Download 1st Law Teacher Notes

Newton’s First Law
Introduction
The previous unit dealt with “kinematics,” a ____description ______ of how an _____object________
moves. This unit deals with “dynamics,” an _____explanation___ of _______motion______. Whereas kinematics
examines how an object moves, dynamics looks at the forces which cause this motion – why the object moves.
Forces:
-
can be defined as __a push/pull on an object (measured in Newtons)___
-
are vectors since they ___have both magnitude and direction______
-
often involve contact between objects (ex., friction, pushing, pulling)
Aristotle, a famous and very influential Greek philosopher in the years 384 BC to
322 BC concluded that when there are no forces acting on an object, the object
would not move (or alternately, a moving object must have forces acting on it in
order to keep it moving). This conclusion seemed so obvious that no one
questioned it for nearly 2,000 years. Galileo (in
about 1590) suggested that objects could move
without any forces acting on them. About 50 years
later, Isaac Newton extended Galileo’s work with
forces and produced what some scientists say was
one of the greatest scientific books ever published.
His book, “The Mathematical Principles of Natural
Philosophy” contained his laws of motion as well as
a new mathematics that today we call “calculus.”
Aristotle
Newton
Net Force
Many different forces can act on an object at the same time. For example, if you are pulling a box along the
ground on a windy day, there are several forces acting on the box. There is the force of __gravity___, the
force of the _____ground pushing up_______, the force of ____friction____ from the ground, the
force of wind ___friction____, and the force of you _____pulling__________ (see diagram).
The length of the arrows
Ground force or
normal force
indicate the relative
__strength of ___
Wind friction
Pull force
___the force____.
The net force is the _sum_
Ground
friction
of all these forces.
Force due to gravity (or weight)
Mathematically, the net force on an object can be written as
Fnet = F1 + F2 + F3 + …
where F1, F2, etc. represent the different forces acting on the object.
Sample Question
1) What is the net force acting on an object being pulled east by a 34N and a 73N force and west by a 104N
force?
Balanced and Unbalanced Forces
Balanced forces are forces which cancel each other out. They are usually forces of the ___same_
__size___ but acting in ____opposite___ directions. For example, the ground force and
_____gravity_____ in the above diagram are balanced forces. As well, the pull force could balance the
____ground_____ and ______wind_____ friction forces.
When all forces acting on an object are balanced, then the net force is equal to __zero__. Fnet = 0 When all
forces on an object are not balanced, then the net force is not equal to zero. (Fnet ≠ 0). This object is said to have
“_____unbalanced____” forces acting on it. For example, forces on the box would be unbalanced if the “pull
force” is greater than the ______ground/wind_____ friction forces.
Balanced forces
Unbalanced Forces
F1
F1
F4
F4
F2
ob ject
obj ect
F2
F3
F3
Newton’s 1st Law of Motion
Newton’s 1st law of motion deals with balanced forces acting on an object. That is, all the forces acting on an
object must add up to __zero__ (Fnet = 0).
1st Law
When the net force acting on an object is zero, then:
i) a moving object continues to move with a ____constant velocity______.
ii) an object at rest remains ___at rest____.
This means that when an object is not moving or when it is moving at a constant velocity, the sum of all the
forces acting on it is ____zero___.
Situation 1
A pencil is lying, at rest, on your desk.
F des k
The net force must be __zero__. There
are two forces acting on the pencil:
_gravity____ acting down and the desk
___pushing up__. These two forces
are ___balanced___ (equal and opposite).
pencil
desk
F gravity
Situation 2
An eraser is being pushed across your desk
at a constant velocity. The net force must be
F de sk
F pu sh
F fri cti on
era ser
_zero_. There are four forces acting on
F gra vi ty
the eraser: __gravity__, the desk
_pushing up_, you __pushing__
the eraser, and the __force__ of
friction.
Exercises (Newton’s First Law)
1a) Aristotle's view of motion (an object at rest has no forces acting on it and any moving object has unbalanced
forces acting on it) seemed so natural. Why? What easy experiment could anybody do to support his view?
1b) What "flaw" in Aristotle's reasoning did Galileo see? What experiment could be designed to support Galileo's
view?
1c) In our present state of high technology, where could one go where there are essentially no friction forces to
'prove' Galileo's hypothesis?
1d) Explain why the second statement in Newton’s First Law is redundant
(unnecessary).
2. What is the net force on an object being pulled East by a 55 N force as well as a 75 N force and West by a 67 N
force and a 34 N force?
3. An object has the following four forces acting on it: 50 N North, 20 N South, 35 N East and 35 N West. What
would be the size and direction of a fifth force needed to produce a net force of zero?
4. A 54 kg student is sitting at rest in a chair. What force and in what direction is the chair exerting on the
student?
5. A car is moving at constant velocity along a horizontal road. The road is pushing
up with a magnitude of 1.2x104 N and there is friction force of 800 N.
a) What is the mass of the car?
b) With what force is the engine exerting on the car?
6a) State in words Newton's 1st law of motion.
6b) What is meant by "net force = zero”? Give an example.
7. In what direction does friction always act?
Friction, baby!
Figure 1.
8 Consider a book at rest on a table top.
A vector force diagram is shown in
figure 1. Fg is the force of gravity and
Ft on b is the force of the table on the
Figure 2.
F
F
t on b
bo ok
bo ok
ta ble
book.
t on b
ta ble
Fg
Fg
8a) what can be said about the
magnitude of the two forces? Why?
8b) Suppose that you push the book to the right along the table top at a constant velocity. In figure 2 above, add
additional force vectors for this situation. Give your force vectors meaningful labels.
9. A 250 gram mass is lying at rest on a desk. We all know that gravity is acting on the mass and many people
(those that haven't taken physics 11) might say that gravity is the only force acting on the mass.
a) Explain why gravity can not be the only force acting? What is the other force?
b) What is the value of the magnitude (size) of this other force?
10. A small plane is traveling horizontally at a constant velocity. What can be said about the net forces acting on
the plane in both the horizontal and vertical directions.
Yes, friction force is at play when you get
a massage!
11a A 4.5 kg desk is being pushed across a floor at a b) Calculate the coefficient of friction for this
constant velocity of 0.50 m/s with a push force
motion.
of 40 N. What is the magnitude of the force of
friction? (don't be fooled)
12. A 1200 kg car is being pushed (in neutral) along a level road at constant speed. If the coefficient of rolling
friction is 0.20, what force must be exerted on the car to keep it moving? What is the magnitude of the force
of friction acting on the car?
13. Given the vector diagrams shown describe the type of motion that may result. In some cases there may be
more than one interpretation.
a)
b)
F1
F1
F4
F3
-F 1
F2
c)
d)
F1
F1
-F 2
F2
-F 1
-F
-F
2F
-F 1
Numerical Answers:
2) 29 N East, 3) 30 N South, 4) 529 N upward, 5a) 1.2x103 kg, 5b) 800 N, 9b) 2.5 N up, 11a) –40 N, 11b) 0.91,
12) 2.4x103.