Newton’s Laws of Motion Chapter 3 & Chapter 4

Newton’s First Law of Motion: Inertia The first law of motion states that an object at rest will remain at rest and an object in motion will remain in motion unless acted upon by an unbalanced force.

Inertia Newton called the tendency of objects to remain in motion or to stay at rest inertia. EXAMPLE: A car moving in a turn and the passenger presses against the door because he is still moving in a straight line. A car moves, you fall backwards. It stops, you fall forwards. Your body has inertia.

Question: Which would have greater inertia? A toy car or a real car? Why would there be a difference?

Answer: The more mass an object has, the greater its inertia is. A much greater force would be needed to change the real cars velocity.

History of Inertia Aristotle ( B.C.) Believed that the “natural” motion of celestial objects was circular, while terrestrial objects tend to “naturally” fall. He also believed that object need force to maintain motion

History of Inertia Galileo ( ) Claimed that no force was needed to keep an object in uniform, straight-line motion forever. It is the force of friction that slows an object down.

History of Inertia Newton ( ) During the years of 1665 and 1666, Isaac Newton developed three laws that describe the states of motion. He also created the mathematical equations needed to solve them. Newton was moved by his family away from Oxford to avoid the “Black Death” also known as the plague. The significance of his contribution was perhaps best expressed by the Apollo crew as they were hurtling towards the moon. They radioed a message to mission control saying: “We would like to thank the person who made this trip possible...Sir Isaac Newton!”

Friction Chapter 5

Friction: A Force Opposing Motion  Friction is a force that acts in a direction opposite to the motion of the moving object.  Friction will cause a moving object to slow down and finally stop.

Sliding Friction  When solid objects slide over each other, the type of friction that results is sliding friction.  EXAMPLE moving a box across the surface of a table.

Rolling Friction  The friction produced by objects such as wheels or ball bearing is called rolling friction. Rolling friction tends to be less than sliding friction.  EXAMPLE roller blades, bikes, and cars.

Fluid Friction  Fluid friction is usually less than sliding friction. Water and air are considered fluids. The force exerted by fluid is called fluid friction.  EXAMPLE air resistance to a falling object and a boat moving through the water.

Lubricants  Slippery fluids like grease, reducing friction.

Newton’s Second Law of Motion: Force = mass x acceleration

Newton’s Second Law of Motion This law explains how force and acceleration are related. Newton’s second law of motion show how force, mass, and acceleration are related. No force, no motion!

Physicist like Aristotle of Greece, Sir Isaac Newton (he was responsible for totally understanding gravity) and Galileo have conducted experiments on gravity. Legend has it that in the late 1500’s, the famous Italian scientist Galileo dropped two cannonballs at exactly the same time from the top of the Leaning Tower of Pisa in Italy. According to the scientific theories of the day, the more massive ball should have landed first. But Galileo wanted to disprove this theory. His hypothesis was that all object fall at the same rate no matter what their masses are. Galileo’s experiment displays the basic laws of nature that govern the motion of falling objects.

Formula Force = Mass x Acceleration (N) (kg) (m/sec 2 ) 1N = 1 kg x 1 m/sec 2

Formula F=ma Force a=F/m Acceleration m=F/a Mass F ma

Classwork Practice Problems: kg x 1 m/sec kg x 9.8 m/sec 2 3. If a 60 kg person on a 15 kg sled is pushed with a force of 300 N, what will be the person’s acceleration? 4. A bowling ball rolled with a force of 15 N accelerates at a rate of 3 m/sec 2 ; a second ball rolled with the same force accelerates 4 m/sec 2. What are the masses of the two balls?

Falling Objects The acceleration of a falling object is due to the force of gravity between the object and the Earth. Near the surface of the Earth, the acceleration due to the force of gravity (g) is 9.8 m / sec 2

Gravitational Forces of Common Objects Moon: 1/6 the Earth’s gravity Saturn: 95 times the Earth’s gravity Sun: 330,000 times the Earth’s gravity

Air Resistance: Experiments: 1. Air pressure and inertia breaks a pencil. 2. Galileo’s famous cannonball experiment.

Terminal Velocity When a falling body no longer accelerates. It has reached terminal velocity. Sky divers reach terminal velocity at about 190 km / hr. At this point, the sky divers no longer feel the sensation of falling.

Newton’s Third Law of Motion: Action & Reaction

Newton’s 3 rd Law of Motion The third law of motion states that for every action, there is an equal and opposite reaction. In short, for every force there must be an equal opposite force. All forces come in pairs. The third law of motion states that for every action, there is an equal and opposite reaction. In short, for every force there must be an equal opposite force. All forces come in pairs. The reaction engine of rockets is an excellent application of the third law of motion. The fuels push against the sides of the rocket and escapes out of the bottom. The gases move downward and the rocket will move in the opposite direction, or upward. The reaction engine of rockets is an excellent application of the third law of motion. The fuels push against the sides of the rocket and escapes out of the bottom. The gases move downward and the rocket will move in the opposite direction, or upward.

Discussion and Practice Turn to page 110 and look at figure Which of Newton’s three laws of motion explains why they jumper lands in the water, not the dock? Turn to page 110 and look at figure Which of Newton’s three laws of motion explains why they jumper lands in the water, not the dock?

Summary of Newton’s Laws of Motion