lessons 7 newtons laws

7/29/2019 Lessons 7 Newtons Laws

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Newtons laws of motion


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Newtons Laws of Motion

Thats

me!


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Newtons 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).
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Newtons 1st Law

If there is no

resultant force acting

on an object, it willmove with constant

velocity. (Note the

constant velocity

could be zero).

Does this make

sense?


5/29

Newtons 1st Law

If there is no

resultant force acting

on an object, it willmove with constant

velocity. (Note the

constant velocity

could be zero).

Can you copy it

whilst you think

about it?


6/29

Newtons 1st law

Newtons first law was actually discovered by

Galileo.

Newton nicked it!


7/29

Newtons first law

Galileo imagined a marble rolling in a very

smooth (i.e. no friction) bowl.


8/29

Newtons first lawIf you let go of the ball, it always rolls up theopposite side until it reaches its original height

(this actually comes from the conservation of

energy).


9/29

Newtons first lawNo matter how long the bowl, this alwayshappens


10/29

Newtons first lawNo matter how long the bowl, this alwayshappens.

constant velocity


11/29

Newtons first lawGalileo imagined an infinitely long bowl wherethe ball never reaches the other side!


12/29

Newtons first lawThe ball travels with constant velocity until its reachesthe other side (which it never does!).

Galileo realised that this was the natural state of objects

when no (resultant ) forces act.

constant velocity


13/29

Another example

Imagine Mr Porter cycling at

constant velocity.


14/29

Newtons 1st law

He is providing a pushing force.

Constant velocity


15/29

Newtons 1st law

There is an equal and opposite

friction force.

Constant velocity

Pushing force

friction


16/29

Newtons second law

Newtons second law concerns examples

where there is a resultant force.

I thought of this

law myself!


17/29

Lets 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


18/29

Newtons 2nd law

Now lets imagine what happens if

he pedals faster.

Pushing force

friction


19/29

Newtons 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


20/29

Newtons 2nd law

Now imagine what happens if he stops pedalling.

friction


21/29

Newtons 2nd law

He slows down (decellerates). This is a negative

acceleration.

friction


22/29

Newtons 2nd law

So when there is a resultant force,

an object accelerates (changes

velocity)

Pushing force

friction

Mr Porters Porche


23/29

Newtons 2nd law

There is a mathematical relationship

between the resultant force and

acceleration.

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

FR = maIts physics,

theres always a

mathematical

relationship!


24/29

Newtons 2nd law

There is a mathematical relationship

between the resultant force and

acceleration.

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

FR = maCan you

copy thistoo?


25/29

An example

What will be Mr Porters

acceleration?

Pushing force (100 N)

Friction (60 N)

Mass of Mr Porter and bike = 100

kg


26/29

An example

Resultant force = 10060 = 40 N

FR= ma

40 = 100aa = 0.4 m/s2

Pushing force (100 N)

Friction (60 N)

Mass of Mr Porter and bike = 100 kg


27/29

Newtons 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 ontable (body B)

Table (body B) exerts force on

hand (body A)


28/29

Dont worry!

We wont do Newtons 3rd law until next year!
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Thats all folks!

lessons 7 newtons laws

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