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Force and Motion –

Essential Question: What makes objects move the way they do?

8.1 — An object’s inertia causes it to continue to moving the way it is moving unless it is acted upon by a force.

GRADE-LEVEL CONCEPT 8.1.a

The motion of an object can be described by its position, direction of motion and speed.

(Students should understand that ...)
1. An object is said to be in motion when its position changes in relation to a point of reference. An object’s motion can be described and represented graphically according to its position, direction of motion, and speed.
2. Speed describes the change in an object’s position over a period of time, and is measured in units such as meters per second or miles per hour.
3. Average speed takes into account the different speeds at which an object moves over a period of time. Average speed is calculated by dividing the total distance traveled by the change in time, regardless of any changes in motion or direction during its travel.
4. Motion of objects can be represented on a distance vs. time line graph, with distance traveled as the vertical (“y”) axis and time as the horizontal (“x”) axis. The steepness and slant of the motion line vary depending on the speed and direction of the moving objects. A straight horizontal line indicates an object at rest.

C22. Calculate the average speed of a moving object and illustrate the motion of objects in graphs of distance over time.

GRADE-LEVEL CONCEPT 8.1.b

An unbalanced force acting on an object changes its speed and/or direction of motion.

(Students should understand that ...)
1. In order for an object to change its motion, a push/pull (force) must be applied over a distance.
2. Forces can act between objects that are in direct contact, or they can act over a distance. There are forces of attraction, such as gravity or magnetism, and forces of resistance, such as friction and drag (air resistance). Forces are measured in Newtons or pounds using scales.
3. Forces can act simultaneously on an object from all directions with different strengths (magnitudes). When the magnitude and direction of all the forces acting on an object are combined, or added together, the total force (net force) determines the object’s motion. Forces in opposite directions are subtracted; forces in the same direction are added.
4. If the strength of all the forces acting on an object from one direction is equivalent to the strength of the forces from the opposite direction, then the forces cancel each other out, and are said to be balanced. Balanced forces keep an object moving with the same speed and direction, including keeping it at rest.
5. If the net force acting on an object is not zero, then the forces are said to be unbalanced, and the object’s speed or direction will change, changing its motion (acceleration). Acceleration is any change in motion, and occurs when something speeds up, slows down or changes direction. On a position time graph, this would be indicated by a change in the steepness of the motion line, or by a curved line.
6. The greater the unbalanced force on an object, the greater its change in motion (acceleration). The greater the mass of an object, the greater the force needed to change its acceleration. Given the same amount of force, an object with a greater mass will change acceleration less. The total net force acting on an object can be determined by measuring its mass and change in motion (acceleration).

C23. Describe the qualitative relationships among force, mass and changes in motion.

GRADE-LEVEL CONCEPT 8.1.c

Objects moving in circles must experience force acting toward the center.

(Students should understand that ...)
1. Some objects continuously change direction without changing speed, causing them to move in a circular path. Circular motion is caused by a constant unbalanced force that is constantly changing direction and pulling towards the center. If there were no force pulling the object toward the center, it would continue to move in a straight line in the direction it was moving before the force was removed.

C24. Describe the forces acting on an object moving in a circular path.

Grade Level Expectations

(Students should be able to ...)

1. Demonstrate how forces, including friction, act upon an object to change its position over time in relation to a fixed point of reference.
2. Calculate the average speed of an object and distinguish between instantaneous speed and average speed of an object.
3. Create and interpret distance-time graphs for objects moving at constant and nonconstant speeds.
4. Predict the motion of an object given the magnitude and direction of forces acting upon it (net force).
5. Investigate and demonstrate how unbalanced forces cause acceleration (change in speed and/or direction of an object’s motion).
6. Assess in writing the relationship between an object’s mass and its inertia when at rest and in motion.
7. Express mathematically how the mass of an object and the force acting on it affect its acceleration.
8. Design and conduct an experiment to determine how gravity and friction (air resistance) affect a falling object.
9. Illustrate how the circular motion of an object is caused by a center seeking force (centripetal force) resulting in the object’s constant acceleration.

SCIENTIFIC LITERACY TERMINOLOGY: Motion, point of reference, speed, constant speed, average speed, position-time graph, slope, force, friction, gravity, inertia, mass, acceleration, balanced/unbalanced forces, net force, circular motion