A force is the push or a pull on the object. A force changes the motion of a moving object.
A force can be strong or weak and it always has a direction.
The earth is a planet that goes around the sun in the circular orbit. The speed of its motion doesn’t change and the direction of its motion does. The earth has a layer of gas above its surface, which helps to support life.
Atoms are the building blocks of matter. At the center of an atom is its nucleus, which has a positive electric charge. Outside the nucleus are electrons, which have a negative charge.
Asteroids and comics are object that orbits our sun, but are smaller than planets. Asteroid that pass close to the earth are called the near-earth objects. The layer of gases, just above the earth surface, is our atmosphere. The atmosphere is made up of gases, such as oxygen and hydrogen. Without an atmosphere, there will be not life on the earth.
Asteroid n.[天]小行星;海盘车;海星 adj.星状的
orbit n.轨道;眼眶;势力范围;（人生的）旅程，生活过程
vt.& vi.在…轨道上运行，环绕轨道运行
vi.盘旋;绕轨道运行
vt.绕轨道而行;进入轨道
The two lines on the left intersect each other at a right angle which is 90 degrees. The 2 lines on the right are parallel, which means they never cross.
The adjacent sides of rectangle are perpendicular, which means they meet at a 90-degree angle. The opposite sides of the rectangle are equal a parallel, which means they never cross. The area of a rectangle is the length times the width.
Intersect vt.横断，横切，横穿 vt.& vi.（指线条、道路等）相交，交叉
A circle is the 2 dimensional figures with radius and circumference which is the distance around the circle. The area of the circle is π R square, where r is the radius and Pi is the ratio of the circle’s circumference to its diameter.
Radius n.桡骨;半径范围;半径（距离）;用半径度量的圆形面积
Dimensional adj.维的;尺寸的;<物>量纲的;<数>因次的
A sphere is a 3 dimensional figure with the a ratio and volume, which is the three-fourth πR cube. All points on the surface of the sphere are equidistance from its center.
Cube n.立方形，立方体;立方，三次幂;小房间
vt.求…的立方;把（食物）切成小方块;测定…的体积
equidistance 等距离的
A right triangle is a 2 dimensional figure with 3 sides, 2 of which are perpendicular. To find its area, multiply the length of the 2 perpendicular sides, A and B and divided by 2, A equates one half A B.
Perpendicular
adj.垂直的，成直角的;直立的，险陡的;[建]垂直式的;〈谑〉直站着的
n.垂直线，垂直面;直立，直立姿势，廉直;垂直测器，锤规;[建]垂直式建筑，绝壁
Equate vt.等同;使相等;相当于
the electric force between the nucleus and the electrons holds the atom together.
A force can increase or decrease the speed of an object, and it can change the direction of motion.
