Science Topic 3 Lessons 1 & 2

Lesson 1

• An object is in motion if its position changes when compared to another object.

• A reference point is a place or object used to determine whether something is in motion. (Because motion is determined by a reference point that can change, motion is relative).

• While objects move relative to each other, they can also speed up, slow down, and change direction. The motion of an object can change when one or more forces act on the object.

• A force is described by its strength and by the direction in which it acts.

• Forces can be classified as either contact forces or noncontact forces. Contact forces are those applied only when one object actually touches another.

• A noncontact force is a force applied to an object whether it touches the object or not.

• More than one force can act on an object. When the forces on an object are unbalanced, there is a non-zero net force acting on the object The net force on an object is the combination of all the forces acting on that object.

• If the forces act in the same direction, the net force is the sum of the forces. If the forces act in the opposite directions, the net force is the difference in strength of those forces.

• If the net force turns out to be zero, the forces are balanced. Otherwise, the forces are unbalanced.

Lesson 2

• The speed of an object is the distance the object moves per unit of time. Speed is a type of rate. (A rate tells you the amount of something that occurs on changes in one unit of time).

• To calculate the speed of an object, divide the distance the object trabels by the amount of time it takes to travel that distance.

• The speed at a particular instant in time is called instantaneous speed. To calculate the average speed, divide the total distance traveled by the total time.

• To describe an object’s motion, you also need to know its direction. The speed at which an object travels in a given direction is called velocity.

• Speed and velocity are not the only ways to describe motion. Scientists define acceleration as the rate at which velocity changes. A change in velocity can involve a change in speed, direction, or both.

• In science, when an object accelerates, it increases speed, decreases speed, or changes direction.

• When the term acceleration is used, it means one of 2 things — any change in speed or any change in direction.

• Even an object that is traveling at constant speed is accelerating when it changes direction.

• If an object is not changing direction, you can describe its acceleration as the rate at which its speed changes.