# 物理力学笔记

[VAMPIRE13] 2008-6-28 6:14:49

Forces and motion

Representing motion

Speed, distance and time

Speed (meter per sec)= distance(m)/ time taken (sec)

Diagram: vertical axis is the distance travelled form the start. Horizontal axis is the time from the start.

When an object is stationary, the line is horizontal

When an object is moving at a steady speed, the ling is straight, but sloped.

The steeper the line, the greater the speed.

Velocity-time graphs

Explain for objects moving with a constant velocity or a constant acceleration (The rate of change of velocity, measured in meters per second squared. Acceleration= change of velocity/ time taken)

When an object is moving with a constant velocity, the line on the graph is horizontal. When an object is moving with a constant acceleration, the line on the graph is straight, but sloped.

The steeper the line, the greater the acceleration.

Acceleration= change of velocity/ time taken

Force, mass and acceleration

Balanced and unbalanced forces

When all the forces acting on an object cancel out, we say that they are balanced. If the forces are balanced:

-          A stationary object remains stationary

-          A moving object keeps on moving at the same speed in same direction.

The longer the arrow, the bigger the force

Unbalanced forces

When all the forces acting on an object do not cancel out, we say they are unbalanced.

-          A stationary object begins to move in the direction of the unbalanced force.

-          A moving object speed up, slows down or changes direction depending on the direction of the unbalanced force.

Unbalanced forces cause objects to accelerate.

The bigger the force, the greater the acceleration.

The bigger the mass of the object, the smaller the acceleration.

Force (Newton, N) = mass (kg) * acceleration (m/s squared)

Other elements that affect force.

1. Air resistance.
2. contact force
3. Friction
4. Gravity