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Newton’s Laws of Motion
1st Law – Law of Inertia
2nd Law – Force = Mass x Acceleration
3rd Law – Action-Reaction Everything to do with motion, forces, gravity, speed, or acceleration is
governed by these laws.
Newton’s First Law of Motion“Law of Inertia”
An object will stay at rest unless acted on by an unbalanced force
An object in motion will stay in motion unless acted on by an unbalanced force
Inertia
Bike Race Crash
Inertia
Inertia – the tendency of an object to resist change in motion (whether at rest or moving)
Table Cloth Inertia Egg Drop Inertia
What does this mean?
An object will “keep doing what it was doing” unless acted on by an unbalanced force
If the object was sitting still, it will remain stationary
If it was moving at a constant velocity, it will keep moving
It takes force to change the motion of an object
Mass and Inertia
The amount of inertia an object has depends on its mass
The greater the mass the greater the inertiaMass is measured in
grams or kilograms
The soccer ball has a little mass and therefore a little inertia
When kicked it is easy to put it into motion because it has low inertia
The brick wall has a large mass and therefore a lot of inertia
When kicked it is difficult to put it into motion because it has high inertia
Objects at rest remain at rest unless acted on by a net force.
Since the train is so heavy, it is difficult to change its speed.
Since the baby carriage is so small, it is very easy to change its speed.
Flick a pencil
Flick a binder
Which was easier to put into motion?
Objects in motion remain in motion in a straight line (unless acted upon by an outside force).
Since the train is so heavy, it is difficult to stop it once it is moving.
Since the soccer ball is so small, it is very easy to stop it once it is moving.
Stop a pencil
Stop a binder
Which was easier to stop?
Two crates, one heavy and one light, are at rest on a waxed floor. Which crate will need a greater force to provide the same acceleration? Use the concept of inertia to explain your answer.
The heavy crate has more inertia, so it requires more force to make it accelerate.
Section Review
What is inertia? How is it related to Newton’s first law of motion?
Inertia is the tendency of an object to resist any change in its motion. Newton’s first law is called the law of inertia. The law states that an object in rest stays in rest and an object in motion stays in motion unless acted on by an outside force.
Friction and Gravity
Newton’s First Law of Motion states that an object in motion will stay in motion unless acted on by an outside force.
Two forces that act on objects are friction and gravity.
Friction
Friction acts in a direction opposite to the object’s direction of motion
Without friction, the object would continue to move at a constant speed forever
The strength of the force of friction depends on two factors:
1. the types of surfaces
2. how hard the surfaces push together.
Types of Friction
1. Sliding Friction – solid surfaces slide over each other
2. Rolling Friction – when an object rolls over a surface
Force of friction is less in rolling friction than with sliding friction
3. Fluid Friction – when an object moves through a fluid
Force of friction is less in fluid friction than with sliding friction
Explain the role of friction in walking on a sidewalk versus walking on a
sidewalk covered with ice.
Friction between your shoe and the sidewalk allows you to push against the sidewalk. Ice decreases this friction.
Gravity
Gravity is the force that pulls objects toward each other
Force and Gravity
Air ResistanceObjects falling through air
experience a type of friction called air resistanceAs surface area increases,
air resistance increases.
Eventually, air resistance equals gravity
The greatest velocity an object reaches is called terminal velocity In a vacuum, all objects fall
at the same rate
Resultant Force Podcast
Weight vs. Mass
Weight – a measure of the force of gravity on an object. (weight is a force) (Weight on the moon is 1/6 of your weight on Earth)
Mass – a measure of the amount of matter in that object.
Weight = Mass x Acceleration due to gravity
Units of Measure:
Weight – measured in Newtons
Mass – measured in Kg
Acceleration – measured in m/s2
(50 Kg person weighs 50kg x 9.8m/s2 = 490 Newtons)
Your Weight on Other Worlds
Newton’s 1st
Law of Motion
Two teams are playing tug of war. They are both exerting equal force on the rope in opposite directions. This balanced force results in no change of motion.
A soccer ball is sitting at rest. It takes an unbalanced force of a kick to change its motion.
Some Examples from Real Life
Check Your Understanding
Ben Tooclose is being chased through the woods by a moose
If Ben takes a zigzag pattern through the woods, he will be able to use the large mass of the moose to avoid being caught
Explain this in terms of inertia and Newton's first law of motion
Answer
The large mass of the bull moose means that the bull moose has a large inertia.
Thus, Ben can more easily change his own state of motion (make quick changes in direction) while the moose has extreme difficulty changing its state of motion.
Physics for better living!
Force, Mass, and Acceleration
Newton’s Second Law of Motion
The acceleration of an object is inversely proportional to the mass of the object and directly proportional to the force
Units of measure:
Acceleration = m/s2
Mass = Kg
Newton = Kg x m/s2
Newton - is the force needed to accelerate one kg of mass at 1 meter/sec 2 (1 N = 1kg x 1m/s2)
Newton’s 2nd
Law
Force = mass x accelerationF = ma
Inversely Proportional
Inversely proportional: As the mass
increases, the acceleration decreases
They both do opposite things
Directly Proportional
Directly proportional: As the force increases,
the acceleration increases
They both do the same thing
Push the shopping cart
Shopping Cart Example
Graphing Newton’s 2nd Law
For the same force: more mass
causes less acceleration
less mass causes more acceleration
Graphing Newton’s 2nd Law
For the same mass: more force
causes more acceleration
less force causes less acceleration
Graphing Newton’s 2nd Law
For the same acceleration: more mass needs
more force less mass needs
less force
What does F = ma say?F = ma basically means that the force of an object
comes from its mass and its acceleration.
Something very small (low mass) that’s changing speed very quickly (high
acceleration), like a bullet, can still have a great force.
Something very small changing speed very slowly will have a very weak force.
Something very massive (high mass) that’s changing speed very slowly (low acceleration), like a glacier, can still have great force.
A Little Practice
How much force will you need to accelerate a 200kg wagon at 5m/s2?
1000 N
Newton’s Second Law Summary
• How much force will you need to accelerate a 10,000kg train at 5m/s2
50,000 N
Question 1
When the net force on an object increases, how does the object’s acceleration change?
The acceleration increases.
Question 2 Suppose you know the acceleration
of a shopping car as it rolls down a supermarket aisle. You want to find the net force with which it was pushed. What other information do you need in order to find the force?
You need to know the mass of the shopping cart.
Question 3 Suppose you doubled the force acting
on an object. In what way could you change its mass to keep its acceleration?
If you double the net force acting on an object, you have to double the mass of the object to keep the acceleration unchanged.
Newton’s Second Law
Action and Reaction
Newton’s 3rd LawAction-Reaction
Newton’ s 3rd Law of Motion
All forces occur in pairs, and these two forces are equal in strength and opposite in direction
For every action there is an equal and opposite reaction
Reacti
on
Acti
on
• Right now, gravity is pulling you down in your seat, but Newton’s Third Law says your seat is pushing up against you with equal force
• This is why you are not moving. There is a balanced force acting on you– gravity pulling down, your seat pushing up
What does this mean?
Equal but Opposite
Ice Skaters (push off of each other with equal force in opposite directions):
The speed at which the two skaters move depend on their masses. (the greater the mass the slower the speed)
What happens if you are standing on a skateboard and push against a wall? You slide in the opposite direction (away from the wall), because you pushed on the wall but the wall pushed back on you with equal and opposite force.
Why does it hurt so much when you stub your toe? When your toe exerts a force on a rock, the rock exerts an equal force back on your toe. The harder you hit your toe against it, the more force the rock exerts back on your toe (and the more your toe hurts).
Think about it
Check Your Understanding While driving down the road,
an unfortunate bug strikes the windshield of a bus
The bug hits the windshield and the windshield hits the bug.
Which of the two forces is greater: the force on the bug or the force on the bus?
Answer
Trick Question! Each force is the same size. For every action, there is an equal ... (equal!). The fact that the bug splatters only means
that with its smaller mass, it is less able to withstand the larger acceleration resulting from the interaction.
Check Your Understanding
A gun recoils when it is fired.
The recoil is the result of action-reaction force pairs.
As the gases from the gunpowder explosion expand, the gun pushes the bullet forwards and the bullet pushes the gun backwards.
The acceleration of the recoiling gun is ...
Answer
The force on the gun equals the force on the bullet (Newton’s 3rd Law)
Yet, acceleration depends on both force and mass (Newton’s 2nd Law)
The bullet has a greater acceleration due to the fact that it has a smaller mass.
Remember: acceleration and mass are inversely proportional (Newton’s 2nd Law)
Question 3 Suppose you and a friend, who has exactly
twice your mass, are on skates. You push away from your friend. How does the force with which you push your friend compare to the force with which your friend pushes you? How do your accelerations compare?
The force with which you push your friend is equal and opposite to the force with which your friend pushes you. You accelerate faster than your friend and roll back further because your mass is less.
Lego - Newton’s Laws of MotionLaws of Motion Rap
Newton’s 3rd Law
Review Newton’s First Law (Inertia):
Objects in motion tend to stay in motion and objects at rest tend to stay at rest unless acted upon by an unbalanced force.
Newton’s Second Law (F=ma): Force equals mass times acceleration
(F = ma).
Newton’s Third Law: For every action there is an equal and opposite
reaction.
