Forces and Motions

1. Forces and Motions

2. Forces What is a Force? • Anything that changes the state of restor motion of an object • It’s what causes ACCELERATION • has magnitude and direction • Therefore force is a vector

3. Force cont. A force can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction. Units = Newtons (N)

4. Forces can be COMBINED: Net Force: the combination of all of the forces acting on the object Balanced Forces: When the forces on an object are balanced, the netforce is zeroand there is nochange in the object’s motion. Warning: Doesn’t always mean the object is at rest; Example: Tug of war, Pushing piano, car traveling at constant velocity

5. Unbalanced Forces • Results when net force acting on an object is NOTequaltozero • When an unbalanced force acts on an object, the object accelerates in the direction of the net force • Fig 14, p.332 http://www.darvill.clara.net/enforcemot/friction.htm *(moves in direction of net force)

6. Force • A push or pull on an object • Units: Newton (N) = 1 kg x m/s2 • Net Force (Fnet) : sum of all of the forces that are acting on an object • Balanced Forces: Fnet = 0 • Object is at rest or moving with constant velocity • Unbalanced Forces: Fnet is not equal to 0, • Object is accelerating or decelerating

7. Types of Forces • Applied – contact force in the direction the object is moving • Tension- caused by a rope, cable, etc., directed away from the object • Friction- opposes the motion of objects, must have contact • Normal- caused by a surface • Force of gravity- force at a distance, caused by attraction between two objects

8. Two Types of Friction • Static • Friction of an object at rest • Greater than kinetic friction (harder to get an object moving than to keep it moving) • Friction is greatest when surfaces are rough ex. the force that is keeping this block from sliding downhill • Kinetic • Friction of a moving object

9. Types of Kinetic Friction • Sliding • Two surfaces moving past each other • Rolling • Movement of a circular object on a surface • Ex. Cart on wheels • Fluid Friction • Movement of an object through a liquid or gas • Ex. Air resistance

10. FRICTION is defined as is defined as is defined as is defined as & an example is & an example is & an example is & an example is STATIC SLIDING ROLLING FLUID The friction between surfaces that are stationary force that exists when objects slide past each other force that exists when a round object rolls over a flat surface (usually less than sliding friction) force that exists when an object moves through a fluid (air, water) a book sitting on a table -hockey puck on ice -child going down a slide -a sled down hill -a roller blade on a sidewalk -bowling ball on bowling alley -a car driving down the road -swimmer swimming through pool

11. How can we decrease friction? Watch this demo and see… http://www.scottso.net/examples_encfri.htm Can you think of a situation in which you would want to increase friction?

12. Gravity is a force:natural phenomenon in which objects that have mass are attractedto one another • Gravity is an attractive force  pulls objects together • Earth’s gravity acts downward toward the center of the Earth. • There is an upward force that balances gravity • AIR RESISTANCE

13. Falling Objects have two forces acting on them: • Gravity causes objects to accelerate downward • airresistance acts in the direction oppositetothemotion & reduces acceleration.

14. Free Fall: the motion of a body when only the force of gravity is acting on the body • Free fall acceleration of an object is directed toward the center of Earth • Because free fall acceleration results from gravity, its symbol is g • Acceleration due to gravity on Earth = 9.8 m/s2 Formula for objects in free fall: d = ½ gt2

15. In a vacuum, two objects would accelerate at the same rate because both are in freefall (Fig 8, p.354) Question: What other force is not present in vacuum that would affect acceleration? Answer = air resistance

16. Drawing Free Body Diagrams • Identify the types of forces acting on the object • Use a dot or a box to represent the object • Use arrows to represent the direction and size of the force • -change the size of the arrow if more force is being applied in one direction versus the other • Label the arrows with the type of force

17. Free Body Diagrams • How will the object move in the diagram to the right? • Answer: Object may be at rest and not move or it could be moving at a constant velocity since all the forces are balanced and the net force is zero

18. Free body diagram practice • Free Body diagram practice

19. II.) Newton’s First Law of Motion A) Historical Development 1) Aristotle (384 BC- 322 BC): Incorrectly proposed that force is required to keep an object moving

20. 2) Galileo (1564 – 1642): Concluded that moving objects not subjected to friction or other force, would continue to move indefinitely; Disproved Aristotle

21. 3) Newton (1643 – 1727): Defined mass and force; Introduced 3 Laws of Motion

22. C) Newton’s 1st Law of Motion 1) According to Newton’s 1st Law, the state of motion of an object does not change as long as the net force is zero. a) Basically saying that an object at rest will stay at rest unless a force acts on it 1) Example: Soccer ball will remain (atrest) on the grass unless a force is acted on it

23. 2) Sometimes called the “LawofInertia” a) Inertia: the tendency of an object to resist being moved or, if the object is moving, to resist a change in speed or direction until an outside force acts on the object 1) Car crash: You continue forward because of inertia “Science and the Consumer” p.348

24. How is inertia related to mass?P 347 • Mass is a measure of inertia. • Who would you rather be tackled by…a toddler or a defensive lineman? • What is easier to move? An empty garbage can or a garbage can full of lead? Why? • The empty garbage can has less mass= less inertia= less resistance to being accelerated.