The Force and Newton’s 2nd Law
What is a force?
A force by definition is :
The push or pull on an object by interacting with one or more
other objects! Pushing or pulling are the essentials over what
we will be going over.
Newton’s 2nd Law
So we now know that forces represent the push
or pull on an object… but how can this "force"
be defined? With an equation!
Newton's Second Law states that:
• F = MA
• Where F is the "Total Force on an object", M is
the "Mass of an object", and A is the
"Acceleration of an object"
What does this F=MA mean?
Look at the equation and think about it.
What this equation is telling us is that the force on an
object is equal to the object’s mass multiplied by its
• So, the amount of force on an object depends on
both its mass and its acceleration.
• Also, keep in mind that all three variables, or letters,
have units applied to them.
Units of the Force Equation
(These units are the Newton (N) for Force,
Kilogram(Kg) for Mass, and Meters per Second
Squared (M/S2) for Acceleration.)
We will be working with this equation
quite often in the future, so don't worry
about not being able to absorb all this
information at once.
Think about it
Are any forces acting on you or I when we’re
completely still, with no other physical objects
interacting with us?
The answer is…
Yes!! The force of gravity is acting on each and every
one of us. Even if there’s no physical object
interacting with you, the Earth’s gravity still affects
The force of gravity can also be thought of as the
acceleration of gravity multiplied by your mass.
Acceleration of gravity is:
g = -9.8 m/s2 (Acceleration ”a” is replaced by “g”)
So, for the force of gravity, the force would be:
Fgravity = Massobject x -9.8m/s2
What is the mass of an object?
The mass of an object is a property/trait that we
all have, that can never ever EVER change! We
are all made up of mass.
If a piece of paper were to be crumpled up, its
shape has changed, but the amount of mass
that it has is still the same.
Let’s say a car is moving with a force of 4000 Newtons. The car has an
acceleration of 4 m/s2 and a mass of 1000kg. When we double that force,
what happens to that car’s acceleration and its mass?
Think about it #2
Again, mass can NEVER change.
So by doubling the force, the mass stays the
same, and we double the acceleration. Thus,
the car’s acceleration is now 8 m/s2.
More about Forces
So far we’ve talked about how a force can be
described as a push or pull by a single object.
For many forces on an object, we either add or
subtract all the forces acting on that object,
depending on each force’s direction.
If two forces are in the same direction, they will
If they act in the opposite direction, they will
In this lesson, we will only be talking about forces
coming from the left or right.
Think about it #3
a) If I push a car with a force of 150 N to the right, and my friend helps me
push with a force of 100 N to the right as well, how much total force
would we apply to the car?
b) What if I pushed the car with 150 N to the right, and my friend pushes the
car 100N to the left? What direction do you think the car would move?