Do NowLook around the room and think about the objects you see in terms of force. A force is always exerted by one object on another object. Then, answer the following questions in your science journal:

Where do you see a force happening in the room right now? Which object is exerting the force, and which is receiving it?

Do NowWhat is a force?

How does a force produce motion?

What does a net force of zero mean?

Write your answers in your science journal:

Big Picture

PS2.A: Forces and Motion The motion of an object is determined by the

sum of the forces acting on it; if the total force on the object is not zero, its motion will change. The greater the mass of the object, the greater the force needed to achieve the same change in motion. For any given object, a larger force causes a larger change in motion. (MS-PS2-2)

Objectives• Describe forces, and explain how forces act on objects.

• Determine the net force when more than one force is acting on an object.

• Compare balanced and unbalanced forces.

• Describe ways that unbalanced forces cause changes in motion.

•Explain friction.

Agenda

Today we will:Day 1:Review homeworkLearn about forces by taking notes from

teacherDay 2:Group and individual work calculating forces

Vocabulary

forcenet forceunbalanced forcefriction

Forces Acting on Objects

• In science, a force is simply a push or a pull exerted on an object in order to change the motion of the object. All forces have both size and direction.

• A force can change the acceleration of an object. This acceleration can be a change in the speed or direction of the object.

• Scientists express force using a unit called the newton (N).

Forces Acting on Objects, continued

• Unseen Sources and Receivers of Forces It is not always easy to tell what is exerting a force or what is receiving a force. For example, you cannot see what exerts the force that pulls magnets to refrigerators.

• You cannot see that the air around you is held near Earth’s surface by a force called gravity.

Determining Net Force

• Usually, more than one force is acting on an object. The net force is the combination all of the forces acting on an object.

• Determining net force depends on the directions of the forces.

Determining Net Force, continued

• Forces in the Same Direction Two forces are added to determine the net force if the forces act in the same direction. The net force will be in the same direction as the individual forces.

• Forces in Different Directions If forces are acting in opposite directions, the net force can be found by subtracting the smaller force from the larger one.

Balanced and Unbalanced Forces

• Balanced Forces When the forces on an object produce a net force of 0 N, the forces are balanced.

• Balanced forces will not cause a change in the motion of a moving object. Balanced forces do not cause a nonmoving object to start moving.

Forces that oppose motion

• Friction is a force that opposes motion between two surfaces that are in contact.

• Friction occurs because the surface of any object is rough. Even surfaces that feel smooth are covered with microscopic hills and valleys.

The Source of Friction

• When two surfaces are in contact, the microscopic hills and valleys of one surface stick to the tiny hills and valleys of the other surface. This contact causes friction.

The Source of Friction

• The Effect of Force on Friction The amount of friction depends on the force pushing the surfaces together. If this force increases, the hills and valleys of the surfaces can come into closer contact.

• The close contact increases the friction between the surfaces. Objects that weigh less exert less downward force than objects that weigh more, as shown on the next slide.

Types of Friction

• Kinetic Friction The word kinetic means “moving.” So, kinetic friction is friction between moving surfaces.

• The amount of kinetic friction between two surfaces depends in part on how the surfaces move. Surfaces can slide past each other, or a surface can roll over another surface.

• Static Friction When a force is applied to an object but does not cause the object to move, static friction occurs.

• The word static means “not moving.” The object does not move because the force of static friction balances the force applied.

• Static friction disappears as soon as an object starts moving, and then kinetic friction immediately occurs.

Friction: Harmful and Helpful

•Some Ways to Reduce Friction One way to reduce friction is to use lubricants. Lubricants are substances that are applied to surfaces to reduce the friction between the surfaces.• Some examples of common lubricants are motor oil, wax, and grease. Lubricants are usually liquids, but they can be solids or gases.• Another way to reduce friction is to make surfaces that rub against each other smoother.

• Some Ways to Increase Friction Making surfaces rougher is one way to increase friction. For example, sand scattered on icy roads keeps cars from skidding.

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