Transcript Newton`s First Law

3.2 Newton’s First Law of Motion
Every year in Canada there are
approximately 160 000 car accidents,
3000 of which are fatal.
Many of these injuries and deaths
involve motorists who are not wearing
seat belts or are driving too fast.
What happens to motorists who are not
wearing seat belts during a collision?
Newton’s First Law will help answer this
question.
3.2 Newton’s First Law of Motion
Early scientists and philosophers thought that a constant net force was required to
keep an object moving. Larger net forces caused objects to move at higher constant
speeds. No net force meant the object would stop.
In the 1600s, Galileo demonstrated, through a simple thought experiment involving a
ball and some ramps, that if there was no friction present, the ball will continue to roll
until it reached the same height as its starting position on the first ramp.
If the second ramp is eliminated, the ball would continue to roll forever because it will
never reach the same initial height of the first ramp. Remember, no friction is present.
3.2 Inertia
Galileo concluded that once an object starts moving, it will continue to move at a
constant velocity if no friction is present.
He used the concept of inertia to explain his conclusion;
• Inertia is the property of matter that causes an object to resist changes in motion.
• Inertia is directly proportional to the mass of the object. (Inertia α mass)
As the mass of an object increases, its inertia increases; As the mass decreases, its
inertia decreases.
3.2 Newton’s First Law
Sir Isaac Newton published Principia Mathematica, a
set of books which included much of his work on
physics, as well as a description of Galileo’s law of
inertia.
The law of inertia is also known as Newton’s First Law
of Motion because it was included with Newton’s other
laws of motion.
Newton’s First Law of Motion states that if the net
force acting on an object is zero, the object will remain
at rest or continue to move with a constant velocity.
3.2 Newton’s First Law
There are many implications of Newton’s First Law:
• If the velocity of an object is constant, then the net
force acting on it is zero; same as if the object were at
rest.
• Objects at rest tend to remain at rest; objects in motion
tend to remain in motion in a straight line with a
constant speed until acted on by an unbalanced force.
• If a net force acts on an object, the velocity of the
object will change in magnitude or direction; the object
will undergo acceleration.
SP #1,3 p.124-126
3.2 Applications of Newton’s First Law
How do headrests prevent injuries?
• During a rear-end collision, the car will suddenly
accelerate forward and so will your body because
the seat exerts a force on your torso.
• In a vintage car with no headrest, no force is applied
to your head. According to Newton’s First Law, it
remains at rest.
• Your head will initially appear to snap backwards,
relative to your body accelerating forward, resulting
in whiplash.
3.2 Applications of Newton’s First Law
How do headrests prevent injuries?
• The head rest in a modern car helps push the
head forward with the rest of the body.
• This helps to prevent whiplash since your neck
does not bend as far back during a rear-end
collision.
3.2 Applications of Newton’s First Law
How do seat belts work?
• One design feature of a seat belt uses the sudden
decrease in velocity to activate a gear mechanism.
• The seat belt strap is attached to a spool, which in
turn is attached to a gear. Beneath the gear is a
pendulum that is free to swing back and forth.
• When the vehicle is moving at a constant velocity,
the pendulum freely hangs straight down.
3.2 Applications of Newton’s First Law
How do seat belts work?
• When the car comes to a sudden stop, the
pendulum swings forward due to its inertia.
• This causes the pendulum to move a metal stop
into the teeth of the gear mechanism, locking the
seat belt in place.
3.2 Homework
Practice # 1, 4, 5 p.126
Questions # 3, 4, 6, 8, 9, 13 p.122