forces. what is force? a force is a push or a pull one body exerts on another body. sometimes it is...
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Forces
What is force?
• A force is a push or a pull one body exerts on another body.
• Sometimes it is obvious a force has been applied, but other forces are not as noticeable.
• Forces can cause a change in motion.
Balanced forces
• When two or more forces act on an object at the same time, the forces combine to form the net force.
• Forces on an object that are equal in size and opposite in direction are called balanced forces.
BALANCED FORCES
• Balanced forces do not cause acceleration. (any change in speed or direction)
Unbalanced forces
• Unbalanced forces cause a change in motion.
• Unbalanced forces cause an object to start moving, stop moving, or change direction. (an unbalanced force will cause an object to accelerate)
Unbalanced forces
• The forces acting on an object may not always be opposite in direction.
• The net force that acts on this box is found by adding the two forces together.
Force DiagramsIn what direction and with how much
force would the object travel?
60 N 30 N
40 N 40 N
20 N 20 N
100 N
50 N
Types of Forces
• A force can be either balanced or unbalanced.
• Common types of force include gravity, magnetism, friction, air resistance, or centripetal force
Friction
• Suppose you give a skateboard a push with your hand.
• According to Newton’s 1st law of motion, if the net force acting on an object is zero, it will continue to move in a straight line with constant speed.
• Does the skateboard keep moving forward with a constant speed after it leaves you hand?
Friction
• Recall that when an object slows down it is accelerating.
• By Newton’s 2nd law of motion, if the skateboard is accelerating, there must be a net force acting on it.
• The force that stops the skateboard is friction.
Friction• Friction is the force that opposes motion between two
surfaces that are touching each other.• The amount of friction depends on; 1: kinds of surfaces 2: the force pressing the surfaces together
Types of FRICTION
• Sliding friction- dry solid surfaces
• Rolling friction- rolling over a surface
• Fluid friction- object moves through a liquid or gas.
Gravity
• Gravity’s attractive force depends on 1. the distance
between the two objects
2. the mass of the two object
Every object exerts a gravitational force on every other object
Gravity
• Isaac Newton formulated the law of universal gravitation, which he published in 1687.
• His law states that gravitational force=
(constant) x (mass 1)x(mass 2) = G m1m2
(distance)2 d2
• According to the law of universal gravitation, the gravitational force between two masses decreases rapidly as the distance between the masses increases.
GRAVITY
• Free fall- when the only force acting on a falling object is gravity. (9.8m/s/s)
• Do all objects regardless of mass fall at this rate?
• What about projectiles?
GRAVITY
• Air resistance- a type of fluid friction that objects falling through air experience.
• Paper vs.. Book• Weight- The measure of the
force of gravity on an object (scale)
• Mass- is a measure of the amount of matter in that object.
Weight
• Weight and mass are NOT the same.
• Weight is a force and mass is a measure of the amount of matter an object contains.
• Weight and mass are related. Weight increases as mass increases.
Weight on different planets
• The weight of an object usually is the gravitational force between the object and Earth.
• The weight of an object can change, depending on the gravitational force on the object.
Air resistance
• When an object falls towards Earth, it is pulled downward by a force called gravity.
• However, a friction-like force called air resistance opposes the motion of objects that move through air.
• Air resistance causes objects to fall with different accelerations and different speeds.
Air resistance
• The amount of air resistance on an object depends on the speed, size, and shape of the object.
• Air resistance, not the objects mass, is the reason feathers and leaves fall slower then pennies or apples.
What about circular motion???
• Bucket Demonstration
• What is the centripetal force for a satellite?
• Centripetal- “center seeking”
• Centripetal force- any force that causes an object to move in a circle.
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