chapter: forces and changes in motion table of contents section 3: the laws of motionthe laws of...
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Chapter: Forces and Changes in Motion
Table of ContentsTable of Contents
Section 3: The Laws of Motion
Section 1: Motion
Section 2: Forces and Motion
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Forces and MotionForces and Motion
22Force
• A force is a push or a pull that one object exerts on another object.
• Objects like floors, chairs, and Earth also exert forces on other objects, including people.
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Forces and MotionForces and Motion
22Force Has Direction and Size
• Just like velocity and acceleration, a force has both size and direction.
• The direction of a force is the direction of the push or pull.
• Pushing or pulling harder increases the size of the force you exert. The size of a force is measured in newtons (N).
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Forces and MotionForces and Motion
22How Forces Combine
• When more than one force acts on an object, the forces combine.
• The combination of all the forces acting on an object is the net force.
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Forces and MotionForces and Motion
22Combining Forces in the Same Direction
• When two forces act in the same direction on an object, like a box, the net force is equal to the sum of the two forces.
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Forces and MotionForces and Motion
22Combining Forces in Opposite Directions
• If two forces of equal strength act on the box in opposite directions, the forces will cancel, resulting in a net force of zero.
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Forces and MotionForces and Motion
22Combining Forces in Opposite Directions
• When two unequal forces act in opposite directions on the box, the net force is the difference of the two forces.
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Forces and MotionForces and Motion
22Balanced and Unbalanced Forces
• When the net force on an object is zero, the motion of the object doesn’t change.
• The forces acting on an object are balanced forces if the net force is zero.
• The forces acting on an object are unbalanced forces if the net force is not zero.
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Forces and MotionForces and Motion
22Contact and Non-contact Forces
• A force is exerted when one object pushes or pulls on another.
• A force that is exerted only when two objects are touching is a contact force.
• Non-contact forces are forces that can be exerted by one object on another even when the objects aren’t touching.
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Forces and MotionForces and Motion
22Gravity
• Gravity is a non-contact force that every object exerts on every other object due to their masses.
• The gravitational force between two objects is an attractive force that tends to pull the two objects closer together.
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Forces and MotionForces and Motion
22Gravity
• The size of the gravitational force between two objects depends on the masses of the objects and the distance between them.
• The gravitational force between two objects increases if the mass of one or both of the objects increases.
• If two objects move closer together, the gravitational force between them increases.
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Forces and MotionForces and Motion
22Mass and Weight
• Because weight is a force, it is measured in newtons.
• Weight is not the same as mass. Mass is the amount of matter an object contains, and is measured in kilograms.
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Forces and MotionForces and Motion
22Friction
• Friction is a contact force that resists the sliding motion of two surfaces that are touching.
• Friction causes a sliding object to slow down and stop.
• Friction also can prevent surfaces from sliding past each other.
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Forces and MotionForces and Motion
22Static Friction
• Static friction is the force between two surfaces in contact that keeps them from sliding when a force is applied.
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Forces and MotionForces and Motion
22Sliding Friction
• Sliding friction exists between all sliding surfaces that are touching.
• Sliding friction is the force that opposes the motion of twosliding surfaces in contact.
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Forces and MotionForces and Motion
22What causes friction?
• When two surfaces are in contact, the surfaces stick to each other where the dips and bumps on one surface touch the dips and bumps on the other surface.
• Friction is caused by the sticking of the two surfaces at these bumps and dips.
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Forces and MotionForces and Motion
22The Buoyant Force
• The buoyant force is a force exerted by a fluid on an object that is in the fluid.
• The buoyant force is always upward.• If you are
floating in water, the buoyant force is large enough to balance your weight.
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Forces and MotionForces and Motion
22Air Resistance
• Air resistance is a contact force that opposes the motion of objects moving in air.
• Just like friction, air resistance acts in the direction opposite to an object’s motion.
• Air resistance is less for a narrow, pointed object than for a wide, flat object.
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22Section CheckSection Check
Question 1
A force is a _______ or a _______.
Force is a push or a pull.
Answer
FL: SC.C.2.3.2
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22Section CheckSection Check
Question 2
A force exerted between two objects that are not touching is known as _______.
The answer is a long-range force. Gravity is a long-range force; although we are not touching the Sun, our planet is being acted on by the Sun to keep it in orbit.
Answer
FL: SC.C.2.3.1
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22Section CheckSection Check
Question 3You try to push a desk across the floor one direction but a friend is trying to push it the opposite direction. As a result, the desk doesn’t move. This is an example of what kind of forces?
A. balanced forcesB. continuing forcesC. extended forcesD. unbalanced forces
FL: SC.C.2.3.6
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22Section CheckSection Check
Answer
The answer is A. When forces are balanced, the net force is zero.
FL: SC.C.2.3.6