Unit 02 Resources

Big Idea

AN OBJECT THAT IS CHANGING ITS MOTION -- THAT IS ACCELERATING -- MUST BE EXPERIENCING AN EXTERNAL NET FORCE IN THE DIRECTION OF THE ACCELERATION.

Objects around us speed up and slow down and change their direction all the time. The only way they can do this is by receiving a force from the outside that is not balanced by other forces.

What should I know?

1. Objects that change their position by different amounts each second must have changing motion.

2. Motion can change in one of three ways: speeding up, slowing down, or turning. All of these are types of acceleration.

3. By comparing an object's velocities at the beginning and end of a time period, we can measure the average amount of acceleration it experiences.

4. Since acceleration is a comparison of two velocities (m/s) in a unit of time (s), the units for acceleration are (meters/second)/second.

5. The units for acceleration are often simplified to m/s^2.

6. The acceleration an object experiences must be caused by an unbalanced external force. This type of force is called a net force.

7. Forces always have four characteristics: a cause, an object, a direction and a size, which we abbreviate CODS.

8. In this class, forces are always measured in Newtons, which is a simplified form of the kg * m/s/s. 

9. If a force appears not to have a cause, it must not be a true force. An example of this is a centrifugal force.

10. Forces come in two types: contact forces and field forces.

11. Contact forces are pushes or pulls that are applied through a touch.

12. Field forces are pushes or pulls that act at a distance. Gravitational, electrostatic and magnetic forces are all field forces.

13. An analysis of the forces acting on an object can be accomplished using a free body diagram, on which all forces are represented as vector arrows drawn in the correct direction and to the correct relative size.

14. Determination of the size and direction of a net force acting on an object can be made by adding the force vectors on a free body diagram.

15. The acceleration an object experiences is directly proportional to the net force causing it.

16. The acceleration that is produced by a net force must be in the same direction as that net force.

17. A given net force acting on objects of different masses produces different accelerations. The acceleration an object experiences is inversely proportional to the mass of the object.

18. NEWTON'S SECOND LAW OF MOTION summarizes points 12 and 13 in the following formula: F = ma.

19. To use Newtons as the unit of measure of force, mass must be measured in kilograms and acceleration in meters/second/second.

20. Weight is another name for the force of gravity between an object and the Earth.

21. Mass and weight are very different concepts, but related through Newton's Second Law of Motion.

22. Converting a mass (in kg) to a weight (in N) can be done by multiplying the mass by a constant 9.81 m/s/s.

23. The constant in point 21 is the gravitational acceleration -- g -- which represents the acceleration experienced by all objects that fall freely near the Earth's surface.

24. Contrary to popular belief, the mass of an object that falls freely does not affect the acceleration it experiences -- every object that is falling freely experiences an acceleration of g, regardless of its mass.

Vocabulary

1. Acceleration -- a change in the velocity of an object. Because it is a change in the velocity, it could mean speeding up, slowing down or changing direction. It is a vector quantity. It is symbolized with a lowercase a. Its units are m/s/s, read "meters per second per second."

2. Gravitational Acceleration -- the acceleration applied to objects by the gravitational pull of the Earth. At the Earth's surface, it is about 9.81 m/s/s for all objects, regardless of their weight. It is symbolized with a lowercase g.

3. Free Fall -- the state in which an object's motion is changing due only to gravitational acceleration.

4. Resistance Forces -- forces that always act in the opposite direction of the motion of the object.

5. Friction -- a resistance force due to the rubbing of surfaces as objects slide past. Unlike other forces, it is often symbolized with a lowercase f.

6. Air Resistance -- a resistance force due to the object's collisions with air particles. It is also called drag.

7. Mass -- the amount of matter in an object. In physics, it has two consequences: (1) it determines the amount of resistance to changes in motion for an object, and (2) it produces and interacts with gravitational fields.

8. Kilograms -- the standard MKS unit of mass. It is abbreviated with a kg.

9. Weight -- a measure of the gravitational pull of the Earth on an object. The more mass an object has, the more it interacts with the Earth's gravitational field, and, therefore, the more it weighs.

10. MKS System of Units -- the subset of metric system units that are most commonly used in physics. The M stands for the meter -- the standard for length measurement. The K stands for the kilogram -- the standard unit for mass. The S stands for seconds -- the standard unit for time.

11. Newtons -- the common unit of all force measurements in the MKS system of units. It is abbreviated with an N or sometimes an n. It is equivalent to the force required to provide a 1 kilogram mass with an acceleration of 1 m/s/s, i.e. 1 N = 1 kg * m/s/s. 

12. Spring Scale -- a measurement device used to judge the size of a force. The longer the spring stretches, the larger the force must be.

13. Newton's Second Law of Motion -- It states, "the acceleration of an object is directly proportional to the net force acting on the object, is inversely proportional to the mass of the object, and is in the direction of the net force." More simply stated, a net force causes a mass to accelerate. The more net force applied, the more it will accelerate. The more mass on which the net force is applied, the less acceleration is produced.

14. Terminal Speed -- the speed at which the upward force of air resistance balances out the downward force of the Earth's gravitational pull. Since there is no longer any net force at this speed, the speed is constant.

15. Terminal Velocity -- the terminal speed, but with the direction given as well.

Notes for this Unit

LINK to sample problem videos