# Forces

## by ksearfoss

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## ForcesPresentation Transcript

• Forces
• Definition: push or pull
• Forces have size (magnitude) and direction
• Forces at a distance– gravity, magnetic
• vs.
• Contact forces– friction, spring, etc.
• Newton’s Laws
• Law of Inertia : An object at rest remains at rest unless acted upon by an external force. An object in motion continues to travel at constant speed and direction unless acted on by an external force.
• This is like the law of laziness . An object wants to keep doing what it is doing.
• Law of Inertia example:
• Remember the bumper cars?
• First bumper car travels forward until acted on by force of still car.
• Second bumper car is still until acted on by force of moving bumper car.
• Objects with greater mass have greater inertia.
• It is harder to start a more massive object moving from rest.
• It is harder to stop a more massive object once it is moving.
• Newton’s Laws
• F = ma: The force on an object is equal to the object’s mass times its acceleration.
• acceleration : the change of an object’s velocity with respect to time (i.e.- object is speeding up or slowing down)
• Newton’s Laws
• Combining Law of Inertia and force equation
• The speed of an object will not change (the object will not accelerate) unless acted on by an external force.
• That force is equal to the object’s mass times its resultant change in speed.
• Newton’s Laws
• Action-Reaction: For every action, there is an equal and opposite reaction  forces occur in pairs.
• First car exerts force on second car to start it in motion. Second car exerts equal force on first car to make it stop.
• Working with forces: (bumper car collision)
• Given: First car has mass m 1 and acceleration a 1 after collision. Second car has mass m 2 .
• What is the acceleration (a 2 ) of the second car after the crash?
• Remember:
• Force (F 1 ) of first car on the second = opposite of force (F 2 ) of second car on the first (Newton’s 3 rd Law)
• F 1 = F 2
• m 1 a 1 = m 2 a 2
• m 1 a 1 = m 2 a 2
• m 2 m 2
• a 2 = m 1 a 1
• m 2
So… what does this mean? If the mass of the second car is greater than the mass of the first car, the acceleration of the second car will be (more or less?) than the acceleration of the first.
• Gravitational Force
• Exists between all objects.
• There are attractive forces between all people and objects in this room.
• Gravity increases with the size of the objects.
• Gravity increases as the objects get closer together.
• Mass vs. Weight
• Mass is a measure of the amount of matter in an object. It is the same at every point in the universe.
• Weight is a measure of the amount of gravitational attraction (force) between two objects. It depends on the masses of both objects and their distance apart.
• Using Newton’s 2 nd Law:
• W = m * g
• (F = m * a)
• where W is the weight of an object
• m is the mass of the object
• g is the acceleration of the object due to gravity
• At the surface of the Earth, g = 9.81 m/s 2 .
• The units for weight (force) are Newtons.
• 1 N = 1 kg*m/s 2
• What is your weight in Newtons?
• Assume your mass (m) equals 60 kg.
• The acceleration due to gravity (g) equals 9.81 m/s 2 .
• 1 N = 1 kg*m/s 2
• W = m*g
• W = 60 kg * 9.81 m/s 2 = 590 Newtons
• What is this weight in pounds?
• 1 lb = 4.45 N
590 N 1 lb = 133 lb 4.45 N
• You feel your weight when the force of gravity is resisted by another object.
• (Your chair is keeping gravity from pulling you closer to the Earth.)
• Weight on the moon:
• The mass of the moon = 1/6 of the mass of the Earth
• So the acceleration due to the gravity of the moon = 1/6 of g
• So your weight on the moon = 1/6 of your weight on the Earth
• What is your weight on the moon?
• moon walk video
• The mass of Jupiter is 2.6 times the mass of Earth.
• What would you weigh on Jupiter?
• W = 2.6 x 133 lb = 346 lb
• Tides– caused by forces Tides are caused by water being pulled away from the Earth’s surface by forces.
• gravitational force of the moon The moon’s gravity pulls on the oceans, creating a bulge of water on the side of the Earth closest to the moon.
• The Earth and moon rotate around a point (center of gravity) off of the center of the Earth.
• centrifugal force from Earth’s rotation Centrifugal force due to the rotation of the Earth pulls water away from the center of gravity (opposite side of moon).
•
• So, the gravitational pull of the moon and centrifugal force produce bulges on opposite sides of the Earth.
• spring tides When the gravitational forces of the Sun and the moon pull along the same line, the tides are extra high and low. These extreme tides occur twice each month (new and full moons) and are called spring tides .
• neap tides When the gravitational forces of the Sun and the moon pull at right angles to each other, the tides are not as high or as low. These mild tides occur twice each month (1 st and 3 rd quarter) and are called neap tides .