Transcript Newton`s First Law - Science

Topic – Physics 2a
Newtons Laws
Key words
Force, mass, acceleration, action, reaction,
tension
Topic – Physics 2a
Newtons first law: Inertia
Topic – Physics 2a
Newtons first law
What is Newton’s first law?
• States that a body will either stay at rest or continue with
a constant speed in a straight line unless it is acted on
by a net force.
• Draw Fig 3.15 page 97
How do satellites move at a constant velocity?
• Since there is no air in space to retard a craft, a satellite will
continue to move at a constant velocity once in orbit as long as it is
not acted on by a net force the opposite direction.
• This also occurs if forces are involved but there net forces are zero∑F=0. They are said to be at equilibrium or forces are balanced
Topic – Physics 2a
Describe how the following
show inertia
Topic – Physics 2a
Answers:Describe how the
following show newtons first law
• Newton’s First Law: Things like to
keep doing what they are doing unless
acted on by net external force.
• Shopping trolley has a large inertia
(large mass) so a large resistance to
change of motion. When it is moving it
wants to keep moving so much harder
to stop and you need a large net
external force acting in the opposite
direction
Again, explain Newton’s First Law.
Here the china is heavy (large inertia) so
wants to stay in place as that is what it is
doing. As long as the table cloth is
pulled quickly, and the friction reduced,
then the china will stay where it is
Topic – Physics 2a
3 and 4
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Trying to push car:
Newton’s First Law: Things like to
keep doing what they are doing
unless acted on by net external
force.
The car is stationary and has a large
inertia (large mass). To overcome
the large inertia a very large net
external force is required.
Passengers in a bus:
Newton’s First Law: Things like to keep doing what
they are doing unless acted on by net external
force.
When a passenger is standing on a moving bus, they
are moving at the speed of the bus. When the bus
stops, unless a net external force is acting on them
(seat or hand hold) they will continue to move
forward and can fall over when they lose balance.
Topic – Physics 2a
Mass and inertia
Activity: Use these ideas to answer the questions below.
1. The law of inertia states that no force is required to maintain
motion. So why do you have to keep peddling your bicycle to
maintain motion when riding somewhere?
• In this case there is a net external force acting on the bicycle –
friction.
2. If you were in a spaceship in space and fired a rocket, how much
force would have to be exerted on the rocket to keep it going?
• None, there is no external force acting on the spaceship
Complete questions 3.3 page 101 Heinemann
Topic – Physics 2a
Newtons second law
• Consider a book sitting on your school desk.
According to Newton’s First Law, the book will
stay there until a net external force is added.
Say you now push that book (you supply the net
external force), the book moves. Newton’s
Second Law of Motion explains the relationship
between the force supplied and the acceleration.
• Newtons second law states: rate of
acceleration is proportional to the force
supplied and inversely proportional to the
mass of the object.
Topic – Physics 2a
• Or more simply,
• Acceleration = Force / Mass
• Force = mass x acceleration - which is more often stated as
ma
F =
• Newton’s Second Law of Motion is also expressed as follows: The
rate of change of momentum of an object is proportional to the
applied force and takes place in the direction of that force.
• This can be explained by investigating momentum.
• F= (mv-mu)
=
F = m(v-u)
∆t
∆t
• Now we know that change in velocity divided by time is acceleration
so substituting this into the above equation results in
F = ma
which, as we have said before, is the simple way of expressing
Newton’s Second Law.
Topic – Physics 2a
Newtons third law
• Newtons third law states:
• states that for every force applied (action), an
equal and opposite force always appears
(reaction), even if no movement results.
• The most important aspect of Newton’s Third
Law is to be aware of the fact that there are
TWO different forces acting. While the value is
identical, the directions are reverse so they are
different forces. The forces act on different
objects, NEVER on the same one, so the forces
can’t cancel each other out.
Topic – Physics 2a
• E.g. body A exerts a force on B – ACTION
body B exerts a force on A – REACTION
• Example: Kicking a ball: A common
misunderstanding of Newton’s Third Law – if I
kick a ball, the ball can’t move as action/reaction
forces are the same. But if the ball is kicked it
accelerates. Does this contradict Newton’s
Third Law?
NO!!!!!!
• The only force on the ball is the kick, so it can
accelerate, the reaction force acts on your foot
and slows it down – that is the force from the ball
on your foot.
Topic – Physics 2a
Task 3
• Activity:
• Why do cars move?
• Action force:
Reaction Force:
• Why do objects fall to earth?
• Action force:
Reaction Force:
• A bug hitting the windscreen of a bus slows the bus
down.
• Action force:
Reaction Force:
Topic – Physics 2a
Task 3
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Activity:
Why do cars move?
Friction of road on tyre exerts a force on the type and the car moves
forwards.
Action force: road on tyre
Reaction Force: tyre on road
Why do objects fall to earth?
Gravity is obviously pulling the object towards the Earth but likewise,
an object is pulling the Earth towards itself as well. It is only that the
mass of the Earth is so much more than the object that the object
moves a much further distance than the Earth
Action force: Earth on object
Reaction Force: Object on
Earth
A bug hitting the windscreen of a bus slows the bus down.
Yes, it does. The bug has a very small mass and very large
acceleration compared to bus
Action force: bug on bus
Reaction Force: bus on bug