Newton’s FIRST Law Of Motion

• Your science book sits on the table until you pick it up. If you hold your book above the ground, then let go, gravity pulls it to the floor.

• An object will speed up, slow down, or turn only if something is pushing or pulling on it.

Acceleration by Force

2

Force• A FORCE is a push or a pull.

• A force can be exerted in different ways. • For instance, a paper clip can be moved

by the force a magnet exerts, the pull of Earth’s gravity, or the force you exert when you pick it up.

• These are all examples of forces acting on the paper clip.

3

4

Net Force

• The combination of all the forces acting on an object is the NET FORCE.

• When more than one force is acting on an object, the net force determines the motion of the object.

Combining forces to form NET FORCE

• If the forces are in the same direction, then the net force is the sum between the two forces.

12 N 8 N

NET FORCE = 12N + 8N = 20N, left

Combining forces to form NET FORCE• If two forces are in opposite directions,

then the net force is the difference between the two forces, and it is in the direction of the larger force.

12 N 8 N

NET FORCE = 12N - 8N = 4N, right

Forces Can Cancel Each Other• A force can act on an object without

causing it to accelerate if other forces cancel the push or pull of the force.

• If you and your friend push on a door with the same force in opposite directions, the door does not move.

• Two or more forces exerted on an object are BALANCED FORCES if their effects cancel each other and they do not cause a change in the object’s motion.

• If the forces on an object are balanced, the net force is zero.

Balanced Forces

• If the forces are UNBALANCED FORCES, their effects don’t cancel each other.

• Any time the forces acting on an object are unbalanced, the net force is not zero and the motion of the object changes.

Unbalanced Forces

• A still object will never move without unbalanced forces acting on it

12N 12N

0N 0N

Balanced Forces = No Change in Velocity

• A moving object will never stop moving without unbalanced forces acting on it

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• Galileo Galilei, who lived from 1564 to 1642, was one of the first to understand that a force doesn’t need to be constantly applied to an object to keep it moving.

• Galileo’s ideas helped Isaac Newton to better understand the nature of motion.

Newton’s First Law of Motion

• Newton was able to explain the motion of objects in three rules called Newton’s laws of motion.

• According to NEWTON’S 1st LAW of MOTION, if the net force acting on an object is zero, the object remains at rest, or if the object is already moving, continues to move in a straight line with constant speed.

Newton’s First Law of Motion

In Other Words…• An object at rest

remains at rest unless an unbalanced force acts on it and causes it to move.

• An object in motion remains in motion unless as unbalanced force acts on it and causes it to stop.

a) Newton’s first law of motion is often called the Law of Inertia.

b) INERTIA is the tendency of all objects to resist any change in motion.

Law of Intertia

Ball wants to keep sitting still

Ball wants to keep moving

• Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever.

Law of Intertia

• Once airborne, unless acted on by an unbalanced force (gravity and air – fluid friction) the golf ball would never stop!

Law of Intertia

c) Mass is a measure of inertia

d) Small mass = less inertia

e) Large mass = more inertia

Law of Intertia, continued

Don’t let this be you. Wear seat belts!!

Because of inertia, objects (including you) resist changes in their motion. When the car

going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 m/hour.

Law of Intertia

Consider the behavior of a soda in a glass filled to the rim while starting a car from rest…

…while bringing a car to rest from a state of motion?

Examples of Newton’s 1st Law

Consider how blood rushes from your head to your feet while quickly stopping when riding on a descending elevator.

Examples of Newton’s 1st Law

Consider a brick being painlessly broken over the hand of a physics teacher by slamming it with a hammer. (CAUTION: do not attempt this at home!)

Examples of Newton’s 1st Law

Consider how to dislodge ketchup from the bottom of a ketchup bottle, it is often turned upside down and, thrusted downward at high speeds and then abruptly halted.

Examples of Newton’s 1st Law

Consider why headrests are placed in cars to prevent whiplash injuries during rear-end collisions.

Examples of Newton’s 1st Law

Consider how while riding a skateboard (or wagon or bicycle), you fly forward off the board when hitting a curb or a rock which suddenly stops the motion of the skateboard

Examples of Newton’s 1st Law

Today, Newton’s three laws of motion are considered to be the backbone of physical science. These ideas were major breakthroughs that set history on a new course. Because of these ideas we now have safety restraints in multiple types of travel.

Newton’s Laws of Motion

ANSWER THE FOLLOWING

SELF CHECK QUESTIONS:

Question 1

You try and push a desk across the floor one direction but a friend is trying to push it the opposite direction. As a result, the desk doesn’t move. This is an example of what kind of forces?

A. balanced forcesB. opposite forcesC. parallel forcesD. unbalanced forces

Question 2What happens any time there are unbalanced forces acting on an object—that is, any time the net force is not zero?

Question 3

Explain Newton’s first law of motion.

Question 4

Newton’s First Law is also referred to as the Law of _________________. Why?

Question 5

List and give an example of the four types of friction:

The answer is A: balanced forces. When forces are balanced, the net force is zero.

Question 1

You try and push a desk across the floor one direction but a friend is trying to push it the opposite direction. As a result, the desk doesn’t move. This is an example of what kind of forces?

Answer

Question 2What happens any time there are unbalanced forces acting on an object—that is, any time the net force is not zero?

AnswerAny time the net force is not zero, the object moves.

Question 3

Explain Newton’s first law of motion.

Answer

Newton’s first law of motion says that when the net force acting on an object is zero an object already in motion will tend to stay in motion and one at rest will tend to stay at rest.

Question 4

Newton’s First Law is also referred to as the Law of _________________.

Answer

Inertia; because inertia is an object’s tendency to resist any change in motion. In other words, an object at rest wants to stay at rest and an object in motion wants to stay in motion (unless acted upon by an unbalanced force)

STUDENTS, BE PREPARED

FOR A 5-10 QUESTION POP

QUIZ ON NEWTON’S FIRST

LAW OF MOTION ON

MONDAY!