# Chapter: Forces and Changes in Motion

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Chapter: Forces and Changes in Motion
Table of Contents Chapter: Forces and Changes in Motion Section 1: Motion Section 2: Forces and Motion Section 3: The Laws of Motion

Forces and Motion 2 Force 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.

Force Has Direction and Size
Forces and Motion 2 Force 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).

Forces and Motion 2 How 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.

Combining Forces in the Same Direction
Forces and Motion 2 Combining 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.

Combining Forces in Opposite Directions
Forces and Motion 2 Combining 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.

Combining Forces in Opposite Directions
Forces and Motion 2 Combining 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.

Balanced and Unbalanced Forces
Forces and Motion 2 Balanced 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.

Contact and Non-contact Forces
Forces and Motion 2 Contact 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.

Forces and Motion 2 Gravity 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.

Forces and Motion 2 Gravity 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.

Forces and Motion 2 Mass 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.

Forces and Motion 2 Friction 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.

Forces and Motion 2 Static Friction Static friction is the force between two surfaces in contact that keeps them from sliding when a force is applied.

Forces and Motion 2 Sliding Friction Sliding friction is the force that opposes the motion of two sliding surfaces in contact. Sliding friction exists between all sliding surfaces that are touching.

Forces and Motion 2 What 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.

Forces and Motion 2 The 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.

Forces and Motion 2 Air 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.

Question 1 Answer 2 A force is a _______ or a _______.
Section Check 2 Question 1 A force is a _______ or a _______. Answer Force is a push or a pull. FL: SC.C.2.3.2

Section Check 2 Question 2 A force exerted between two objects that are not touching is known as _______. Answer 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. FL: SC.C.2.3.1

Section Check 2 Question 3 You 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 forces B. continuing forces C. extended forces D. unbalanced forces FL: SC.C.2.3.6

Section Check 2 Answer The answer is A. When forces are balanced, the net force is zero. FL: SC.C.2.3.6