Name __________________________________________________________________
Period __________
Reading Guide 4.1
We have already learned about Newton’s 1st law of motion back in chapter 1.1. We will use this law through this chapter
along with Newton’s 2nd and 3rd. Let’s start by reviewing Newton’s 1st. (1) Simply stated, objects will continue doing
whatever it is they’re doing unless being acted on by a ___________________ force. Figure 4.1 shows a book sitting on
a table. The arrows drawn are vectors showing the magnitude and direction of the forces acting on the book. (2) What is
creating the upward force? _____________________ What is creating the downward force?__________________
These two forces are acting at the same time on the book. The book isn’t changing what it’s doing and so the book is in
equilibrium. An object doesn’t need to be holding still to be in equilibrium. Remember Newton’s 1st law says things will
keep doing what they’re doing (holding still or moving with a constant velocity) unless acted on by a net force.
Equilibrium just means the forces are in balance and the object doesn’t change its speed or direction. A car driving down
the road at a constant speed is in equilibrium. (3) What is the magnitude of the net force on this car?
__________________________ A lot of people believe that the car is moving because of a force from the engine
pushing the car in the direction its moving, and they would be right. The problem is that their thinking is incomplete.
This cannot be the only force acting on the car in the horizontal direction. (4)If the engine’s force were the only force,
what would the car do?
Another simple way to look at Newton’s 1st Law is: If the net force equals zero –> constant speed. If the net force is
anything other than zero –> changing speed (acceleration). There are two ways to find the net force.
The first way is drawing vectors. We have already learned this method. If force vectors act on a line you can add or
subtract them. If force vectors act at a right angle you will need to use the Pythagorean Theorem. Let’s practice. Draw
pictures of the stories and then determine the net force.
(5) Jack is pushing a box to the left with a force of 83 N but Ann is pushing the opposite direction with a force of 100 N.
There are no other forces acting on the box. Draw a picture and determine the net force.
(6) Jack is still pushing the box to the left with a force of 83 N but now Ann is pushing at a right angle to Jack. Ann still
has a force of 100 N. There are no other forces acting on the box. Draw a picture and determine the net force.
The second way we are able to determine the net force acting on an object is to use Newton’s 2nd law. To really
understand the 2nd law we need to go back a little and remember what acceleration is. Acceleration is the change in
velocity divided by the time needed to make the change. That change in velocity is what acceleration IS but Newton’s 2 nd
law tells us how we GET it. Many people believe a force is the way to get something to move and again that’s correct but
incomplete. Push on the wall of the school sometime and tell me if you can get it to move. Any force isn’t enough to get
an object to start moving. If the wall is as strong as it should be it will in a sense “push back”. The forces will be balanced,
the wall stays in equilibrium and so it doesn’t change what it’s doing. (7) What kind of force is needed to make an object
change what it’s doing? ____________ (you had better know this because it was a previous answer to a question) On
Page 171 at the bottom Newton’s 2nd law is explained. The short version of this law is Fnet=ma. (8)Use the explanation
on page 171 and write this law in your own words.
(9)Take Newton’s 2nd law and solve it for acceleration. a=
(10)Newton was the first to realize that the acceleration we produce when we move something depends not only on
how hard we push or pull (the force) but on the _____________ as well.
Use the 2nd law to help answer the following questions.
(11) If the net force acting on an object increases, what will happen to the object’s acceleration? (increase, decrease, or
nothing)
(12) What if the net force acting on an object remained the same but the mass of the object was able to increase. What
will happen to the object’s acceleration? (increase, decrease, or nothing)
(13) Find the force on an object with a mass of 300 kg that is accelerated at a rate of 2 m/s2.
(14) Determine the mass of an object if you know you are able to create an acceleration of 5 m/s2 by pushing on it with a
force of 200 N.
We also use Newton’s 2nd law when we talk about weight and how it relates to mass. You should have learned in past
classes that all objects fall with the same acceleration regardless of their mass. This law explains why. We can use the
symbol Fg to represent gravitational force. (15) Another word that means gravitational force is _________________.
(16) What is the magnitude of the earth’s gravitational acceleration in SI units?
Newton’s 2nd law works for all objects with mass, acceleration, and force. F=ma. When we are talking about gravity this
equation works here too but it is often written differently. w=mg (17) Use this new variation of Newton’s 2nd Law and
tell me what the different variables stand for.
Use Newton’s 2nd law again to help with the next 2 questions.
(18) If an object on Earth has a mass of 10 kg, how much does it weigh?
(19) If an object on Earth has a weight of 200 N, how much mass does it have?
Next we’ll learn about friction. Many times in physics we will ignore the effects of friction because it simplifies things.
Through the next part of this reading guide we’ll use the book to find out more of what friction is and two types of it.
(20) Friction is a result of what?
(21) Describe Static Friction.
(22) Describe Kinetic Friction.
(23) Which is usually greater – static or kinetic friction?
(24) Describe what is meant by the Normal Force?