Newton s Laws of Motion. Chapter 3, Section 2

Transcription

1 Newton s Laws of Motion Chapter 3, Section 2

2 3 Motion and Forces Inertia and Mass Inertia (ih NUR shuh) is the tendency of an object to resist any change in its motion. If an object is moving, it will have uniform motion. It will keep moving at the same speed and in the same direction unless an unbalanced force acts on it.

3 3 Motion and Forces Inertia and Mass The velocity of the object remains constant unless a force changes it. If an object is at rest, it tends to remain at rest. Its velocity is zero unless a force makes it move. The inertia of an object is related to its mass. The greater the mass of an object is, the greater its inertia.

4 3 Motion and Forces Newton s Laws of Motion The British scientist Sir Isaac Newton ( ) was able to state rules that describe the effects of forces on the motion of objects. These rules are known as Newton s laws of motion.

5 3 Motion and Forces Newton s First Law of Motion Newton s first law of motion states that an object moving at a constant velocity keeps moving at that velocity unless an unbalanced net force acts on it. If an object is at rest, it stays at rest unless an unbalanced net force acts on it. This law is sometimes called the law of inertia.

6 1 Newton s Second Law Force, Mass, and Acceleration Newton s first law of motion states that the motion of an object changes only if an unbalanced force acts on the object. Newton s second law of motion describes how the forces exerted on an object, its mass, and its acceleration are related.

7 Newton s 2 nd Law Newton s Second Law states: An object acted upon by an unbalanced force will accelerate in the direction of the force. If you kick the ball, it starts moving. The ball accelerates only while your foot is in contact with the ball.

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9 Newton s Third Law For every action, there is an equal and opposite reaction.

10 4.3 The Third Law of Motion Newton s Third Law Newton s third law of motion describes action-reaction pairs this way. When one object exerts a force on a second object, the second one exerts a force on the first that is equal in strength and opposite in direction.

11 Action & Reaction Forces Since a force is an interaction between objects, two objects are involved in every force. Call the objects A and B: Action force: A pushes B Reaction force: B pushes A

12 4.3 The Third Law of Motion Action and Reaction When a force is applied in nature, a reaction force occurs at the same time. When you jump on a trampoline, for example, you exert a downward force on the trampoline. Simultaneously, the trampoline exerts an equal force upward, sending you high into the air.

13 4.3 The Third Law of Motion Action and Reaction Forces Don t Cancel According to the third law of motion, action and reaction forces act on different objects. Thus, even though the forces are equal, they are not balanced because they act on different objects.

14 4.3 The Third Law of Motion Action and Reaction Forces Don t Cancel For example, a swimmer acts on the water, the reaction of the water pushes the swimmer forward. Thus, a net force, or unbalanced force, acts on the swimmer so a change in his or her motion occurs.

15 4.3 The Third Law of Motion Rocket Propulsion In a rocket engine, burning fuel produces hot gases. The rocket engine exerts a force on these gases and causes them to escape out the back of the rocket. By Newton s third law, the gases exert a force on the rocket and push it forward.

16 Rocket Propulsion Action-Reaction Pairs The rocket exerts a downward force on the exhaust gases. The gases exert an equal but opposite upward force on the rocket. F Gas F Rocket

17 Newton s Third Law Another Example: Action-Reaction Pairs The hammer exerts a force on the nail to the right. The nail exerts an equal but opposite force on the hammer to the left.