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Forces!!!!

Forces!!!!. Chapter 3. Force. A force is a push or a pull A force can cause the motion of an object to change Force does not always change velocity Balanced forces=forces that are equal in size and opposite in direction

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Forces!!!!

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  1. Forces!!!! Chapter 3

  2. Force • A force is a push or a pull • A force can cause the motion of an object to change • Force does not always change velocity • Balanced forces=forces that are equal in size and opposite in direction • When two or more forces act on an object at the same time, the forces combine to form the net force

  3. Inertia and Mass • Inertia is the tendency of an object to resist change in its motion • Objects have different inertias based on their mass • The greater the mass of an object, the greater the inertia of that object • Ex: A bowling ball takes much more force to move, change directions or slow down, the bowling ball has great inertia. • Ex: A tennis ball can be stopped or started by blowing on it, the tennis ball has small inertia!

  4. Newton---the motion guy • Sit Isaac Newton, a British scientist, was able to state the rules that describe the effects of forces on the motion of objects (known as Newton’s law of motion) • 1st law=inertia • 2nd law=F=ma • 3rd law=action/reaction

  5. Newton’s 1st law • An object moving at a constant velocity keeps moving at that velocity unless an unbalanced net force acts on it • Think----An object in motion stays in motion and an object at rest stays at rest unless acted on by an outside force • Refers to the idea of inertia

  6. Inertia in real life • Motion in a car • Car crashes • Roller Coasters

  7. Factors Affecting Force • Acceleration • The more force, the greater the acceleration • Mass • The more mass, the greater the force

  8. Newton’s 2nd Law • The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. • a = FNET/m • FNET=ma • Force is measured in Newtons (N) • Mass is measured in kilograms (kg) • Acceleration is measured in meters/second2 (m/s2)

  9. Practice Probelm • What force would be required to accelerate a 40 kg mass by 4 m/s2? GIVEN: F = ? m = 40 kg a = 4 m/s2 WORK: F = ma F = (40 kg)(4 m/s2) F = 160 N

  10. Practice Problem • A 4.0 kg shotput is thrown with 30 N of force. What is its acceleration? GIVEN: m = 4.0 kg F = 30 N a = ? WORK: a = F ÷ m a = (30 N) ÷ (4.0 kg) a = 7.5 m/s2

  11. Section Check 2.3 Question 1 A force is a __________. Answer A force is a push or pull. Forces, such as the force of the atmosphere against a person’s body, are not always noticeable.

  12. Section Check 2.3 Question 2 When are forces on an object balanced? Answer When forces are equal in size and opposite in direction, they are balanced forces, and the net force is zero.

  13. Friction • Think back to Newton’s 1st law • Why does a ball rolling on the desk eventually slow down and stop? • Friction=the force that opposes the sliding motion of two surfaces that are touching each other

  14. Practice Problem • Together a bike and its rider have a mass of 275 Kg. The bike is slowed down with an acceleration of –4.5 m/s/s. What is the net force on the bike? • 1.24 E3 N in the direction opposite of motion

  15. Practice Problem • A person with a black belt in karate has a fist that has a mass of 0.70 kg. Starting from rest, this fist attains a velocity of 8.0 m/s in 0.15 s. What is the magnitude and direction of the average net force applied to the fist to achieve this level of performance? • 37.33 N in the direction of motion

  16. Friction • The amount of friction between two surfaces depends on two factors • The kinds of surfaces • The force pressing the surfaces together • Friction is caused by the rough surfaces of objects • ALL objects have a rough surface, even if it’s only on a microscopic level

  17. Types of Friction • Static Friction= the frictional force that prevents two surfaces from sliding past each other • The reason its harder to start pushing than to continue pushing the same object • Sliding Friction=the frictional force that opposes the option of two objects sliding past each other • Rolling Friction=frictional force between a rolling object and the surface it rolls on

  18. Section Check 3.1 Question 1 Newton’s second law of motion states that _________ of an object is in the same direction as the net force on the object. A. acceleration B. momentum C. speed D. velocity

  19. Section Check 3.1 Answer The answer is A. Acceleration can be calculated by dividing the net force in newtons by the mass in kilograms.

  20. Section Check 3.1 Question 2 The unit of force is __________. A. joule B. lux C. newton D. watt

  21. Section Check 3.1 Answer The answer is C. One newton = 1 kg · m/s2

  22. Section Check 3.1 Question 3 What causes friction? Answer Friction results from the sticking together of two surfaces that are in contact.

  23. 3.2 GravityA Basic Force Gravity is one of the four basic forces. The other basic forces are the electromagnetic force, the strong nuclear force, and the weak nuclear force.

  24. 3.2 Weight The gravitational force exerted on an object is called the object’s weight. Because the weight of an object on Earth is equal to the force of Earth’s gravity on the object, weight can be calculated from this equation:

  25. 3.2 Weight and Mass Weight and mass are not the same. Weight is a force and mass is a measure of the amount of matter an object contains. Weight and mass are related. Weight increases as mass increases.

  26. 3.2 Weight and Mass The weight of an object usually is the gravitational force between the object and Earth. The weight of an object can change, depending on the gravitational force on the object.

  27. 3.2 Weight and Mass The table shows how various weights on Earth would be different on the Moon and some of the planets.

  28. 3.2 Weightlessness and Free Fall You’ve probably seen pictures of astronauts and equipment floating inside the space shuttle. They are said to be experiencing the sensation of weightlessness.

  29. 3.2 Floating in Space So what does it mean to say that something is weightless in orbit? When you stand on a scale you are at rest and the net force on you is zero. The scale supports you and balances your weight by exerting an upward force.

  30. 3.2 Floating in Space The dial on the scale shows the upward force exerted by the scale, which is your weight. Now suppose you stand on the scale in an elevator that is falling.

  31. 3.2 Floating in Space If you and the scale were in free fall, then you no longer would push down on the scale at all. The scale dial would say you have zero weight, even though the force of gravity on you hasn’t changed.

  32. Section Check 3.2 Question 1 Gravity is an attractive force between any two objects and depends on __________. Answer Gravity is an attractive force between any two objects and depends on the masses of the objects and the distance between them.

  33. Section Check 3.2 Question 2 Which is NOT one of the four basic forces? A. gravity B. net C. strong nuclear D. weak nuclear

  34. Section Check 3.2 Answer The answer is B. The fourth basic force is the electromagnetic force, which causes electricity, magnetism, and chemical interactions between atoms and molecules.

  35. 3.3 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.

  36. 3.3 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.

  37. 3.3 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.

  38. 3.3 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.

  39. 3.3 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.

  40. 3.3 Question 1 According to Newton’s third law of motion, what happens when one object exerts a force on a second object? Answer According to Newton’s law, the second object exerts a force on the first that is equal in strength and opposite in direction.

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