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Chapter 12

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Force: a push or pull that acts on an object Key Point: a force can cause a resting object to move or it can accelerate a moving object by changing it’s speed or direction.

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Forces are measures in Newtons (N) Can use an arrow to represent the direction and strength of a force The length represents the strength or magnitude of the force

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Forces in the same direction add together Forces in opposite directions are subtracted The net force is the overall force once the forces are combined A. Adding B. Subtracting = =

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BALANCED FORCESUNBALANCED FORCES Combine to produce a net force of zero Key Point: When the forces are balanced the net force is zero, no change in the motion Ex. Beginning of arm wrestling Result is a net force that is not equal to zero Key Point: When an unbalanced force acts on an object the object accelerates Ex. Winning team of tug of war

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All moving objects are subject to friction Friction is the force that opposes motion Acts at the surface where objects are in contact

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1. Static Friction Acts on objects not moving 2. Sliding Friction Opposes motion as an object slides over a surface 3. Rolling Friction When rolling the object and the floor are slightly bend out of shape Acts on objects rolling 4. Fluid Friction Opposes the motion of an object in a fluid Gases or liquids

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Force that acts between any two masses Is an attractive force, it pulls objects together Key Point: Earth gravity acts downward toward the center of the Earth

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Two forces affect falling objects 1. Gravity 2. Air resistance Gravity causes objects to accelerate downward Air resistance acts in the direction opposite to the motion and reduces acceleration Larger surface area the more air resistance

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The constant velocity of a falling object, when the force or air resistance equals the force of gravity

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A thrown object follows a curves path This curved path is projectile motion Key Point: the combination of an initial forward velocity and the downward vertical force of gravity causes the curved path

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Newton’s First Law The state of motion of an object does not change as long as the net force acting on the object is zero. Unless an unbalanced force acts an object at rest remains at rest An object in motion remains in motion with the same speed and direction unless an unbalanced force acts on it

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Inertia – the tendency of an object to resist the change in motion Examples 1. Soccer Ball– remains at rest unless someone kicks it 2. Car collision – people move forward until the seat belt stops them

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An unbalanced force causes an objects velocity to change so the object accelerates Acceleration is directly proportional to the net force acting on it As force increase acceleration increases

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Acceleration also depends on the mass Mass – measure of the inertia of an object and depends on the amount of matter Acceleration depends on force and mass Key Point: The acceleration of an object is equal to the net force divided by the mass

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Acceleration = force/mass Force = mass x acceleration Mass = force/acceleration

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1. A car with a mass of 200 kg accelerates when a traffic light turns green. If the net force on the car is 8000 N, what is the car’s acceleration? 2. A car accelerates at a rate of 30 m/s 2 and its mass is 1375 kg. What is the force acting on the car? 3. A 29 – N force accelerates a boy on a skateboard at 0.4 m/s 2. What was the mass of the boy?

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WEIGHTMASS Measure of the force of gravity Measure of the amount of inertia Amount of matter

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Weight = mass x acceleration due to gravity (9.8) Fg = ma 1. An astronaut has a mass of 40 kg. What would his mass be on earth?

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Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. “Action/Reaction”

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1. Swimming 2. Hammer and Nail

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A product of an object’s mass and velocity Key Point: An object has a large momentum if the product of it’s mass and velocity is large.

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If no net force acts on a systems, then the total momentum does not change Key Point: In a closed system, the loss of momentum in one equals the gain of momentum in another.

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