6th grade science Mrs. Reese Forces and Motion 6th grade science Mrs. Reese

Sir Isaac Newton (1642-1727) While many scientists and philosophers like Aristotle and Galileo tried to observe and discuss how objects move on Earth, Sir Isaac Newton was the first scientist to formulate a set of laws or rules to summarize how all objects move on Earth and in the universe. It is said that Newton came up with these ideas when an apple fell on his head when he was sleeping under a tree!

What is a force? Force is a push or pull one body exerts on another. Another way to describe a force is any action that has the ability to change the motion of an object by giving the object energy. Forces are a big part of physics. Physicists devote a lot of time to the study of forces that are found everywhere in the universe. The forces could be big, such as the pull of a star on a planet. The forces could also be very small, such as the pull of a nucleus on an electron. Forces are acting everywhere in the universe at all times.

Examples of Forces Contact Force - When two objects are in contact, each object exerts a force on the other. Pushing and pulling. Gravity – attraction force Force that pulls everything near the Earth’s surface toward the Earth’s center. Magnetic Force - attraction between magnets. N +S Electrical Force - attraction between charges: protons(+) and electrons (-)

Do we use forces in our body? Yes, during breathing, muscles contract (use force) to move air in and out of the lungs. The heart also contracts (uses force) to pump blood through the blood vessels, carrying oxygen to the muscles.

Forces in Nature Erosion is the process by which fragments of soil and rock are broken off from the ground (sediments) and are carried away. Eventually these fragments are deposited in a new place. Water has the ability to cause erosion if it carries enough force and speed. The more force and speed water has, the greater the impact on changing the shape of the land.

Soil Erosion Due To Water

Forces in Nature

Balanced Forces Balanced forces are forces that are equal in size and opposite in direction. Example: tug-of-war between two people who are pulling with equal strength. The net force is zero! There is no change in motion!

Unbalanced Forces Unbalanced forces are forces that change the velocity (speed and direction) of an object. Example: tug-of-war between two people of unequal strength. Suppose one student is pulling with a force of 30N and the other with 45N, the difference in the NET force is 15N in the direction of the greater force. Hey! Pick on Somebody your own size! 30N 45N

Newton’s First Law: Law of Inertia Inertia is defined as an object’s tendency to resist any change in its motion. If an object is moving, it will keep moving at the same speed and direction unless and unbalanced force acts on it. Example: When you are riding in car, both you and the car are in motion. If the car stops suddenly, inertia will make you keep going forward unless you are wearing your seatbelt!. MORAL: WEAR YOUR SEATBELT!!!

Newton’s First Law: Law of Inertia The opposite is also true. If an object is at rest, then the object will remain at rest unless an unbalanced force acts on it. Example: Imagine if you have a toy car on the floor and you want it to move. Will it move unless you apply a force to it? NO!!!

Newton’s First Law

Does mass affect inertia? The more mass (amount of matter in object) an object has, the greater its inertia. Example: Which has more inertia, a golf ball or bowling ball? Ask yourself, who resists a change in motion the most?

Force of Friction Friction is the force that opposes motion between two surfaces that are touching each other. Friction depends on the type of surfaces that are being pressed together and the force at which they are pressed together. Example: Friction between your shoes and the floor is what allows us to walk! If there was no friction, you would be constantly slipping and falling!

Types of Friction You can have rolling, sliding, and static friction. Static friction is friction between two solid objects that are not moving relative to each other. The static friction force must be overcome by an applied force before an object can move. Rolling friction is the resistive force that slows down the motion of a rolling ball or wheel. This type of friction is typically a combination of several friction forces at the point of contact between the wheel and the ground or other surface. Sliding friction is the friction between moving surfaces.

What type of friction is this? NOT MOVING 

Newton’s Second Law Newton’s Second Law: Newton’s second law describes how force, mass, and acceleration are connected. Newton’s second law states that a force acting on an object causes the object to accelerate in the direction of the force.

Newton’s Second Law The larger the force that is applied on the object, the greater its acceleration. An object with more mass requires more force to accelerate. Objects with more mass accelerate slower than object’s with less mass if an equal force is applied to both objects. Example: semi-truck vs. car

What is the Formula? F = m x a F = Force in Newtons (N) or kg (m/s2) m = mass in kilograms (kg) a = acceleration in meters per second squared (m/s2)

Math Example With what force will a car hit a tree if the car has a mass of 3000kg and it is accelerating at a rate of 2m/s2?

Math Example (Use 5 Steps) A 10 kg bowling ball would require what force to accelerate it down an alleyway at a rate of 3 m/s2? F = ? m = 10 kg F = m x a F = 10 kg x 3 m/s2 a = 3 m/s2 Step 5 solve by hand! 10 x 3 = 30 F = 30 kg m/s2 or F = 30N

Newton’s Third Law Describes action-reaction pairs: When an object exerts a force on a second object, the second one exerts a force on the first that is equal in size and opposite in direction. In other words, for every action force, there is an equal and opposite reaction force. The forces are balanced. It is important to remember that the action-reaction forces always act on different objects.

Example 1

Example 2 Consider the propulsion of a fish through the water. A fish uses its fins to push water backwards. But a push on the water will only serve to accelerate the water. In turn, the water reacts by pushing the fish forwards, propelling the fish through the water.

Example 3 Consider the motion of your automobile to school. An automobile is equipped with wheels which spin backwards. As the wheels spin backwards, they push the road backwards. In turn, the road reacts by pushing the wheels forward.