Chapter 6 Notes

Chapter 6 Forces and Newton's laws of motion

A force is defined as any push or pull. All forces are derived from only four basic forces (also known as interactions). They are gravitational, electromagnetic, strong nuclear force, and weak nuclear force. Forces are carried or transmitted between particles by other particles:

Electromagnetic forces are carried by photons. This force exists between any two charged objects.
The weak nuclear force is carried by three particles known as the weak bosons. It is associated with the radioactive decay of some nucleii.
The strong nuclear force is carried by eight particles known as the gluons. It is the strongest force and binds quarks into protons and neutrons
Gravity is thought to be carried by a yet undetected particle called a graviton. It is the weakest of the forces and is an attractive force between any objects with mass.

There are several current theories which attempt to relate all of these forces as being subsidiary to one single force. Such a theory is the Grand Unifying Theory (GUT).

Newton's First Law of Motion

1) An object in motion will remain in motion until an unbalanced force acts on it. We also say that a body at rest will remain at rest until a forces acts on it. (Actually, if all forces are removed from an object the term "at rest" or "in motion" depends on the reference frame of the observe.) Galileo probably was the real founder of this law, having done much of the work before Newton's time. This law is often called Galileo's Law of Inertia.

(2) Terms to know: inertia, net force, balance and unbalance force.

Newton's Second Law of Motion

F = ma : This is by far the most important equation in first year physics, and along with the basic photosynthesis equation governs much of the existence you have come to enjoy!

NOTE: "F" in this equation represents the NET force!

The first step in solving any force problem is to write down F = ma .

There will be other versions of this formula which we will explore later.

The law states that if an unbalanced force is applied to a mass, that mass will accelerate. Furthermore, the acceleration will be in inverse proportion to the mass.
F = ma is a vector relationship.

The Unit of Force

F = ma = (1.00 kg)(1.00 m/s2) = 1 Newton, (1 N is about the weight of an apple). There are about 4.45 Newtons per pound.

Newton's Third Law of Motion

For every action there is an opposite and equal reaction. Imagine jumping from a small canoe unto a dock.

video: The Mechanical Universe: Newton's Laws of Motion (:15) and The Fundamental Forces (:16)

Mass and Weight

You will not pass this course without a clear understanding of the difference between mass and weight.
Mass is the amount of matter in a substance.

Normally weight is considered the gravitational force of attraction of a small object and a much larger object- between you and the earth. We could easily calculate the force of attraction between the moon and the earth, but to call this a weight would not have a lot of meaning.

weight = m * g

where g is the acceleration of gravity (9.80 m/s2)

In this course you will calculate weight in every conceivable way. For example, how much do you weigh on Mars due to the gravity of Jupiter?
How much does a 200 lb. astronaut weigh on the space shuttle while in orbit? The answer is about 170 lb.

Normal Force:

- “The contact force exerted by a surface on an object.”
- “Perpendicular to and away from the surface.”

Weighing yourself in an accelerating elevator:
Possible Steps:
1. What is your/the elevator’s acceleration?
2. What is your mass?
3. What is the net force?
4. Draw your free body diagram.
5. Write an equation with your net force and all forces acting on you.
6. Solve for the force of the scale.

Net Force

Net force is the sum of all forces acting concurrently upon an object.

Such forces do not necessarily have to act in the same dimension.

The net force might not be in the same direction as any of the individual forces at work.

Net force, like all forces, is vector in nature and will have both magnitude and direction.

Newton’s 2nd law formula, F = ma, should always use the net force for F.


What is the net force on this car?

If the car has a mass of 500.0 kg, what is the acceleration of this car?

Example 2

If the forces are as follows: (mass of the plane = 3.63 x 104 kg)
Lift = 3.56 x 104 N
Gravity = 3.56 x 104 N
Thrust = 4.4 x 104 N
Drag = 9.2 x 102 N

What is the net vertical force?
What is the net horizontal force?
What are the horizontal and vertical accelerations?



You must apply a free body diagram to these problems. On the diagram always indicate the direction of motion; then the force of friction,force due to friction , will be in the opposite direction.

Static friction is the force you must overcome, in order to start moving. Sliding friction (some books call this kinetic friction) is the force you must overcome to maintain motion. The frictional force is never measured directly, it's measured by finding the force required to overcome friction.


where u is the coefficient of friction.normal forceis the normal force, that force pressing upward.


This is an important class of problems. Since an object at constant velocity is not subjected to unbalanced forces, the amount of applied force Fa required to keep it in steady motion is equal, but opposite in direction to the frictional force Ff which tries to retard the motion.

Here is a step by step solution to a friction problem!

 Acceleration When Two Forces Act on an Object

We'll solve problems involving more than two forces; using the free body diagram makes these problem easy.

The Fall of Bodies in the Air

Due to gravity, all objects fall at the same rate. In class we'll discuss Galileo's
thought experiment which proves this statement.

Terminal velocity is when the upward force due to air resistance equals the force
downward due to gravity; therefore acceleration will be zero ( but the object
remains in motion).

*Note: If the object has constant velocity (zero acceleration), the applied forceforce in the x-direction = the force of frictionforce due to friction. In lab, this is the way we determine the frictional force.

 Homework: Set 1: P 144-146, problems 1-4, 20-25

Set #2: problems 26-35

Set#3: problems 36-40

Video: (1)Fundamental Forces, (2) Simple Harmonic Motion

Lab experiment: Coefficient of Friction, Force Table, Simple Pendulum