Newton’s First Law of Motion

unit 6.3year 10

Context• Forces act on us every day, causing

many different effects. In 1687, Isaac Newton asked how these forces act and what interactions occur between them. He then formulated three laws to explain how objects move when a force acts on them. They are often referred to as Newton’s Laws of Motion.

Force• A force is a push, pull or twist that can

cause an object to: – increase its speed (accelerate) –decrease its speed (decelerate) –change its direction–change its shape.

• If any of these things happen, then a force caused it. It is possible, however, for a force to be acting without any of these things happening.

Newton’s First Law• An object at rest stays at rest and an

object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.• sometimes referred to as the law of

inertia.

Newton’s First Law• There are two parts to this statement

- one that predicts the behavior of stationary objects and the other that predicts the behavior of moving objects. The two parts are summarized in the following diagram.

Newton’s First LawForces are balanced

Object at rest(v = 0 m/s/s)

Stay at rest

Object in motion

(v ≠ 0 m/s/s)

Stay in motionSame speed &

direction

Newton’s First Law• The head of a hammer can be tightened

onto the wooden handle by banging the bottom of the handle against a hard surface.

• A brick is painlessly broken over the hand of a physics teacher by smashing it with a hammer. (do not attempt this at home!)

• To dislodge sauce from the bottom of a bottle, it is turned upside down and thrust downward at high speeds and then stopped.

Newton’s First Law

"keep on doing whatthey're doing”

LadderBad driver

Inertia and Mass• Objects tend to "keep on doing what

they're doing." In fact, it is the natural tendency of objects to resist changes in their state of motion. This tendency to resist changes in their state of motion is described as inertia.

Inertia and Mass (Galileo)

Inertia and Mass• Isaac Newton built on Galileo's

thoughts about motion. Newton's first law of motion declares that a force is not needed to keep an object in motion. • Slide a book across a table and watch

it slide to a rest position.

Inertia and Mass• Demonstrations

Check your understanding1. Imagine a place in the cosmos far from all gravitational and frictional influences. Suppose that you visit that place (just suppose) and throw a rock. The rock will…a. gradually stop.b. continue in motion in the same

direction at constant speed.

Check your understanding1. Imagine a place in the cosmos far from all gravitational and frictional influences. Suppose that you visit that place (just suppose) and throw a rock. The rock will…a. gradually stop.b. continue in motion in the same

direction at constant speed.

Check your understanding2. A 2-kg object is moving horizontally with a speed of 4 m/s. How much net force is required to keep the object moving at this speed and in this direction?Answer: 0 N. An object in motion will maintain its state of motion. The presence of an unbalanced force changes the velocity of the object.

Check your understanding3. Supposing you were in space in a weightless environment, would it require a force to set an object in motion?

Check your understanding3. weightless environmentAbsolutely yes! Even in space objects have mass. And if they have mass, they have inertia. That is, an object in space resists changes in its state of motion. A force must be applied to set a stationary object in motion. Newton's laws rule – everywhere!

State of Motion• Inertia is the tendency of an object

to resist changes in its state of motion. The state of motion of an object is defined by its velocity - the speed with a direction. Thus, inertia could be redefined as follows:

Inertia: tendency of an object to resist changes in its velocity.

Check your understanding1. A group of physics teachers is taking some time off for a little putt-putt golf. The 15th hole at the Hole-In-One Putt-Putt Golf Course has a large metal rim that putters must use to guide their ball towards the hole. Mr. B guides a golf ball around the metal rim When the ball leaves the rim, which path (1, 2, or 3) will the golf ball follow?

Check your understanding

Check your understanding• The answer is 2. Once leaving the rim,

the ball will follow an "inertial path" (i.e., a straight line). At the instant shown in the diagram, the ball is moving to the right; once leaving the rim, there is no more unbalanced forces to change its state of motion. Paths 1 and 3 both show the ball continually changing its direction once leaving the rim.

Balanced and Unbalanced Forces

• An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.• what exactly is meant by the phrase

unbalanced force? What is an unbalanced force?

Balanced and Unbalanced Forces

• Consider a book on the desk.• There are two forces acting upon the

book. One force - the Earth's gravitational pull - exerts a downward force. The other force - the push of the table on the book (sometimes referred to as a normal force) - pushes upward on the book.

Balanced and Unbalanced Forces

• Since these two forces are of equal magnitude and in opposite directions, they balance each other. The book is said to be at equilibrium.• It maintains its state of motion, it will

not accelerate while all forces are balanced

Balanced and Unbalanced Forces

• If enough force is applied to the side of the book it slides along the desk until the force of resistance stops the book.• While accelerating (+ or -) the forces

being applied to the book are unbalanced.

Check your understanding4. If the forces acting upon an object are balanced, then the object

a. must not be moving.b. must be moving with a constant velocity.c. must not be accelerating.d. none of the above

Check your understanding4. If the forces acting upon an object are balanced, then the objectThe answer could be A (but does not have to be A) and it could be B (but does not have to be B). An object having balanced forces definitely cannot be accelerating. This means that it could be at rest and staying at rest (one option) or could be in motion at constant velocity (a second option). Either way, it definitely is not accelerating - choice C of your four choices.