Do now! Can you continue the questions you started last lesson? (Page 27 questions 6, 7 & 8. Page 29 questions 1 & 2) Bunny suicide #1.

Do now!

Can you continue the questions you started last lesson? (Page 27 questions 6, 7 & 8.Page 29 questions 1 & 2) Bunny suicide #1

Last lesson

• Motion graphs

No movement

distance

time

speed

time

Constant speed

distance

time

speed

time

Area = distance travelled

Constant acceleration

distance

time

speed

time

Area = distance travelled

Gradient = acceleration

a = (v-u)/t

You did some questions!

Page 27 questions 6, 7 & 8.Page29 questions 1 & 2.

Today’s lesson

• 1.7 Know what a force is

• 1.8 Types of force

• 1.9 & 1.10 Vectors and scalars

• 1.11 Adding forces

• 1.13 Newton’s laws (2nd law)

Forces

• Remember a force is a push (or pull)

Forces

• Force is measured in Newtons

Forces

• There are many types of forces; electrostatic, magnetic, upthrust, friction, gravitational………

Which of the following is the odd one out?

MassSpeedForce

TemperatureDistanceElephant

Scalars and vectors

Scalars

Scalar quantities have a magnitude (size) only.

For example:

Temperature, mass, distance, speed, energy.

1 kg

Vectors

Vector quantities have a magnitude (size) and direction.

For example:

Force, acceleration, displacement, velocity, momentum.

10 N

Scalars and Vectors

scalars vectors

Magnitude (size)

No direction

Magnitude and direction

temperature mass

speed

velocity

force

acceleration

Scalars and Vectors

scalars vectors

Magnitude (size)

No direction

Magnitude and direction

temperature mass

speed

velocity

force

acceleration

Copy please!

Representing vectors

Vectors can be represented by arrows. The length of the arrow indicates the magnitude, and the direction the direction!

Adding vectors

When adding vectors (such as force or velocity) , it is important to remember they are vectors and their direction needs to be taken into account.

The result of adding two vectors is called the resultant.

Adding vectors

For example;

6 N 4 N 2 N

4 m/s

4 m/s 5.7 m/s

Resultant force

Resultant velocity

Adding vectors

For example;

6 N 4 N 2 N

Resultant force

Copy please!

An interesting example

Think of a dog (dead) orbiting the earth with constant speed (in a circle).

An interesting example

At this point, what is its velocity?

velocity?

An interesting example

velocity

An interesting example

velocity?

What is its velocity here?

An interesting example

velocity

As you can see the velocity has changed as it is now going in another direction.

An interesting example

velocity

We have constant speed but changing velocity.

Of course a changing velocity means it must be accelerating! We’ll come back to this in year 12!

Resultant force

Newton’s Laws of Motion

That’s me!

Newton’s 1st Law

If there is no resultant force acting on an object, it will move with constant velocity. (Note the constant velocity could be zero).

Newton’s 1st Law

If there is no resultant force acting on an object, it will move with constant velocity. (Note the constant velocity could be zero).

Does this make sense?

Newton’s 1st Law

If there is no resultant force acting on an object, it will move with constant velocity. (Note the constant velocity could be zero).

Can you copy it whilst you think

about it?

Newton’s 1st lawNewton’s first law was actually discovered by Galileo.

Newton nicked it!

Newton’s first law

Galileo imagined a marble rolling in a very smooth (i.e. no friction) bowl.

Newton’s first lawIf you let go of the ball, it always rolls up the opposite side until it reaches its original height (this actually comes from the conservation of energy).

Newton’s first lawNo matter how long the bowl, this always happens

Newton’s first lawNo matter how long the bowl, this always happens.

constant velocity

Newton’s first lawGalileo imagined an infinitely long bowl where the ball never reaches the other side!

Newton’s first lawThe ball travels with constant velocity until its reaches the other side (which it never does!).

Galileo realised that this was the natural state of objects when no (resultant ) forces act.

constant velocity

Other examplesImagine a (giant) dog falling from a tall building

Other examplesTo start the dog is travelling slowly. The main force on the dog is gravity, with a little air resistance

gravity

Air resistance

Other examplesAs the dog falls faster, the air resistance increases (note that its weight (force of gravity) stays the same).

gravity

Air resistance

Other examplesEventually the air resistance grows until it equals the force of gravity. At this time the dog travels with constant velocity (called its terminal velocity)

gravity

Air resistance

Oooops!

Another example

Imagine Mr Porter cycling at constant velocity.

Newton’s 1st law

He is providing a pushing force.

Constant velocity

Newton’s 1st law

There is an equal and opposite friction force.

Constant velocity

Pushing force

friction

Newton’s second law

Newton’s second law concerns examples where there is a resultant force.

I thought of this law myself!

Let’s go back to Mr Porter on his bike.

Remember when the forces are balanced (no resultant force) he travels at constant velocity.

Constant velocity

Pushing force

friction

Newton’s 2nd law

Now lets imagine what happens if he pedals faster.

Pushing force

friction

Newton’s 2nd law

His velocity changes (goes faster). He accelerates!

Pushing force

friction

acceleration

Remember that acceleration is rate of change of velocity. In other words

acceleration = (change in velocity)/time

Newton’s 2nd law

Now imagine what happens if he stops pedalling.

friction

Newton’s 2nd law

He slows down (decellerates). This is a negative acceleration.

friction

Newton’s 2nd law

So when there is a resultant force, an object accelerates (changes velocity)

Pushing force

friction

Mr Porter’s Porche

Newton’s 2nd lawThere is a mathematical relationship between the resultant force and acceleration.

Resultant force (N) = mass (kg) x acceleration (m/s2)

FR = maIt’s physics,

there’s always a mathematical relationship!

Newton’s 2nd lawThere is a mathematical relationship between the resultant force and acceleration.

Resultant force (N) = mass (kg) x acceleration (m/s2)

FR = maCan you copy this

too?

An example

What will be Mr Porter’s acceleration?

Pushing force (100 N)

Friction (60 N)

Mass of Mr Porter and bike = 100 kg

An example

Resultant force = 100 – 60 = 40 N

FR = ma

40 = 100a

a = 0.4 m/s2

Pushing force (100 N)

Friction (60 N)

Mass of Mr Porter and bike = 100 kg

Newton’s 3rd lawIf a body A exerts a force on body B, body B will exert an equal but opposite force on body A.

Hand (body A) exerts force on table (body B)

Table (body B) exerts force on hand (body A)

Don’t worry!

We won’t do Newton’s 3rd law until next year!

That’s all folks!

Let’s try some questions!

Complete Physics for IGCSE

Read pages 38 and 39 and then try page 37 questions 1, 2 and 3 and page 39 Questions 1 and 2