Force and Motion: Content Introduction

* Consider everything we have done so far on force and motion as the exploration part of the learning cycle.

Motion – Position

A. Position – in 1 dimension it is the place on a number line. The position can be positive or negative. (MKS units: meters)

B. Distance – the actual length traveled.C. Change in position – the final position

minus the initial position for the time interval considered.

0-x +x

Motion –Velocity

A. Speed – how fast something is moving. (MKS units: meters / second)

B. Velocity – speed and direction.C. Average speed – distance traveled

divided by time taken.D. Average velocity – the final position

minus the initial position divided by the time interval considered.

E. When moving only in one direction (no turning around) the average speed and average velocity are the same.

The average velocity between t = 1 and t = 3 is?

A. -1 m/s B. 0.5 m/s C. 1 m/s D. -0.5 m/s

E. 2 m/s

The average velocity between t = 1 and t = 2 is?

A. -1 m/s B. 0.5 m/s C. 1 m/s D. -0.5 m/s

E. 2 m/s

Motion –Acceleration

A. Average acceleration - the final velocity minus the initial velocity divided by the time interval considered. (MKS units: meters / second / second)

B. If the velocity changes in value or direction, the object is accelerating.

Fig. P2.17, p.51

The body is not accelerating from

A. 0 to 2 s B. 4 to 8 s C. 11 to 12 s

D. 2 to 4 s E. 8 to 11 s

Is the object following the blue line accelerating?

A. yes B. no C. not enough information

Is the object following the orange line accelerating?

A. yes B. no C. not enough information

Force and Mass

A. A force is a push or a pull (MKS units: Newtons).

B. Force is an agent of change. A net force produces a change in the motion (acceleration).

C. Mass is the amount of stuff in a body (MKS units: Kilograms).

D. A quart of water has a mass a little more than a kilogram and it takes a little over 10 Newtons to hold the quart of water from falling.

Force – Newton’s Laws

I. A body remains at rest or in a state of uniform motion (constant velocity) unless acted on by a net force.

II. The net force on a body equals the mass of the body times its acceleration.

III. If body A acts on body B, then body B acts back on body A with a force of equal value, but opposite in direction.

Force – Facts

A. Unbalanced forces produce a change in the motion (acceleration). This is the same as a net force.

B. Balanced forces don’t produce a change in the motion.

Force – Examples

A. Because F = ma, the same force acting on a small mass and a large mass will produce the greater acceleration on the smaller mass. (Wacker Demo)

B. Because F = ma, a bigger force acting on the same mass will produce a greater acceleration.

C. The force of gravity produces a constant acceleration.

Time (s)V

elo

cit

y (

m/s

)

321 4

-9.8

-19.6

-29.4

-39.2

Freefall

For an object at rest on a surface (e.g., a book on a table), the surface (e.g., table) does not exert an upward force.

The 'force of motion' can be used up.

If an object is more active (moves faster) than another object, it also has more force.

Falling Washers Demonstration

What questions do you have on the content readings?