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

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.

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).

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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

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

22Section CheckSection Check

Answer

The answer is A. When forces are balanced, the net force is zero.

FL: SC.C.2.3.6