Newton’s 2nd Law
We know that in order for an object at rest to
move it has to have a force on it.
For a moving object to stop it has to have a force
Most of what we were dealing with last chapter
was stuff that needed no forces on it (for
example a rock in motion in space needs no
force to keep moving).
For our next chapter we will be dealing with
objects that have forces that make them stop
moving or start moving.
• An object that is accelerating is changing its
• Any object that has a force on it can be
• Usually when we push on something, our
force is not the only force on the object.
• For an object to be accelerated, it must have a
total force on it that is not zero.
If two people pushed on an object in opposite
directions, each with a force on 10 N, what
would be the total force on the object?
This is a case where forces on an object do not
necessarily mean that the object’s motion will
If you push on a grocery cart that is already
moving with 3 N of force and the ground has a
frictional force of 3 N, the cart will move but it
will not be able to have its motion changed
because the total force on it is zero. The cart
will just move at a constant speed.
• The more mass an object has the more force is
needed to accelerate it.
• Big masses have small accelerations. This is
called an inverse relationship.
• Newton came up with a law that tells us how
hard an object must be pushed to produce a
certain acceleration. It is Newton’s Second
• The equation for Newton’s Second Law is:
force equals mass times acceleration or
F = ma
Objects in motion on Earth are affected by
friction, whether it is friction between two
surfaces or friction due to the air (air
Practice Problems. Do these in your notes.
1. What is the force needed to accelerate a 60
kg object at 4 m/s2?
2. How much acceleration will a 90 N force give
a 75 kg object?
3. The weight of an object is 35 N. If it
accelerates at 16 m/s2, how much force was
used to push it?
4. The force on an object is 15 N. If the force
causes an acceleration of 43 m/s2, what is the
mass of the object? What is the weight of
the object in Newtons?