laws of motion. chapter six: laws of motion 6.1 newton’s first law 6.2 newton’s second law ...

LAWS OF MOTION

Chapter Six: Laws of Motion

6.1 Newton’s First Law

6.2 Newton’s Second Law

6.3 Newton’s Third Law and

Momentum

Chapter 6.1 Learning Goals

Describe how forces cause changes in motion.

Demonstrate and describe Newton’s first law.

Explain the meaning of net force.

6.1 Force changes motion

A force is a push or pull, or any action that is able to change motion.

6.1 Law of inertia Newton’s first law says that objects

continue the motion they already have unless they are acted on by a net force.

If the net force is zero, an object at rest will stay at rest.

If an object is acted upon by unbalanced forces, its motion will change.

6.1 Net forceNewton’s first law is

often written in terms of the net force:

“An object at rest will stay at rest and an object in motion will continue in motion at constant velocity UNLESS there is a net force.”

According to these vectors, in what direction

is the net force?

6.1 Force changes motion

Forces can be used to increase or decrease the speed of an object, or to change the direction an object is moving.

6.1 Law of inertia Inertia is the

property of an object that resists changes in motion.

Objects with more mass have more inertia and are more resistant to changes in their motion.

Which ball has more inertia?

A car drives along the highway at constant velocity. Find the car’s weight and the friction force if the engine produces a force of 2,000 newtons between the tires and the road and the normal force on the car is 12,000 N.

Solving Problems

1. Looking for: …weight of car in newtons, force due to

friction

2. Given: …ForceN = 12,000N (up); …ForceE = 2,000N (forward)

3. Relationships: Newton’s 1st Law:

net force = zero at constant velocity; so ForceN = ForceW and ForceE = ForceF

Solving Problems

FE = 200 N

4. Solution Draw a free body

diagram. There is no net force

upward, so the weight of the car is an equal downward force of −12,000 N.

The forward engine force balances the friction force so the friction force is −2,000 N.

Solving Problems

FF = -200 N

FW

= -

12,0

00N

FN

= 1

2,0

00N

Chapter Six: Laws of Motion

6.1 Newton’s First Law

6.2 Newton’s Second Law

6.3 Newton’s Third Law and

Momentum

Chapter 6.2 Learning Goals

Define Newton’s second law by relating force, mass, and acceleration.

Apply Newton’s second law quantitatively.

Describe the relationship between net force and acceleration.

Investigation 6A

Key Question: What is the relationship between force

and motion?

Newton’s First and Second Laws

6.2 Newton’s second law

Newton’s first law tells us that motion cannot change without a net force.

According to Newton’s second law, the amount of acceleration depends on both the force and the mass.

6.2 The newton The S.I. unit of

force (newton) is defined by the second law.

A newton is the amount of force needed to accelerate a 1 kg object by 1m/s.

6.2 Newton’s second law There are three main ideas related

to Newton’s Second Law:

1. Acceleration is the result of unbalanced forces.

2. A larger force makes a proportionally larger acceleration.

3. Acceleration is inversely proportional to mass.

6.2 Newton’s second law

Unbalanced forces cause changes in speed, direction, or both.

6.2 Acceleration and forceThe second law says

that acceleration is proportional to force.

If force is increased or decreased, acceleration will be increased or decreased by the same factor.

6.2 Acceleration and directionAnother important factor of the second

law is that the acceleration is always in the same direction as the net force.

6.2 Acceleration and massThe greater the mass, the smaller the acceleration for a given force.

This means acceleration is inversely proportional to mass.

6.2 Acceleration, force and massThe acceleration caused by a

force is proportional to force and inversely proportional to mass.

The stronger the force on an object, the greater its acceleration. Force is

directly proportional to acceleration.

If twice the force is applied, the acceleration is twice as great.

The greater the mass, the smaller the acceleration for a given force. Mass is

inversely related to force.

An object with twice the mass will have half the acceleration if the same force is applied.

6.2 Applying the second law Keep the following

important ideas in mind:1. The net force is what

causes acceleration.2. If there is no

acceleration, the net force must be zero.

3. If there is acceleration, there must also be a net force.

4. The force unit of newtons is based on kilograms, meters, and seconds.

A car has a mass of 1,000 kilograms. If a net force of 2,000 N is exerted on the car, what is its acceleration?

1. Looking for: …car’s acceleration

2. Given …mass = 1,000 kg; net force = 2,000 N

3. Relationships: a = F / m

4. Solution: 2, 000 N ÷ 1,000 kg = 2 N/kg = 2 m/s2

Solving Problems

Chapter Six: Laws of Motion

6.1 Newton’s First Law

6.2 Newton’s Second Law

6.3 Newton’s Third Law and

Momentum

Chapter 6.3 Learning Goals

Describe action-reaction force pairs.

Explain what happens when objects collide in terms of Newton’s third law.

Apply the law of conservation of momentum when describing the motion of colliding objects.

Investigation 6B

Key Question:What happens when equal and opposite

forces are exerted on a pair of Energy Cars?

Newton’s Third Law

6.3 Newton’s Third Law

Newton’s Third Law (action-reaction) applies when a force is placed on any object, such as a basketball.

6.3 The Third Law: Action/Reaction Newton’s Third Law

states that every action force creates a reaction force that is equal in strength and opposite in direction.

There can never be a single force, alone, without its action-reaction partner.

6.3 The Third Law: Action/Reaction It doesn’t matter

which force you call the action and which the reaction.

The forces do not cancel because we can only cancel forces acting on the same object.

One force acts on the ball, and the other force acts on the hand.

6.3 Action and reaction

When sorting out action and reaction forces it is helpful to examine or draw diagrams.

Here the action force is on the ________________, and the reaction force is on the _______________.

Solving Problems

A woman with a weight of 500 newtons is sitting on a chair.

Describe one action-reaction pair of forces in this situation.

1. Looking for: …pair of action-reaction forces

2. Given …girl’s forceW = -500 N (down)

3. Relationships: Action-reaction forces are equal and opposite and act on

different objects.

4. Solution Draw a free body diagram The downward force of 500 N exerted by the woman on

the chair is an action. Therefore, the chair acting on the woman provides an

upward force of 500 N and is the reaction.

Solving Problems

Fw = -500 N

Fc = 500 N

6.3 Collisions Newton’s third law tells us that any time

two objects hit each other, they exert equal and opposite forces on each other.

The effect of the force is not always the same.

6.3 MomentumMomentum is the mass of a object times its velocity.

The units for momentum are kilogram-meter per second (kg·m/s).

6.3 MomentumThe law of

conservation of momentum states that as long as the interacting objects are not influenced by outside forces (like friction) the total amount of momentum is constant or does not change.

6.3 MomentumThe result of a

skateboarder throwing a 1-kg ball at a speed of -20 m/sec is that he and the skateboard with a total mass of 40 kg move backward at a speed of +0.5 m/sec (if you ignore friction).

We use positive and negative numbers to show opposite directions.

6.3 CollisionsWhen a large truck

hits a small car, the forces are equal.

The small car experiences a much greater change in velocity much more rapidly than the big truck.

Which vehicle ends up with more damage?

Solving Problems

If an astronaut in space were to release a 2-kilogram wrench at a speed of 10 m/s, the astronaut would move backward at what speed?

The astronaut’s mass is 100 kilograms.

1. Looking for: … the velocity of the astronaut (backward)

2. Given …velocity1 = 10 m/s; mass1= 2 kg;

...mass2 = 100 kg;

3. Relationships: m1v1 = m2v2

4. Solution Draw a free body diagram.

Solving Problems

Investigation 6C

Key Question:Why do things bounce back when they

collide?

Collisions

Forensic EngineeringHuman bodies are not

designed to handle the impact of crashing into a stationary object after traveling through space at the speed of a car.

The study of how vehicles move before, during, and after a collision is called vehicular kinematics.