Download Newton`s 2 nd Law

Topic: Forces and Motion
Aim: Describe Newton’s 3 Laws of Motion.
Do Now: Last question on the back of KE and PE
reading notes
HW: Ditto – How do waves travel through matter?
ONLY COMPLETE PART B.
CL Waves due Thursday
Energy and Waves Exam on Tuesday
Decide whether each item below describes kinetic
energy or potential energy.
1. An airplane circling in preparation for a landing
2. A pile of coal that will be used for fuel
3. A flag blowing in the wind
4. A hatbox stored on the top shelf of a closet
5. A hammer held above a nail
6. A marble rolling down the aisle of your science
classroom
7. A new car battery
8. Glucose stored in plants as a result of
photosynthesis
9. An ant crawling across a sandwich at a picnic
10. An eyelash fluttering
Calculate the net force in each diagram. State the
direction the object will move.
800N – 400N
= 400N
down
400N
1200N
1200N – 800N
= 400N
up
800N
800N
2 people are pulling at opposite ends of a rope.
The person pulling on the right side of the rope
is applying a force of 50N while the person on
the left is applying a force of 70N. What will
happen to the rope?
1. It will move towards the right.
2. It will move towards the left.
3. It will fall to the ground.
4. It will stay in place.
2 people are pulling at opposite ends of a
rope. If they are pulling on the rope with
equal but opposite forces, what will
happen to the rope?
1. It will stay in place between the 2
people.
2. It will move towards the right.
3. It will move towards the left.
4. It will fall to the ground.
Newton • British scientist
(1642–1727)
• Describe all states of
motion (rest, constant
motion, acceleration) &
how forces cause them
Inertia
• Tendency of object to stay
at rest or in motion unless
acted on by an outside
force
• Greater mass, greater
inertia
The body is motionless at the start
of a race. The athlete must overcome
their inertia to start moving!
NEWTON’S 1ST LAW OF MOTION
• LAW OF INERTIA: An object at
rest will stay at rest, OR an object
in motion will stay in motion unless
acted on by an outside force
Air friction is the force that slows
the ball & gravity pulls the ball back
to Earth.
• Example: Soccer ball remains at
rest unless acted on by an
outside force (kicking)
The law of inertia can explain what happens
in a car crash. When a car traveling about
50 km/h collides head-on with something
solid, the car crumples, slows down, and
stops within approximately 0.1s.
FIRST LAW: Objects resist a
change in motion - an effect you can
feel in a starting or stopping car.
•Acceleration: how fast
velocity (speed) is
changing
Newton’s 2nd Law: Changing an object’s
speed and direction depends on the mass of
the object and the amount of force exerted
on it.
NEWTON’S 2nd LAW
•
F = mass x acceleration
(N) (kg)
(m/s2)
• Example = A 52kg water skier is
being pulled by a speedboat. The
force causes him to accelerate at
2 m/s2. Calculate the force that
causes this acceleration.
–F = ma
–F = 52kg
x
–F = 104.0 N
2 m/s2
1. Identify the difference between the two pictures.
2. Which will have greater acceleration? Support
your answer.
SECOND LAW:
LARGE FORCE  GREATER ACCELERATION
than a small force.
SMALL MASS  GREATER ACCELERATION
than a large mass.
NEWTON’S 3rd LAW
• For every action there is an equal
and opposite reaction
THIRD LAW: Every action
has an equal and opposite
reaction.
If A acts on B, then B acts back on A
In a rocket engine, burning fuel produces hot
gases. The rocket engine exerts a force on
these gases and causes them to escape out the
back of the rocket. The gases exert a force on
the rocket and push it forward.
A swimmer “acts” on the water, the
“reaction” of the water pushes the
swimmer forward.
Whenever one object exerts a force on
another object, the other object exerts an
equal but opposite force on it. Hammer pushes
on stake. Stake pushes on hammer. The
hammer acts, the stake re-acts.
Let’s summarize…
1. Describe Newton’s 1st law of motion.
2. Identify an example of an outside force that
can act on an object.
3. State the equation that makes up Newton’s 2nd
law of motion.
4. Describe the relationship between force and
acceleration.
5. Describe the relationship between mass and
acceleration.
6. Describe Newton’s 3rd law of motion.
A driver steps on the gas and the car gradually
accelerates to 50km/h. The driver then slams on
the brakes to avoid hitting a box on the road. As
the car comes to a stop, the driver’s body appears
to lurch forward in the seat until restrained by the
seat belt. Which law best explains why the driver’s
body lurches forward when brakes are suddenly
applied?
1. Newton’s 1st Law
2. Newton’s 2nd Law
3. Newton’s 3rd Law
4. The law of gravity
The tendency of a stationary
object to resist being put into
motion is known as
1. acceleration
2. gravity
3. inertia
4. velocity
Which of the following wagons should
move faster if pushed with the same
force?
1. A wagon containing a small boy
2. A wagon containing a small bird
3. A wagon containing a big dog
4. A wagon containing an elephant
1. A chair exerts a force of 20N on the floor.
What is the force that the floor exerts on the
chair?
1. 10N
2. 20N
3. 21N
4. 40N
2. Which one of Newton’s Law is taking place in
the question above?
1. Which of Newton’s three laws
of motion is illustrated?
2. State the law of motion that
describes the example shown in
the diagram.
3. Which of Newton’s laws deals
with the acceleration of moving
objects?