NEWTON’S THREE (3) LAWS OF MOTION

THREE (3) LAWS OF MOTION

• LAW OF INERTIA• LAW OF

ACCELERATION• LAW OF ACTION-

REACTION

LAW OF INERTIAAn object at rest will remain at rest unless acted on by an unbalanced force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

What does this mean?

This means that there is a natural tendency of objects to keep on doing what they're doing. All objects resist changes in their state of motion. In the absence of an unbalanced force, an object in motion will maintain this state of motion.

What is the motion in this picture?

What is the unbalanced force in this picture?

What happened to the skater in this picture?

This law is the same reason why you should always wear your

seatbelt. Seat belts are used to provide safety for passengers whose motion is governed by Newton's laws. The seat belt provides the unbalanced force which brings you from a state of motion to a state of rest. Perhaps you could speculate what would occur when no seat belt is used.

1. Blood rushes from your head to your feet while quickly stopping when riding on a descending elevator.

2. The head of a hammer can be tightened onto the wooden handle by banging the bottom of the handle against a hard surface.

3. A brick is painlessly broken over the hand of a physics teacher by slamming it with a hammer. (CAUTION: do not attempt

this at home!) 4. 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.

5. Headrests are placed in cars to prevent whiplash injuries during rear-end collisions.

Applications of Newton's first law

Check Your Understanding

1. The group of physics teachers are 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 which putters must use to guide their ball towards the hole. Mr. S 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?

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.

a. 0 N b. 0.5 N c. 2.0 N d. 8.0 N

e. depends on the speed.

ANSWER: If an object is in motion, then it will stay in motion with those very same motion characteristics. It doesn't take any force to maintain that same state of motion. In fact, the presence of a force would "ruin" such a state of motion and cause an acceleration.

2. A 4.0-kg object is moving across a friction-free surface with a constant velocity of 2 m/s. Which one of the following horizontal forces is necessary to maintain this state of motion?

INERTIA

• " 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: the resistance an object has to a change in its state of motion.

• Newton's conception of inertia stood in direct opposition to more popular conceptions about motion.

• The dominant thought prior to Newton's day was that it was the natural tendency of objects to come to a rest position. Moving objects, so it was believed, would eventually stop moving; a force was necessary to keep an object moving. But if left to itself, a moving object would eventually come to rest and an object at rest would stay at rest;

•Thus, the idea which dominated people's thinking for nearly 2000 years prior to Newton was that it was the natural tendency of all objects to assume a rest position.

Galileo and the Concept of Inertia

Forces Don't Keep Objects Moving

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Mass as a Measure of the Amount of Inertia

All objects resist changes in their state of motion. All objects have this tendency - they have inertia. But do some objects have more of a tendency to resist changes than others? Absolutely yes! The tendency of an object to resist changes in its state of motion varies with mass. Mass is that quantity which is solely dependent upon the inertia of an object. The more inertia which an object has, the more mass it has.

ANSWER:According to Newton's first law, the rock will continue in motion in the same direction at constant speed.

Check Your Understanding

1. 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 willa. gradually stop. b. continue in motion in the same direction at constant speed.

2. 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 NAn object in motion will maintain its state of motion. The presence of an unbalanced force changes the velocity of the object.

3. Mac and Tosh are arguing in the cafeteria. Mac says that if he flings the Jell-O with a greater speed it will have a greater inertia. Tosh argues that inertia does not depend upon speed, but rather upon mass. Who do you agree with? Explain why.

Tosh is correct. Inertia is that quantity which depends solely upon mass. The more mass, the more inertia. Momentum is another quantity in Physics which depends on both mass and speed. Momentum will be discussed in a later unit.

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

Absolutely 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!

5. Ben Tooclose is being chased through the woods by a bull moose which he was attempting to photograph. The enormous mass of the bull moose is extremely intimidating. Yet, if Ben makes a zigzag pattern through the woods, he will be able to use the large mass of the moose to his own advantage. Explain this in terms of inertia and Newton's first law of motion.

The large mass of the bull moose means that the bull moose has a large inertia. Thus, Ben can more easily change his own state of motion (make quick changes in direction) while the moose has extreme difficulty changing its state of motion. Physics for better living!

Balanced and Unbalanced Forces

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Determining the Net Force

Drawing Free-Body Diagrams

1. A book is at rest on a table top. A free-body diagram for this situation looks like this:

2. A girl is suspended motionless from the ceiling by two ropes. A free-body diagram for this situation looks like this:

             

 

3. An egg is free-falling from a nest in a tree. Neglect air resistance. A free-body diagram for this situation looks like this:

4. A flying squirrel is gliding (no wing flaps) from a tree to the ground at constant velocity. Consider air resistance. A free-body diagram for this situation looks like this:

5. A rightward force is applied to a book in order to move it across a desk with a rightward acceleration. Consider frictional forces. Neglect air resistance. A free-body diagram for this situation looks like this:

6. A rightward force is applied to a book in order to move it across a desk at constant velocity. Consider frictional forces. Neglect air resistance. A free-body diagram for this situation looks like this:

7. A force is applied to the right to drag a sled across loosely-packed snow with a rightward acceleration. A free-body diagram for this situation looks like this:

8. A car is coasting to the right and slowing down. A free-body diagram for this situation looks like this:

LAW OF ACCELERATION

Acceleration is produced when a force acts on a mass. The greater the mass (of the object being accelerated) the greater the amount of force needed (to accelerate the

object).

What does this mean?

Everyone unconsciously knows the Second Law.

Everyone knows that heavier objects require more force to move the same distance

as lighter objects.

However, the Second Law gives us an exact relationship between force, mass, and acceleration. It can be expressed as a mathematical equation:

or

FORCE = MASS x ACCELERATION

This is an example of how Newton's Second Law works:

Mike's car, which weighs 1,000 kg, is out of gas. Mike is trying to push the car to a gas station, and he makes the car go 0.05 m/s/s. Using Newton's Second Law, you can compute how much force Mike is applying to the car.

Answer = 50 newtons

Practice #1

An applied force of 50 N is used to accelerate an object to the right across a frictional surface. The object encounters 10 N of friction. Use the diagram to determine the normal force, the net force, the mass, and the acceleration of the object. (Neglect air resistance.)

Fnorm = 80 N

m = 8.16 kg

Fnet = 40 N, right

a = 4.9 m/s/s, right

Practice #2

An applied force of 20 N is used to accelerate an object to the right across a frictional surface. The object encounters 10 N of friction. Use the diagram to determine the normal force, the net force, the coefficient of friction ("mu") between the object and the surface, the mass, and the acceleration of the object. (Neglect air resistance.)

Fnorm = 100 N

m = 10.2 kg Fnet = 10 N, right

"mu" = 0.1

a =0.980 m/s/s, right

(a = Fnet / m = (10 N) / (10.2 kg) = 0.980 m/s/s.)

Practice #3

A 5-kg object is sliding to the right and encountering a friction force which slows it down. The coefficient of friction ("mu") between the object and the surface is 0.1. Determine the force of gravity, the normal force, the force of friction, the net force, and the acceleration. (Neglect air resistance.)

 Fgrav = 49 N

Fnorm = 49 N

Ffrict = 4.9 N

Fnet = 5 N, left; a = 0.98 m/s/s, left

( Fgrav = 50 N; Fnorm = 50 N; Ffrict = 5 N;

Fnet = 5 N, left; a = 1 m/s/s, left )

a = Fnet / m = (4.9 N) / (5 kg) = 0.98 m/s/s.

Check Your Understanding1. Determine the accelerations which result when a

12-N net force is applied to a 3-kg object and then to a 6-kg object.

2. A net force of 15 N is exerted on an encyclopedia to cause it to accelerate at a rate of 5 m/s2. Determine the mass of the encyclopedia.3. Suppose that a sled is accelerating at a rate of 2 m/s2. If the net force is tripled and the mass is doubled, then what is the new acceleration of the sled?

4. Edwardo applies a 4.25-N rightward force to a 0.765-kg book to accelerate it across a table top. The coefficient of friction between the book and the tabletop is 0.410. Determine the acceleration of the book.

5. In a physics lab, Kate and Rob use a hanging mass and pulley system to exert a 2.45 N rightward force on a 0.500-kg cart to accelerate it across a low-friction track. If the total resistance force to the motion of the cart is 0.72 N, then what is the cart's acceleration?

Finding Individual Forces

Free-body diagrams for four situations

                                                      

If Newton's second law were applied to their falling motion, and if

a free-body diagram were constructed, then it would be seen that the 1000-kg baby elephant

Free Fall and Air Resistance

In situations in which there is air resistance, more massive objects fall faster than less massive objects. But why?

LAW OF ACTION-REACTION

For every action there is an equal and opposite re-

action.

What does this mean?

This means that for every force there is a reaction force that is equal in size, but opposite in direction. That is to say that whenever an object pushes another object it gets pushed back in the opposite direction equally hard.

Let's study how a rocket works to understand

Newton's Third Law.

                                         

The rocket's action is to push down on the ground with the force of its powerful engines, and the reaction is that the ground pushes the rocket upwards with an

equal force.

Identifying Action and Reaction Force Pairs

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Baseball pushes glove leftwards

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Enclosed air particles push balloon wall outwards

Thank you