the bridge?
Hooke's Law
The straight line section of the graph shows the region where Hooke's Law is obeyed.
After that point the object displays plastic behaviour.
Elastic behaviour means...

Plastic behaviour means...
Full transcript