**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...