Physical Science
Chapter 6
Section 1
Newton's First Law of
Motion
Force changes motion
* A force is a push or pull, or any action that is able to change motion.
Law of inertia
*Newton’s first law says that objectscontinue 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.
Net force
* Newton’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?
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.
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.
Practice Problem
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.
Classwork
*6.1 Section Review
Homework
* Newton's First Law Worksheet
*Read 6.2
Physical Science
Chapter 6
Section 2
Newton's Second Law
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.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/s2.
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.
Newton’s second law
* Unbalanced forces cause changes in speed, direction, or both.
Acceleration and force
*The 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.
Acceleration and direction
*Another important factor of the second law is that the acceleration is always in the same direction as the net force.
Acceleration and mass
* The greater the mass, the smaller the acceleration for a given force.* This means acceleration is inversely proportional to mass.
Acceleration, force and mass
* The stronger the force on an object, the greater its acceleration.
*Force is directly proportionalto 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
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?
Classwork
*6.2 Section Review
Homework
Physical Science
Chapter 6
Section 3
Newton's 3rd Law & Momentum
Newton’s Third Law
* Newton’s Third Law (action-reaction) applies when a force is placed on any object, such as a basketball.
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.
The Third Law: Action/Reaction
One force acts on the ball, and the other force acts on the hand.
* 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.
Action and reaction
Practice Problem
* A woman with a weight of 500 newtons is sitting on a chair.
*Describe one action-reaction pair of forces in this situation.
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.
Momentum
*Momentum is the mass of a object times its velocity.
* The units for momentum are kilogram-meter per second ( kg·m/s ).
Momentum
*The 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.
Momentum
Collisions
*When 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?
Practice Problem
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.
