Forces and Motion

1. Forces and Motion Prentice Hall Physical Science: Concepts in Action Chapter 12

2. May the Force be with you • Forces exist all around us • There are 4 types of forces: gravitational, electromagnetic and strong and weak nuclear forces • Which do you think would be the strongest? • Gravity is actually the weakest but has a far reaching effect and nuclear forces (attractions) are the strongest with an effect only on things nearby

3. What is Force? • Force: a push or pull that acts on an object by changing its speed or direction • Can cause a resting object to move • Can accelerate a moving object

4. How is force measured? • Spring scale • Stretch of the spring depends on the mass of the object acting on it • Ex: vegetable scale at the grocery store • Unit of Force • Newton (N) • 1 kg to accelerate 1 m/s2 • (named after Sir Isaac Newton)

5. How is force represented? • Use arrows • Direction • Strength • Length represents strength or magnitude • The scale with more apples, greater mass, has a longer arrow. The arrow is pointed downward due to mass is below the balance pulling downwards.

6. Combining Forces • Have you ever tried to push a broken down car alone? • Did you ask for help? Why? • Forces in the same direction are addedtogether • Force in the opposite direction are subtracted • Net Force the overall force acting on an object after all the forces are combined

7. Balanced vs. Unbalanced Forces • Balanced • Combine to produce a net force of zero • No change in the object’s motion • Ex: tug of war, arm wrestling • Unbalanced • Net force equals the size of the larger force minus the size of the smaller force • Net force does not equal zero • Causes an object to accelerate

8. Representing Forces • Forces can add together or subtract from one another • A- 2 forces acting in the same direction add • B- forces in opposite directions subtract from one another • C – forces that are equal in size and opposite in direction result in no net force

9. Friction • All moving objects are subject to friction • Friction is a force that opposes the motion of objects that touch as they move past each other • Acts at the surface where objects are in contact • 4 types of friction: • Static friction • Sliding friction • Rolling friction • Fluid friction

10. 4 Types of Friction • Static friction • Force that acts on objects that are not moving • Always acts in the direction opposite to that of the applied force • Ex: taking a step • Sliding friction • Force that opposes the direction of motion of an object as it slides over a surface • Less force is needed to keep an object movingthan to start it moving • Ex: pushing a potted plant

11. 4 Types of Friction • Rolling friction • The friction force that acts on rolling objects • Causes change in shape at the point of rolling contact • Ex: furniture • Fluid friction • The force that opposes the motion of an object through fluid • Increases the speed of the object moving through the fluid • Ex: cake batter • Fluids (gas and liquids) • Fluid friction acting on an object moving through the air is known as air resistance

12. Gravity • Gravity is a force that acts between two masses • Attractive force that pulls objects together • Earth’s gravity exerts a force of attraction on every other object that is near Earth • Acts downwards towards the center of the earth Because the boulder is at rest, what do you Know about the net force acting on it?

13. Gravity and Falling Objects • Gravity causes objects to accelerate downward • Air resistance (fluid friction) acts in the direction opposite to the motion and reduces acceleration • The flying squirrel to the right takes advantage of air resistance to slow its fall by increasing it’s surface area. • Terminal velocity is the constant velocity of a falling object when the force of air resistance equals the force of gravity. (in other words, top speed)

14. Projectile Motion • Projectile Motion: motion of a falling object after given an initial forward velocity • Air resistance and gravity are the only forces • The combination of an initial velocity and the downward vertical force of gravity causes the ball to follow a curved path The 2 balls fall with the same acceleration and hit the ground at the same time

15. 12:2 Newton’s 1st Law of Motion • Sir Isaac Newton studied motion and gave us the basic laws of motion • The 1st Law of Motion states that the motion of an object does not change as long as the net force acting on the object is zero. • Sometimes also called the Law of Inertia • Inertia Tendency of an object to resist change in its motion • In other words… An object at rest stays at rest, an object in motion stays in motion at the same direction and speed (until something acts on it)

16. Newton’s 1st Law of Motion Crash Dummy: Example of inertia

17. Newton’s 2nd Law of Motion • How do unbalanced forces affect the motion of an object? • It causes the velocity to change (accelerate) • Ex: you apply a netforce to a ball when you throw it • The harder you throw, the more the ball accelerates. • The acceleration is directly proportionalto the net force acing on it. • Newton’s Second Law of Motion The acceleration of an object is equal to the net force acting on it divided by the objects mass

18. Newton’s 2nd Law of Motion • Mass is a measure of inertia of an object and depends on the amount of matter the object contains • If you double the mass of an object, it cuts the acceleration in half • The acceleration of an object is always in the samedirection as the net force • Newton’s 2nd Law applies when a net force acts in the opposite direction of object’s motion • Force produces deceleration and reduces speed • Ex. Seat belts • Force decelerates the passenger in order to prevent injury • Units for Acceleration are equivalent (mean the same thing) • N/kg=m/s2

19. Newton’s 2nd Law of Motion

21. Weight and Mass • Are weight and mass the same? • Weight & Mass are Different • Weight The force of gravity acting on an object • Product of the mass and acceleration due to gravity • Unit is Newtons (N)

22. Weight and Mass

23. Newton’s 2nd Law of Motion • How would the acceleration of a chain of two carts compare with the acceleration of a single cart if the same force acted on both? • When the same force acts, the single cart accelerates 8x faster than the chain of eight carts • As massincreases, acceleration has to decrease in proportion to the increase in mass

24. Newton’s 2nd Law of Motion • How would the force have to change in order to have the same acceleration for the eight carts as for one cart? • The force would have to be 8x greater • How would another force directed to the left on the cart affect the cart’s acceleration? • The acceleration would depend on the net force. • The net force would be the force acting to the right minus the force acting to the left.

25. Newton’s 2nd Law of Motion • 1.A boy pushes forward a cart of groceries with a total mass of 40.0 kg. What is the acceleration of the cart if the net force on the cart is 60.0 N? • 2.What is the upward acceleration of a helicopter with a mass of 5000 kg if a force of 10,000 N acts on it in an upward direction? • a=F/m • = 10000 N/5000 Kkg • = 2 m/s2 a=F/m = 60.0 N/40.0 kg = 1.50 m/s2

26. Newton’s 2nd Law of Motion 3.An automobile with a mass of 1200 kg accelerates at a rate of 3.0 m/s2 in the forward direction. What is the net force acting on the automobile? (Hint: Solve the acceleration formula for force.) • a=F/m F=ma • = 1200 kg(3.0 m/s2) • = 3600 N • 4.A 25-N force accelerates a boy in a wheelchair at 0.5 m/s2 What is the mass of the boy and the wheelchair? (Hint: Solve Newton's second law for mass.) • a=F/m m=F/a • = 25 N/0.50 m/s2 • = 50 k/=g

27. Newton’s 3rd Law of Motion & Momentum • Newton’s 3rd Law – when an object exerts a force on a secondobject, that object exerts an equal and opposite force on the first object • A force cannot exist alone. Forces always exist in pairs. • These are called action and reaction pairs (forces)

28. Action and Reaction Forces • Pressing your hand against a wall produces a pair of forces • Your hand exerts a force on the wall – the action force • The wall exerts an equal and opposite force against your hand – the reaction force • Using a hammer to drive a nail into a piece of wood produces forcepairs • Hammer applies force to the nail • Action forces drives nail in wall • Nail exerts equal and opposite force • This force brings the motion of the hammer to a stop

29. Action and Reaction Forces Do Not cancel • These forces that are produced by the two objects are action and reaction forces. • Sometimes action and reaction forces result in motion – sometimes they don’t. • Action and reaction forces do NOTcancel themselves out. • Only when equal and opposite forces act on the same object do they result in a netforce of zero

30. Momentum • Momentum is the product of an object’s mass and its velocity • An object at rest has a momentum of zero. • Momentum = Mass x Velocity • Measured in kilogram-meters/second (kg m/s) • p = mv (p is momentum) • An object has a large momentum if the product of it’s mass and velocity is large

31. Law of Conservation of Momentum • What happens to momentum when objects collide? • Momentum is conserved – the total momentum does notincrease or decrease • If no net force acts on a system, then the total momentum of the system does not change • A ‘system’ is part of the world underinvestigation (ie two cars colliding) • The rest of the world around the system is called the ‘surroundings’ • A closed system of objects means that there are no outside forces or objects acting on the objects in the system. • In a closed system, the loss of momentum by one object equals the gain of momentum by the second object.

32. Law of Conservation of Momentum

33. Universal Forces- Electromagnetic • Observations of planets, stars or other galaxies suggest that the fourforces exist throughout the universe • Electromagnetic force is associated with charged particles • Electric and magnetic force are 2 different parts of the electromagnetic force • Electric and magnetic are the only forces that both attract and repel

34. Electric and Magnetic Forces • Electric forces act between charged objects such as electrons or protons • Objects with opposite charges attract • Objects with like charges repel • Clothes often acquire electric charges in the dryer • Clothes with opposite charges tend to cling together • Magnetic forces act on some metals, poles of magnets and moving charges

35. Nuclear Forces • If the nucleus of an atom has protons (+) packed into a small space, why don’t they repel each other and break apart the nucleus? • Two forces, the strong-nuclear force and the weak-nuclear force act within the nucleus to hold it together. • The strong NF overcomes the electric force of repulsion between the protons in the nucleus • Acts only on neutrons and protons. 100x strong than repulsive forces • The weak nuclear force is involved in radioactive processes and is weaker than the strong NF

36. Gravitational Force • Gravitational force an attractive force that acts between any two masses • Weakest universal force • Most effective over large distances • Newton’s law of universal gravitation • Says that every object in the universe attracts every other object • The greater the mass of an object, the greater the gravitational force. • The moredistance between two objects, the less the gravitational force • Gravitational forces quicklab

37. Centripetal Force • How is the moon kept in orbit around earth? It has inertia and should move in a straight path until acted on by a force • Earth’s gravitational force continuously pulls the object toward it • Centripetal force: center-directed force that constantly changes the direction of an object and causes it to move in a circle

38. Centripetal Force • If the Earth causes the moon to stay in a circular orbit, what force does the moon apply on the earth? • Gravitational pull from moon causes two bulges in earth’s oceans • Earth rotates 2x a day below these bulges resulting in two high and two lowtides per day on Earth • Examples affected by centripetal force: • Satellites • Examples (there are too many to list) of satellite uses include: • communication • tracking Earth’s weather and climate