Download Newton`s 2nd Law – Note Sheet

Newton’s 2nd Law – Note Sheet
CP Physics
A ___________ is a push or a pull that is exerted on an object – causing its
motion to change. There are two ways that an object’s motion can be changed –
we can either change its ___________ or change its speed. In either case,
what we are really changing is the object’s ____________. Another name for
a change in velocity is __________________.
Newton’s 2nd Law takes into account the ____________ that is applied to an object and relates it
to how the objects motion will change.
In words, Newton’s 2nd Law states that:
When an ________________ force acts on an object, the object
___________________ such that the product of the mass and the acceleration equal the
unbalanced force.
What does that mean????? It is best re-written as a mathematical equation. Using the words from
the definition above, we can re-write Newton’s 2nd Law another way. Since a product is found by
__________________ two quantities, we must multiply the _____________ and the
________________ of an object and set it equal to the ___________________. This makes
Newton’s 2nd Law read as:
___________ = ____ ____
Before we continue any further, let’s see what we can tell about an objects motion just from this
equation. First of all, we will assume that the mass of the object will always stay the same. As it is
written, the equation is solved for ____________. If the force on the object increased, and the
mass remained unchanged, then the acceleration of the object would have to (increase/decrease) in
order the maintain the equality. If both quantities increase, they are said to be ______________
proportional to one another. Notice that the quantities that are directly proportional are both
__________________ and are on (the same/opposite) sides of the equation.
Now let’s solve the equation for acceleration. Written that way, the equation becomes
_________________. If we wanted to make a statement about how the mass and the
acceleration are related, we would notice that acceleration is in the _____________________
while mass is in the ______________________. This means that acceleration and mass are
________________________ proportional to one another. In words that means if one of those
quantities increases, the other quantity must (increase/decrease). Notice that acceleration is still
(directly/inversely) proportional to the force. So, if we want an object to have a higher rate of
acceleration we could either apply (more/less) force or (increase/decrease) its mass. It is also
very important to note that the acceleration of the object is in the (same/opposite) direction as
the force that causes the acceleration.
What are the UNITS of force?!?!?! There are actually two units for force. The SI unit for force
is the ___________ (N) – named after Isaac Newton. This type of unit, which can be measured
directly, is called a ___________________ unit.
We can also find the unit of force using the equation for Newton’s 2 nd Law.
F = ma
_______ x _______ = ________
When units are determined by deriving them from other measured quantities they are called
_______________ units. So, the units for force can be written as either the fundamental unit
__________ or the derived unit ________________.
Sample Problems:
What is the net force required to accelerate a 1500-kg race car at 3 m/s2?
A cannon ball that has a mass of 55 kg if fired from a canon with a velocity of 770 m/s. The cannon
barrel is 1.5 m long. What is the force on the canon ball while it is in the barrel?
Now is also a good time to differentiate between mass and weight. Mass is the quantity of
________________ in an object. Remember that mass is a _______________ quantity, since it
is concerned only with magnitude and not ________________. Another important difference
between these two quantities is that the mass of an object (does/does not) change with the
location of the object. In other words, if you had a mass of 50 kg on earth and you traveled to the
moon, your mass when you reached the moon would be _________. There are two types of mass _____________________ and __________________. We will mostly be concerned with the
__________________ mass, which can be found by finding the amount of force needed to
accelerate an object. To do this, we will use Newton’s (1st, 2nd, 3rd) law.
Weight is the force of a body of matter due to the gravitational attraction of some other body of
matter – most commonly for us, this “other” body will be the _______________. If there is no
gravitational force present, an object (would still have/would not have) a weight. If the force of
gravity in a location is different than it is on earth, then an object experiencing this new force of
gravity would not have the same (mass/weight), but would have the same (mass/weight). Since
weight is a force, it is a _______________ quantity and the units of weight are _______.
We can use Newton’s 2nd Law to find the weight of an object. However, to do so we will use special
symbols. Since all objects accelerate towards earth because of gravity, we replace the “a” for
acceleration in the equation with ______. The force of gravity is present whether an object is
falling, resting on the ground, or being lifted. The force of gravity is called ______________. In
our equation, we can replace the “F” that represents force with ___________ (____).
Rewritten as the equation for weight, F = ma becomes _________ = ______ _______
Now do practice problems: page92, # 1-3 and page 94 #’s 5 - 7