Forces & Motion

1. Forces & Motion Chapter 12

2. TUG-O-WAR TIME!!!

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

4. Measuring Force • Spring Scales • The stretch of a spring scale depend on the weight (type of force) acting on it

5. Units of Force • Measured in newtons (N) • 1 N = the force that causes a 1 kilogram mass to accelerate at a rate of 1 meter per second squared • 1 N = 1 km•m/s2 • This unit is named after Sir Isaac Newton (1642-1727) • Scientist who explained how force, mass and acceleration are related

6. Force Diagrams • Use arrows to represent the direction and strength of a force (like a vector!)

7. Spring Scale Activity Choose five objects on your table. Attach a string to your objects if necessary. Use the spring scale to determine the weight (in newtons) of your objects. Draw a force arrow for each object that is to scale relative to each other force arrow.

8. Combining Forces • Back to tug-o-war… • You can combine force arrows to show the result of how forces combine • Forces in the same direction add together • Forces in the opposite direction subtract from one another

9. Net Force • The overall force acting on an object after all the forces are combined

10. Balanced Forces • Sometimes the net force acting on an object is zero. • Balanced Forces • Forces that combine to produce a net force of zero • Results in NO CHANGE in an object’s motion

11. Common Example • Two people locked in an arm wrestling match • Tug-o-War match with two evenly matched teams! • Two football players pushing against one another at the line of scrimmage

12. Unbalanced Forces • Results when the net force acting on an object is NOT equal to zero • When an unbalanced force acts on an object, the object accelerates

13. Combining Forces = Adding Forces = Subtracting Forces = 0 Equal and opposite forces

14. Friction • A force that opposes motion of objects that touch as they move past each other. • Acts at the surface where objects are in contact (includes all solids, liquids, and gases) • Friction is important! • Without friction every surface would be impossibly slippery • Food would slide right off your fork • Walking would be impossible • Cars would slide around with their wheels spinning

15. Four main types of friction • Static Friction • Sliding Friction • Rolling Friction • Fluid Friction

16. Static Friction • The friction force that acts on objects that are not moving • Always acts in a direction opposite to that of the applied force

18. Sliding Friction • A force that opposes the direction of motion of an object as it slides over a surface • LESS than static friction • This means that once an object is moving, less force is needed to keep the object moving than to start it moving

20. Rolling Friction • The friction force that acts on rolling objects • When a round object rolls across a flat floor, both the object and the floor are bent slightly out of shape at the point of contact • 100 – 1000 times less than static or sliding friction • This is why movers use wheeled dollies to move heavy objects!

22. Fluid Friction • Liquids and mixtures of air are known as fluids • Fluid friction results when fluids (like liquids and air) oppose motion of an object • Example, when you stir cake batter you can feel fluid friction • Fluid friction increases as the speed of the object moving through the object increases

23. Air Resistance • Fluid friction acting on an object moving through the air • At higher speeds air resistance is a significant force • For example, swimmers, cyclists and even runners wear slick racing suits to reduce air resistance

24. http://Wing Suits... (Start @ 1:00 min

25. Types of Friction Foldable Activity!

26. Gravity • A force that acts between any two masses • An attractive force (it pulls objects together) • Unlike friction, gravity can act over large distances (think skydiving!)

27. Gravity (continued) • Earth’s gravity acts downward toward the center of the Earth

28. Falling Objects • Both gravity and air resistance affect the motion of a falling object • Gravity causes objects to accelerate downward • Air resistance acts in the direction opposite to the motion, reducing acceleration

30. Flying Squirrels… • As objects fall they accelerate (gain speed) • As speed increases, air resistance increases • If an object falls long enough, the upward force of air resistance eventually will equal the downward force of gravity • Forces are balanced, acceleration is zero and the object continues falling at a constant velocity

31. Terminal Velocity • Constant velocity of a falling object when the force of air resistance equals the force of gravity

32. Projectile Motion • The motion of a falling object (projectile) after it is given an initial forward velocity • The only forces acting on a projectile are gravity and air resistance

34. Projectile Motion (Continued) • The combination of an initial forward velocity and the downward vertical force of gravity causes the ball to follow a curved path