Acceleration and Momentum

Science, Grade 6


Table Of Contents: Acceleration and Momentum

1. Acceleration
Acceleration is the rate at which an object's velocity changes. A change in velocity can be an increase or decrease in speed, or a change in direction. A race car accelerates, gaining speed, when it starts a race. The car decelerates, or has negative acceleration, when it slows down and stops to re-fuel. A race car traveling around the track at a constant speed also has acceleration because it is changing direction.
2. Calculating Acceleration
If an object is changing speed, but not changing direction, you can calculate acceleration. Basically, acceleration equals the change of speed of an object, over a specific amount of time. Imagine a roller coaster car at the peak of a steep drop. The initial speed of the car is 0 m/sec, and its speed at the bottom of the peak is 30 m /sec. It takes three seconds to travel from the top to the bottom. The roller coaster accelerated at 10 m/sec2.
3. Acceleration on a Distance vs. Time Graph
Acceleration can be identified on a distance versus time graph. The slope of the line on this graph represents an object’s speed. A straight line indicates no change in speed, and therefore the object is not accelerating. A curved line indicates that the object’s speed is changing. A curve with a slope that is becoming steeper, tells us that the object is accelerating. While a line curved in the opposite direction indicates that the object is decelerating.
4. Graphing Acceleration-Speed vs. Time
The acceleration of an object can be shown on a graph with the x axis as the time and the y axis as the speed. Data points are plotted indicating the speed of the object at different times while it is traveling from one point to another. If the line is straight, then the acceleration was constant. The slope of the line represents the object’s acceleration. A steeper slope would indicate a faster acceleration.
5. Momentum
Momentum refers to the amount of motion that is taking place. To calculate the momentum of an object, multiply its mass by its velocity. A pick-up truck with a mass of 3000 kg is moving at 40 km/hr. The truck’s momentum is 120,000 kg-km/hr. Momentum is also described by its direction. An object’s momentum is always in the same direction as its velocity.
6. Conservation of Momentum
When groups of objects are involved, and there is no outside force, the total momentum of the group does not change. This is called the law of conservation of momentum. For example, when bumper cars collide, the momentum is transferred from one car to another, while the total momentum is conserved.