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Forces and Motion

Science, Grade 6

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Table Of Contents: Forces and Motion

1. Motion

2.1. What Is Motion?
An object is in motion if it changes its position relative to another object. You can describe the characteristics of motion in many ways including distance, speed, velocity, acceleration and momentum.
2.2. Speed
Speed describes the distance an object travels during a certain amount of time. Speed is a measurement of a rate. If a cyclist traveled a distance of 15 kilometers in an hour's time, the bicycle was moving at a speed of 15 kilometers per hour.
2.3. Calculating Speed
Speed equals the distance traveled divided by the time it took to travel. Many objects in motion do not travel at a constant speed. A car traveling between two cities moves at different speeds along the way. To calculate the average speed of an object, use the total distance traveled divided by the total time.
2.4. Graphing the Speed of an Object
The motion of an object can be shown on a graph with the x axis as time and the y axis as distance. To graph the motion, plot the data points that indicate the distance traveled at a particular time, and then draw a line through the points. The slope of this line represents the speed of the object. A steeper slope indicates a faster speed.
2.5. Velocity
To completely describe an object in motion, you need to know both the speed and direction it is traveling. Velocity is defined as the speed of an object in a given direction. Velocity can provide important information. For example, although knowing the speed of a hurricane is informative, actually knowing the velocity of the storm is critical for people to determine if they are in the predicted path of destruction.

2. Pause and Interact

3.1. Review
Solve each problem. Use the whiteboard text tool to show your work and answers.
3.2. Acceleration and Momentum
Follow the instructions in the instructions tab.

3. Acceleration and Momentum

4.1. 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.
4.2. 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.
4.3. 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.
4.4. 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.
4.5. 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.
4.6. 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.

4. Pause and Interact

5.1. Review
Solve each problem. Use the whiteboard text tool to show your work and answers.

5. Force

6.1. What Is a Force?
A force is a push or a pull. When you kick a soccer ball, you are exerting a force on the ball to make it move. A force is described by its strength and its direction. A force’s strength is measured in units called Newtons, named after Sir Isaac Newton. A force is represented by an arrow pointing in the direction of the force, with the length of the arrow indicating the strength of the force.
6.2. Combining Forces
Forces can act in combination and produce what is referred to as net force. When two forces are exerted in the same direction, you calculate net force by adding the two forces together. If two equal forces are opposed to each other, they tend to cancel each other out. These are called balanced forces. Two unequal opposing forces will produce a net force in the direction of the larger force.
6.3. Friction
When two surfaces are rubbed against each other, a force called friction is exerted. Friction acts in the opposite direction of an object’s motion. When an object is in motion, friction can slow it down and cause it to stop. The strength of friction depends on the types of surfaces that are in contact, the amount of contact between the surfaces, and the amount of force pushing the surfaces against each other.
6.4. Types of Friction
There are several types of friction, including static, sliding, rolling and fluid. Static friction occurs when a force is exerted on an object, but the object does not move. When you push a box across a floor, sliding friction occurs in the opposite direction. Rolling friction, seen in wheels and balls, is easier to overcome than sliding friction in similar materials. Fluid friction occurs when an object moves through a fluid, such as when a boat moves through water or when an airplane flies through the air.

6. Pause and Interact

7.1. Review
Use the whiteboard tools to complete the activity.
7.2. Forces and Friction
Follow the instructions in the instructions tab.

7. Gravity

8.1. Law of Universal Gravitation
The law of universal gravitation, proposed by Sir Isaac Newton, states that all objects in the universe attract each other through gravity. Gravity is a type of force that pulls objects toward each other. The amount of gravitational force between two objects depends on their masses and the distance between them. The strength of gravity increases as mass increases, and the strength of gravity decreases as the distance between objects increases.
8.2. Mass and Weight
An object’s mass refers to the amount of matter an object contains, while an object’s weight is the measure of the effect of gravity on its mass. The mass of a person is the same on the Moon and the Earth, but the person weighs less on the Moon than the Earth because the Moon exerts a smaller gravitational force than the Earth.
8.3. Gravity and Motion
When gravity is the only force acting on an object, the object is said to be in free fall. This can be observed if objects are placed in a vacuum. On Earth, all objects in free fall accelerate at the same rate of 9.8 m/s2, even if the objects have different masses. Typically, objects don’t fall within a vacuum, but instead they fall through the air and experience resistance, which is a type of fluid friction acting in the opposite direction of gravity. Objects with greater surface area will experience more air resistance and accelerate more slowly than objects with less surface area.

8. Pause and Interact

9.1. Review
Use the whiteboard tools to complete the activity.

9. Newton’s Laws

10.1. Sir Isaac Newton
Sir Isaac Newton was an English mathematician, physicist and astronomer who lived in the late 1600s. He is considered to be one of the most influential scientists of all time. Newton proposed the law of universal gravitation as well as the three basic laws of motion, which have influenced the study of physics for over 300 years.
10.2. Newton's First Law of Motion
Newton’s first law is also known as the law of inertia. This law states that an object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force. Inertia is an object’s resistance to a change in motion. For example, when a car brakes, a passenger’s body will tend to move forward due to inertia. A seatbelt provides an opposing force to this inertia.
10.3. Newton's Second Law of Motion
Newton’s second law states that the acceleration of an object depends on the mass of the object and the net force affecting the object. When the same amount of force is exerted, an object with smaller mass accelerates faster than an object with larger mass. If the amount of force on an object is increased, then the acceleration will increase.
10.4. Second Law of Motion Equation
The equation that describes Newton’s second law is acceleration equals net force divided by mass. In this example, the cart has a force of 200 N and a mass of 40 kg. The acceleration is equal to 5 m/s2. Rearranging this equation to net force equals mass times acceleration is useful when you need to find the amount of force that is affecting an object.
10.5. Newton's Thrid Law of Motion
Netwon’s third law states that for every action, there is an equal and opposite reaction. This is a simple way of saying that when one object exerts a force on another object, the second object responds with a force in the opposite direction. For example, when air is released from a ballon, the balloon is pushed in the opposite direction.

10. Vocabulary Review

11.1. Forces and Motion Vocabulary Matching
Sir Isaac Newton was an English mathematician, physicist and astronomer who lived in the late 1600s. He is considered to be one of the most influential scientists of all time. Newton proposed the law of universal gravitation as well as the three basic laws of motion, which have influenced the study of physics for over 300 years.

11. Virtual Investigation

12.1. Forces and Motion
Momentum refers to the amount of motion that is taking place. To calculate the momentum of an object, multiply its mass times its velocity. When groups of objects are involved, and there is no outside force, the total momentum of the group does not change. This is referred to as the law of conservation of momentum. In this virtual investigation, you can choose different moving truck scenarios to see how mass and velocity affect momentum. You will observe what happens to the momentum of trucks when they collide and when they connect together. Enter the data you collect in the data sheet (icon in the top right). Remember, this data sheet is draggable and scrollable. Be aware that if you move outside of the Virtual Investigation to other parts of the multimedia lesson, your data will not be saved.

12. Assessment

13.1. Forces and Motion
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