Forces can be thought of as pushes and pulls. They are an interaction between two objects- you can touched without being touched. Here are some of the forces in these interactions and how we identify them.
Gravity: exerts a force on you (pull) which holds you to the surface of the earth and occurs when forces are extended through a force field
Friction: forces parallel to the surface (pull)
-Weight affects the friction by determining how gravity on an object can move.
- Velocity affects the surface type (coefficient).
- friction is calculated by the formula: mk=coefficient
Normal Force: when two surfaces touch, the force perpendicular to the surfaces are called normal forces. It also keeps an object balanced and from going through the ground.
Tension: extended links or material such as string or chain exert tension forces on an object (push)
Push or Pull: When we touch something, a combination of friction and normal forces are present, put for simplicity we refer to them as push of pull forces.
Non-contact forces: When two objects interact, they exert forces through a force field. For example, there is a gravitational force on the earth and the moon, even though they do not touch. Aside from gravitational, electric and magnetic are also non-contact forces.
Direction of Forces:
Gravity always points down towards the earth. The normal force is perpendicular to the surface and the frictional force is parallel to the surface.
Rules of Labeling Forces:
When you label forces, we want to include the type of interaction between the two objects, what object the force is acting on, and what object the force is by. You would use the notation of a large capital F followed by the kind. Here is an example of how the push force would be written.
Fpush
Normal force is always perpendicular to gravity and friction is always parallel to gravity. The force of weight or gravity is always going down. Tension is only drawn when on a rope, extended link, or chain.
Axis:
The axes should be drawn titled when the object is on an angle. The forces would be the same, the lines would just be titled on a slant. If the axis is not tilted, you don't need to add components.
Rules for Action-Reaction Pairs:
Every action has an equal and opposite reactions.The identification of these pairs are two statements stating who is pushing on who and the direction.
Understanding and Deconstructing the Force Diagram:
STEP ONE: Shrink the shape of the object down to a point. Place it at an intersection of a set of coordinate axes with the direction of the axes parallel to the direction of motion.
STEP TWO: Indicate all points at which there is contact between the object and its surroundings. Construct qualitative vectors (vector: a quantity having direction as well as magnitude, especially as determining the position of one point in space relative to another) to represent these forces.
STEP THREE: Indicate which forces are equal in magnitude to other forces. Keep in mind the forces left must equal the forces right and the forces down must equal the forces up. Uses hashes to indicate which forces are equal and congruent to one another.
STEP FOUR: Represent all relevant forces with a labeled vector. Remember the rules of labeling forces.
Newton's Law of Physics; How is this applicable to our unit?
Newton's First Law: An object either remains at rest or continues to move at a constant velocity, unless acted upon by an external force.[
Newton's Third Law: When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.
Activity: Bowling Ball Motion and Hovercrafts
What did we learn?
In result of Newton's first law, if the hovercraft or bowling ball was moving at constant velocity, no forces are acting upon it. The object in motion will stay in motion until an external force acts upon it. Once the person pushed the hovercraft and lost contact with it, no force was acting upon the hovercraft. The hovercraft kept moving until the person on the other side of the gym stopped it (external force).
Understanding Weight:
In order to find the weight of an object, you will use the formula:
W=mg
The M represents the mass (kg) and the G represents the gravitational constant (9.8m), which we round to 10. When you solve for the weight, the units will be Newtons. The mass measures the amount of matter and weight is a measure of the pull of gravity.
Example Problems:
1. Q:You are driving down the road and you have a cup of coffee without a lid. You come to a stop and your coffee spills all over you. Using an FBD, explain why the coffee went all over you.
A: Newton's first law acts in this situation. It states that an object in motion stays in motion. When the car stops, the coffee was still moving forward, therefore when the car brakes, the cup comes to a stop which causes the liquid to spill. Without the lid, there is no force to push against the coffee to keep it from spilling.
No force keeps the ball moving. The only force acting upon the ball is gravity. I learned in the hovercraft experiment, that once an object is moving there is no force keeping it in motion. This also is true because of Newton's first law: an object in motion stays in motion. (The diagram on the right is 2.) and diagram on the left is 3.).
4.) Use FBD's to show how a team can win a tug of war. Explain your reasoning.
The diagram below represents Marcy (in black), who is losing the tug of war competition. The determining factor in her loss is friction. Newton's third law is applied here because each side is pulling and receives an equal and opposite force. Therefore the pull/tension is equal.
Debra (gray) is winning because she has greater friction. The more friction the team has, the less Marcy will be able to pull on her. (Note: the only force that is different is the friction, which is greater in the diagram below. The pull is equal) (Microsoft would not let me draw uneven arrows)
5.) How do seatbelts keep you safe? Use Newton's 1st law in your answer.
Seatbelts provide an unbalanced force in the vertical direction, which slows the car down when you keep moving forward. You keep moving forward because of Newton's 1st law (an object in motion stays in motion). The seat belt is the external force that stops the motion of your body.
6.) Solve for the horizontal and vertical components of tension for the fishing line.
Decoding Triangles:
The component in the FBD are always the hypotenuse when trying to solve for the horizontal/ vertical components. The opposite it opposite the angle. The adjacent is next to the hypotenuse and the angle you are solving for. In order to solve, you will use SOH CAH TOA. Below is the equation determining whether you use Sin, Cosine, or Tangent.
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