LAWS OF MOTION Chapter Six Laws of Motion

LAWS OF MOTION

Chapter Six: Laws of Motion Ø 6. 1 Newton’s First Law Ø 6. 2 Newton’s Second Law Ø 6. 3 Newton’s Third Law and Momentum

Chapter 6. 1 Learning Goals ØDescribe how forces cause changes in motion. ØDemonstrate and describe Newton’s first law. ØExplain the meaning of net force.

6. 1 Force changes motion Ø A force is a push or pull, or any action that is able to change motion.

6. 1 Law of inertia Ø Newton’s first law says that objects continue the motion they already have unless they are acted on by a net force. Ø If the net force is zero, an object at rest will stay at rest. Ø If an object is acted upon by unbalanced forces, its motion will change.

6. 1 Net force ØNewton’s first law is often written in terms of the net force: Ø“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 Ø 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 Ø Inertia is the property of an object that resists changes in motion. Ø Objects with more mass have more inertia and are more resistant to changes in their motion. Which ball has more inertia?

Solving Problems 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: Ø …weight of car in newtons, force due to friction 2. Given: Ø …Force. N = 12, 000 N (up); Ø …Force. E = 2, 000 N (forward) 3. Relationships: Ø Newton’s 1 st Law: Ø net force = zero at constant velocity; so Force. N = Force. W and Force. E = Force. F

FE = 200 N FN = 12, 000 N Ø Draw a free body diagram. Ø There is no net force upward, so the weight of the car is an equal downward force of FF = -200 N − 12, 000 N. Ø The forward engine force balances the friction force so the friction force is − 2, 000 N. FW = -12, 000 N 4. Solution Solving Problems

Chapter Six: Laws of Motion Ø 6. 1 Newton’s First Law Ø 6. 2 Newton’s Second Law Ø 6. 3 Newton’s Third Law and Momentum

Chapter 6. 2 Learning Goals ØDefine Newton’s second law by relating force, mass, and acceleration. ØApply Newton’s second law quantitatively. ØDescribe the relationship between net force and acceleration.

Investigation 6 A Newton’s First and Second Laws ØKey Question: Ø What is the relationship between force and motion?

6. 2 Newton’s second law Ø Newton’s first law tells us that motion cannot change without a net force. Ø According to Newton’s second law, the amount of acceleration depends on both the force and the mass.

6. 2 The newton Ø The S. I. unit of force (newton) is defined by the second law. Ø A newton is the amount of force needed to accelerate a 1 kg object by 1 m/s.

6. 2 Newton’s second law Ø 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 Ø Unbalanced forces cause changes in speed, direction, or both.

6. 2 Acceleration and force ØThe second law says that acceleration is proportional to force. ØIf force is increased or decreased, acceleration will be increased or decreased by the same factor.

6. 2 Acceleration and direction ØAnother important factor of the second law is that the acceleration is always in the same direction as the net force.

6. 2 Acceleration and mass ØThe greater the mass, the smaller the acceleration for a given force. ØThis means acceleration is inversely proportional to mass.

6. 2 Acceleration, force and mass ØThe acceleration caused by a force is proportional to force and inversely proportional to mass.

ØThe stronger the force on an object, the greater its acceleration. Ø Force is directly proportional to acceleration. Ø If twice the force is applied, the acceleration is twice as great.

ØThe greater the mass, the smaller the acceleration for a given force. Ø Mass is inversely related to force. Ø An object with twice the mass will have half the acceleration if the same force is applied.

Ø 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.

Solving Problems Ø 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: Ø …car’s acceleration 2. Given Ø …mass = 1, 000 kg; net force = 2, 000 N 3. Relationships: Ø a=F/m 4. Solution: Ø 2, 000 N ÷ 1, 000 kg = 2 N/kg = 2 m/s 2

Chapter Six: Laws of Motion Ø 6. 1 Newton’s First Law Ø 6. 2 Newton’s Second Law Ø 6. 3 Newton’s Third Law and Momentum

Chapter 6. 3 Learning Goals ØDescribe action-reaction force pairs. ØExplain what happens when objects collide in terms of Newton’s third law. ØApply the law of conservation of momentum when describing the motion of colliding objects.

Investigation 6 B Newton’s Third Law ØKey Question: What happens when equal and opposite forces are exerted on a pair of Energy Cars?

6. 3 Newton’s Third Law Ø 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 Ø Newton’s Third Law states that every action force creates a reaction force that is equal in strength and opposite in direction. Ø There can never be a single force, alone, without its actionreaction partner.

6. 3 The Third Law: Action/Reaction Ø It doesn’t matter which force you call the action and which the reaction. Ø The forces do not cancel because we can One force acts on the only cancel forces ball, and the other force acts on the hand. acting on the same object.

6. 3 Action and reaction Ø 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 ØA woman with a weight of 500 newtons is sitting on a chair. ØDescribe one action -reaction pair of forces in this situation.

Solving Problems 1. Looking for: 2. Given 3. Relationships: 4. Solution Ø Ø Ø Fc = 500 N …pair of action-reaction forces …girl’s force. W = -500 N (down) Fw = -500 N Action-reaction forces are equal and opposite and act on different objects. 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.

6. 3 Collisions Ø Newton’s third law tells us that any time two objects hit each other, they exert equal and opposite forces on each other. Ø The effect of the force is not always the same.

6. 3 Momentum ØMomentum is the mass of a object times its velocity. ØThe units for momentum are kilogrammeter per second (kg·m/s).

6. 3 Momentum ØThe 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 Momentum ØThe 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 We use positive and you ignore friction). negative numbers to show opposite directions.

6. 3 Collisions ØWhen a large truck hits a small car, the forces are equal. ØThe 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 ØIf 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? ØThe astronaut’s mass is 100 kilograms.

Solving Problems 1. Looking for: Ø … the velocity of the astronaut (backward) 2. Given Ø …velocity 1 = 10 m/s; mass 1= 2 kg; Ø. . . mass 2 = 100 kg; 3. Relationships: Ø m 1 v 1 = m 2 v 2 4. Solution Ø Draw a free body diagram.

Investigation 6 C Collisions ØKey Question: Why do things bounce back when they collide?

Forensic Engineering ØHuman bodies are not designed to handle the impact of crashing into a stationary object after traveling through space at the speed of a car. ØThe study of how vehicles move before, during, and after a collision is called vehicular kinematics.