# Forces and Motion Book Chapter 12

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Forces and Motion Book Chapter 12
Newton’s 1st and 2nd Laws Newton’s 3rd Law & Momentum Universal Forces

Forces A force is a push or pull that acts on an object.
A force can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction.

Measuring Force Forces are measured in Newtons (named for Sir Issac Newton). One Newton is the force that causes a 1 kilogram mass to accelerate at 1 m/s2.

Combining Forces Forces are vectors, so you can use an arrow to represent it. The arrow has to be proportional to the force and in the same direction. When you combine forces by adding or subtracting, you are finding the Net Force. The net force is the overall force acting on an object after all forces have been combined.

Balanced and Unbalanced Forces
When the forces on an object are balanced, the net force is zero and there is no change in the object’s motion. When an unbalanced force acts on an object, the object accelerates.

Friction Friction is a force that affects motion by slowing an object down. It is always opposite to the direction of motion and affects how much an object can accelerate. Friction is present because of irregularities in the surfaces of objects that are in contact with one another. Fluids like water and air also exert a friction force on a moving object.

Four Types of Friction Static Friction – friction force that acts on objects that are not moving Sliding Friction – Force that opposes the motion of an object as it slides over a surface Rolling Friction-friction that acts on rolling objects Fluid Friction-opposes motion as an object moves through a fluid

Friction Direction of motion FRICTION BETWEEN TIRES AND ROAD

Gravity Gravity is an attractive force that acts between any two masses. Gravity causes objects to accelerate downward towards the center of the earth.

Falling and Air Resistance

Falling and Air Resistance
As an object falls faster, the force of air resistance increases until it equals the weight of the object. At this time, the net force equals zero and the object stops accelerating. This final speed is called terminal velocity.

Falling and Air Resistance

Discovering the Reason for Motion
Aristotle- Greek scientist who thought that force was necessary to maintain motion See Aristotle’s ideas on motion.

Discovering the Reason for Motion
Galileo – Italian scientist who did experiments that helped correct misconceptions about force and motion

Discovering the Reason for Motion
Newton – scientist who introduced several laws describing force and motion.

Newton’s 1st Law Law of Inertia
Everything in motion stays in motion, and everything at rest stays at rest unless an outside force acts upon it. Everything in motion and at rest has inertia.

Law of Inertia

Law of Inertia

Law of Inertia

Mass Mass determines the amount of inertia possessed by an object.
Mass is the amount of matter in an object.

Mass is NOT Volume Volume is the amount of space an object takes up.
EXAMPLE: A pillow will have a volume larger than a car battery but less mass.

Mass is NOT weight Weight is the force of gravity acting on a body
Objects in space will still have mass but will not have weight. Metric Units: mass - kilogram weight - Newton On the surface of the earth, mass and weight are proportional. 1 kg = 9.8 Newtons

Calculating the weight of an object
Since weight depends on the mass of an object AND the gravitational pull of the earth, we can say that Weight = mass x gravity Or Weight = mg The weight of an object is measured in Newtons. Because weight depends on gravity, it is also a force.

What is the weight of a 500 gram rock?
1. Convert to standard units 500 grams = .5 kg 2. Remember the constant for gravity is 9.8 m/s2. 3. Plug into the equation and solve. Weight = mg Weight = (.5 kg)(9.8 m/s2) Weight = 4.9 N (Newtons)

Questions 1. Your empty hand is not harmed if it bangs against the wall, but it is harmed if you are carrying a heavy load. Why? 2. Does a person diet to lose mass or to lose weight? 3. Can the force of gravity on a 1 kg mass ever be greater than on a 2 kg mass? Explain how. 4. A car at a junk yard is compressed until its volume is less than 1 cubic meter. Has its mass changed? Has its weight changed? Has its volume changed? Explain. 5. If you jump up in a bus that is moving at a constant velocity, will you land farther back in the bus? Explain. Return to Home Page

Newtons 2nd Law of Motion
Law of Acceleration What causes an object to accelerate? -or change its state of motion? NET FORCE

What is a Net Force? A net force is the combination of all forces acting on an object Applied Forces Net Force 15 N 5 N 10 N 5 N 5 N 10 N

Equilibrium Applied Forces Net Force 0 N
If the net force is zero, the motion of the object will not change. This is a condition called EQUILIBRIUM. If an object is in equilibrium, we have to go back to the Law of Inertia.

Acceleration Acceleration is directly related to the net force.
Acceleration ~ Net Force The larger the net force is acting on an object, the greater the acceleration of the object.

Acceleration Acceleration is inversely proportional to the mass of an object Acceleration ~ Try to push a 250 pound crate. Now try to push a 25 pound crate. Which could you move faster? 1 mass

Newton’s 2nd Law of Motion
The acceleration of an object is directly proportional to the net force acting on it, is in the same direction as the net force, and inversely proportional to the mass of the object. a = F m OR F = ma

F a m

Newton’s 3rd Law of Motion
Action / Reaction Law of Interaction Newton realized that force is not a thing in itself, but part of an interaction between one thing and another.

Forces always occur in pairs
One force is called the action force and the other the reaction force. Action: Object A exerts a force on Object B. Reaction: Object B exerts an equal and opposite force on Object A.

Newton’s 3rd Law For each action force, there is an equal and opposite reaction force. Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.

Action / Reaction Forces
Action: Tire pushes road Reaction: Road pushes tire Action: Rocket pushes gas Reaction: Gas pushes rocket Action: Earth pulls ball Reaction: Ball pulls Earth

Action / Reaction Forces
Consider the apple at rest on the table. If we call the gravitational force exerted on the apple action, what is the reaction force? Are there any other action/reaction forces present?

Action / Reaction Forces
If a cement truck and a car have a head-on collision, which vehicle will experience the greater impact force? A. The cement truck B. The car C. Both the same D. ….it depends on other factors

Momentum Momentum is inertia in motion. Momentum = (mass)(velocity)
It is harder to stop a large truck than a small car when both are moving at the same speed because the truck has a larger mass and larger momentum

Calculating Momentum

Example Which would be harder to stop….a .25 kg ball thrown at you at 40 m/s or a 7 kg bowling ball rolled at 1 m/s? It would be harder to stop the baseball; it has more momentum.

Law of Conservation of Momentum
In the absence of an external force, the total momentum of a system remains unchanged. This is an extension of Newton’s 3rd Law (Mass)(Velocity before) = (Mass)(Velocity after) See an example of the Law of Conservation of Momentum (and Energy)

Conservation of Momentum

Conservation of Momentum

Universal Forces Electromagnetic forces, Nuclear forces, and Gravitational forces are all universal forces. Electromagnetic force is associated with charged particles. Electric force and magnetic force are the only forces that can both attract and repel.

Nuclear Forces Two forces, the strong nuclear force and the weak nuclear force, act within the nucleus to hold it together. The strong nuclear force acts only on the neutrons and protons in the nucleus. The weak nuclear force acts over a short range and affects all particles, not just protons and neutrons.

Gravitational Force Gravitational force is an attractive force that acts between two masses. The bigger the masses and the closer they are, the more the attraction force. Newton’s Law of Universal Gravitation states that every object in the universe attracts every other object.

Gravitational Force Gravity is the weakest universal force, but it is the most effective over long distances. Earth’s gravitational force keeps the moon in a nearly circular orbit. The gravitational pull of the moon on the Earth causes ocean tides.

Satellites When an artificial satellite is put into orbit, its inertia and the pull of the Earth keep it in orbit. If the satellite is too close to the atmosphere, friction slows it down and it crashes into the Earth.

Satellite without Gravity

Launch speed less than 8000 m/s
Launch speed equals 8000 m/s

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