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What are Forces?. A brief Introduction to Physics. Newton. Newton had three really important Laws Inertia: All objects at rest or in motion tend to stay at rest or in motion unless acted upon by force.
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What are Forces? A brief Introduction to Physics
Newton • Newton had three really important Laws • Inertia: All objects at rest or in motion tend to stay at rest or in motion unless acted upon by force. • The sum (∑) of all Net forces (Vector sum) on an object is = mass times the acceleration of that object. • For Every Action there is an equal and opposite reaction
Newton’s Second Law of Motion • Net Force = mass X acceleration • ∑F = ma • Force and acceleration are directly proportional. The greater the force, the greater the acceleration. • Mass and acceleration are inversely related. The greater the mass of an object the less the acceleration if the same force is applied. • Think about a truck trying to accelerate versus a car.
The Third Law: • For every reaction there is an equal and opposite reaction. • This means that there has to be a reaction force when I press on the door. • More importantly I press more than the door can the door breaks.
Free Body Diagram • We express the forces on an object with a Free Body Force Diagram. • The vectors representing forces on an object always come from the center of mass.
Force Diagram Practice • What is the force diagram of a box that is being pushed (neglecting friction)? • What is the force diagram of a box being dropped (neglecting air resistance)? • What is the force diagram of a box being pushed across the floor with friction
Cool balanced forces • Balance Video! • http://youtu.be/6aB8I63JFB8
Equilibrium • Equilibrium is a nice way of saying that all the forces are balanced. So lets look around the room and see what we can see where the forces are balanced? • What does this mean about moving objects at a constant speed? • What does this mean about accelerating objects?
What is friction? • Friction is a force which opposes motion. • Friction is an important force in our lives. We rely on friction in many ways. An athlete usually wears shoes which provide him or her with a greater friction between the shoe and the surface. We rely on friction as an important aspect of our motion. In what other ways does friction play a role in our everyday lives?
What causes friction? • There are two factors which affect friction between two surfaces: • Kind of surfaces in contact (rough or smooth) • Amount of force pressing the surfaces together. The rougher the surface and the stronger the force between the surfaces, the greater the amount of friction.
From Glencoe Physical Science What is static friction? • If you were trying to move a box like pictured below and the box did not move, this would be static friction. • Static friction is friction between two surfaces which are not moving past each other.
Glencoe Physical Science What is sliding friction? • Sliding friction occurs when a force is great enough to overcome the static friction. • What is rolling friction? • Rolling friction is the friction which enables wheels to turn and objects to move. If there were no friction, turning wheels would not enable an object to move.
Air Resistance – another type of friction?? • Air resistance is a force which opposes a moving object. This can be easily seen with falling objects. When a piece of paper and a crumple piece of the same type of paper are dropped, they fall at the same rate. • A penny and a feather dropped in air will not fall at the same rate, but a penny and feather dropped in a vacuum (no air) will.
What is gravitational acceleration? • When objects fall they accelerate toward the ground. • Using Newton’s 2nd Law we can see this relationship: F = ma • The force caused by gravity acts upon a certain mass to cause it to accelerate toward the ground at a constant rate. • Acceleration due to gravity is 9.8 m/s2
Gravity causes weight • The weight of an object is caused by the force of attraction between the earth and objects on the surface of the earth. Gravitation force is equal to weight, therefore…
How much does a person with a mass of 70.0 kg weigh on the earth? Weight = mass X 9.8 m/s2 Weight = 70.0 kg X 9.8 m/s2 Weight = 686 N Remember that 1 N = (1 kg) (1m/s2)
What is weight? • When an astronaut moves away from the earth they “lose” weight. Weight is caused by the force of attraction between the earth and objects on its surface. The greater the distance an object has from the earth, the less the force of attraction which would exist.
What is the difference between mass and weight? • Mass is the amount of matter in an object. This does not change in an object as it moves away from the earth. • Weight is caused by gravity. Therefore, the greater the distance, the less the force of attraction. The less the distance the greater the force of attraction. • Your weight is dependent upon the mass of the earth. If you were able to go to the planet, Jupiter, your weight would be 27 time greater, because the mass of Jupiter is 27 times greater.
What causes “weightlessness”? • Even in the space shuttle, there is a force of attraction exerted by the earth on the shuttle and its contents. • When the space shuttle temporarily “falls” toward the earth, the contents of the space shuttle appear to be weightless, but in fact they are falling with the space shuttle. This is called “freefall”.
What is Newton’s 3rd Law of Motion • For every action there is an equal but opposite reaction. • When one 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. • Action-Reaction Law
Two objects in contact with one another and moving must use the action-reaction law. Rocket propulsion involves the action-reaction law. The explosion from one end of the rocket causes a force in that direction. This causes an unbalanced force in the opposite direction causing the rocket to accelerate.
Single Block This Can be expressed as an equation too.
Block on a table f>Fa Moving Fa
