Forces and Newton’s First Law



Forces and Newton’s First Law

Thus far, we have studied the motion of objects. The study of motion is known as Mechanics. However, we were not interested, yet, about what caused the motion. This branch of mechanics is known as Kinematics. Thus far, we have been interested in how something moves. Did it accelerate, in what direction, etc? But now let’s probe deeper. Why did the object move? Did someone push the moving object? Maybe something that was holding the object up suddenly let it go. What caused the motion? This question leads us into the 2nd branch of Mechanics. This branch, which studies the causes of an object’s motion, is known as Dynamics.

So what actually “causes” motion, or a change in motion? The answer is: FORCES! How can we define the term “force”? A force is ANYTHING that could cause an object to accelerate. In the SI unit system, we measure forces using a unit called a Newton (N). This is a derived unit, and [pic]. In the English system of units, we measure forces in pounds (lb). 1 pound is equal to approximately 4.44 N.

There are two categories of forces. The first type is known as a CONTACT FORCE. This includes pushes and pulls. The second type is a FIELD FORCE. This type of force does not require physical contact, and includes gravitational field forces, electric field forces, and magnetic field forces.

People have been interested in the motion of objects for thousands of years. In 350 B.C. Aristotle, a Greek philosopher, studied the motion of objects and concluded the following: “It requires a continuous pushing or pulling” to keep an object such as a rolling stone moving. When the pushing or pulling is no longer applied, the stone comes to rest……A FORCE is required to produce a constant velocity” (Heath, pg. 128) In summary, He concluded that with increased force, an object moves faster. With decreased force, it moves slower. With no force, the object will stop. Sounds very logical right? Aristotle’s views on motion dominated the study of motion for almost 2000 years.

During the Renaissance, Galileo Galilei, the great Italian physicist, used two different “thought experiments” about the motion of an object on an inclined plane to explain motion.

Experiment #1: Galileo imagined a ball rolling down a sloped plane. He figured that it would speed up. He then imagined a ball rolling up a slope. He figured that it would slow down. He reasoned that a ball rolled across a horizontal surface would neither speed up nor slow down, but rather continue to move with a constant velocity.

Experiment #2: Galileo again imagined a ball rolling down a sloped plane. However, this time, he allowed the ball to roll up a plane afterward. He reasoned that no matter what the slope of the two planes, the ball would always attain the same height (the height it was rolled down from equals the height it rolls up to). Therefore, he concluded that if no“up” plane were present at the bottom of the “down” plane, the ball would roll on FOREVER, trying to but never reaching the height from which it were dropped.

Both of Galileo’s thought experiments contradicted his observations of the same events in real-life. However, he attributed the differences to “Resistance”, or what we today call friction. Both thought experiments occurred on a frictionless surface. Galileo argued

that it was just as ‘natural’ for an object to move with a constant speed as it is to be at rest. This contradicted Aristotle’s view.

Finally, in the 17th century, the Englishman Isaac Newton came along. He began his theories of motion by looking at a concept that he called “Inertia”. Inertia is a property of an object. It can be thought of as ‘laziness’. Objects tend to keep doing what they are doing. It takes force to make an object start moving or change direction. The more massive an object is, the larger the force that is required for a given change.” (Holt, teacher’s addition) Mass is a measure of inertia. Galileo was the first person to formalize this concept. However, Newton used it to develop his famous “Laws of Motion”, which he formally published in his book Principia Mathematica. This book is widely considered to be the greatest scientific work ever published.

Newton’s “First Law” of motion, otherwise known as “Newton’s Law of Inertia”, informally states that

This law can be stated in layman’s terms as follows

Notice that the term “IN MOTION” doesn’t mean any old motion, but rather means “constant velocity”. What does the term “unbalanced” mean? It means that when all the forces upon an object are looked at, the NET FORCE IS NOT EQUAL TO ZERO. Also internal forces have no effect on an object’s motion. Only external forces affect motion.

But if this is in fact a “Law” of motion, why don’t we see this happen in the “real world”. When an object is pushed, it slows down, even when no other forces act upon it. Or so it

would seem. FRICTION is the “invisible” force that acts on objects to slow them down. However, in a friction-free case, Newton’s 1st Law will be “observed” to hold true. And, it holds true in all cases, we simply have a hard time “observing” it because of friction.

Examples of Newton’s 1st Law include:

• A person (without a seatbelt) going through the windshield in a car accident.

• A magician pulling the tablecloth off a table but leaving the place-setting.

• Getting a car to start moving vs. keeping it rolling once it’s started.

• Punching a light “speed bag” vs. punching a “heavy bag”.

So, why is Newton’s 1st Law often called the “Law of inertia”? To answer this, we first need to define the term “inertia”. Inertia is …

For example, when a heavy grocery cart is pushed quickly down the super market aisle, it is difficult to stop, or eve slow down. Why? Because is has a lot of inertia. It has a great tendency to resist a change in its current motion. On the other hand, a textbook is very easy to slide accelerate across a desk. It has very little inertia, and thus has very little tendency to resist this change in its motion. The bottom line is this: all objects have a natural tendency to either stay at rest or to keep moving at a constant velocity. Remember, these are the two natural states of motion for an object (according to both Galileo and Newton). Some objects have more tendency, while others have less. An object’s MASS is a measure of its inertia. Which only makes sense, since massive objects are much harder to either move from rest or to budge from a state of constant velocity.

Therefore, in summary:

Forces are capable of causing accelerations and are measured in N or lb.

Objects naturally want to stay at rest or in a state of constant velocity.

The term “constant velocity” implies that Fnet = 0……

…. and if Fnet = 0, then an object will stay at rest or keep a constant velocity.

If Fnet = 0, then a = 0. Inertia can be quantitatively measured by measuring an

object’s mass.

Newton’s First Law Worksheet

1. Fully and clearly state Newton’s 1st law

2. An object is at rest. What does the object have that tends to keep it at rest?

3. What would it take to force the object to move? Be specific, using Newton’s first law.

4. An object is moving along at a constant velocity of 100 cm/s. What is the external force acting on the object? Explain using Newton’s 1st Law.

5. An object is sliding with a velocity of 10 m/s [S] along a perfectly frictionless surface. Instantly, a 45 N [N] force acts upon the object. Based on your knowledge of Newon’s 1st Law, what do you think will happen? Will the object remain in motion or will its motion change? If the motion changes, what kind of change occurs?

6. An object is sliding with a velocity of 4 m/s [NE] along a perfectly frictionless surface. Instantly, 3 forces act upon the object. The forces are 5N [W], 15N [E], and 10N [W]. Based on your knowledge of Newon’s 1st Law, what do you think will happen? Will the object remain in motion or will its motion change? If the motion changes, what kind of change occurs?

7. An object is at rest on a perfectly frictionless surface. Instantly, a 10 N [W] force acts upon the object. Based on your knowledge of Newon’s 1st Law, what do you think will happen? Will the object remain in motion or will its motion change? If the motion changes, what kind of change occurs?

8. According to Newton’s 1st Law, there are two “natural” states of motion that an object tends to be in. What are they?

9. Convert 10 N to lbs. Then, convert 85 lbs to Newtons. (recall that 1 N = ____ lbs)

10. A car is driving along a straight road. A passenger is sitting in the front-passenger seat, which just so happens to be greased with Crisco. There are no doors on the car and no seatbelts (illegal? Maybe.). The car comes to a hard turn in the road and attempts to complete the turn without slowing down. Explain, using Newton’s 1st Law, what will happen to the passenger in the front seat.

11. In real-life, objects in motion don’t tend to stay in motion. What is the unbalanced force that keeps most objects that are in motion from remaining in motion indefinitely? Give three different examples of this type of force.

12. The SI unit of Force is the Newton. Since this is not a “base” SI unit but rather a “derived” unit, we must always remember what “base” units make up the Newton. Write 1N in terms of only kg, sec, and m. (look it up on the internet if you need to … google it ()

13. Give two different examples of “contact” forces. Then, give two different examples of “field” forces.

14. Find the net force in each situation below. Make sure to give both a magnitude and a direction. AND YES, YOU WILL NEED A CALCULATOR ON THIS PROBLEM (

a) 15 N [right] and 42 N [left] b) 65 N [West] and 50 N [North] c) 32 N [E 30 N] and 16 N [South]

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Lyzinski Physics

CRHS-South

AN OBJECT IN MOTION (OR AT REST) TENDS TO STAY IN MOTION (OR AT REST) UNLESS ACTED UPON BY AN UNBALANCED, EXTERNAL FORCE.

#1

OBJECTS WILL STAY AT REST OR AT A CONSTANT VELOCITY UNLESS FORCED TO CHANGE.

THE TENDANCY OF AN OBJECT TO RESIST A CHANGE IN ITS CURRENT MOTION STATE.

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