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LAWS OF MOTION

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

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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.

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6.1 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.”

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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.

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6.1 Law of inertia Inertia is a 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.

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Sample 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.

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**Looking for: Given: Relationships: Solving**

…weight of car in newtons, force due to friction Given: …ForceN = 12,000N (up); …ForceE = 2,000N (forward) Relationships: Newton’s 1st Law: net force = zero at constant velocity; so ForceN = ForceW and ForceE = ForceF

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**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. FN = 12,000N Ff = N FE = 2000 N FW = -12,000N

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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.

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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/s2.

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6.2 Newton’s second law There are three main ideas related to Newton’s Second Law: Acceleration is the result of unbalanced forces. A larger force makes a proportionally larger acceleration. Acceleration is inversely proportional to mass.

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

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**6.2 Acceleration and force**

The second law says that acceleration is directly proportional to force. If force is increased or decreased, acceleration will be increased or decreased by the same factor.

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**6.2 Acceleration and direction**

Another important factor of the second law is that the acceleration is always in the same direction as the net force.

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6.2 Acceleration and mass The greater the mass, the smaller the acceleration for a given force. This means acceleration is inversely proportional to mass.

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**6.2 Acceleration, force and mass**

The acceleration caused by a force is directly proportional to force and inversely proportional to mass.

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**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.

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**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.

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**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.

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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.

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**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.

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**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. The action/reaction forces act on different objects. One force acts on the ball, and the other force acts on the hand.

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

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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.

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6.3 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. F= ma

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**Newton’s Laws of Motion**

First: Law of Inertia Second: F = ma Third: Action & Reaction (forces occur in pairs)

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