Newton’s Laws The concept of force is simply a push or pull. This idea is made more quantitative with units of newtons and pounds. Students sometimes consider the force due to gravity and weight to be two different forces. This lesson reinforces the fact that weight IS the force due to gravity. The force of gravity is related to but distinct from the mass of an object.

Explaining motion mass force Isaac Newton was the first person to explain WHY objects move the way they do. He based his laws of motion on two key concepts: mass force Point out to students that in chapters 2 and 3 they learned how to describe motion (using physics vocabulary like displacement, velocity, and acceleration). In chapter 4 they will begin the work of learning how to explain WHY objects move they way they do. This can be likened to the difference in biology between the study of taxonomy and science of genetics.

What is mass? All matter has mass and takes up space. A solid rock has mass. So do gases and liquids. With your hand out the window of a moving car, you feel the mass in the air pushing against you. These next two slides review the concept of mass, which was introduced in Chapter 2.

Types of forces The concept of force: A force is a push or a pull. Forces can cause an object to change its motion. Can you give some examples of forces? Examples of forces: weight friction tension from rope force from a spring electric force Students may also mention atomic forces, magnetic forces, gravity, air resistance, buoyancy, etc. People can also apply forces by pushing or pulling on objects.

Units of force Quantitative: Force is measured in newtons. When students learn Newton’s second law, F = ma, it will provide a useful tool for remembering the definition of a Newton: 1 N = (1 kg) (1 m/s2).

Newton’s first law The first law has two parts. The second part is not so obvious. the second part of Newton’s first law is a conceptual hurdle. Students believe that a force is required to keep an object in motion, and will often invent such forces where they do not exist. Free-body diagrams are excellent for revealing these errors.

Understanding the first law Motion only changes through the action of a net force. If the net force is zero – there can be no changes in motion. This slide summarizes the key idea of the lesson.

The law of inertia Newton’s first law is also known as the law of inertia. Inertia is the tendency of an object to resist changes in the speed or direction of its motion. The inertia of an object is related to its mass. The more massive an object is, the more inertia it has, and the more it will resist having its motion changed. This slide summarizes the key idea of the lesson.

The meaning of the second law Net force (N) Acceleration(m/s2) Mass (kg) Velocity must change if a net force acts on an object. According to the second law, a net force on an object causes it to accelerate. If an object accelerates, its velocity must change. The second law is introduced.

This does NOT necessarily mean there is no motion! The meaning of the second law Net force (N) Acceleration(m/s2) Mass (kg) The net force is zero on an object with constant velocity. If velocity stays constant, then the acceleration is zero—so the net force must also be zero. This does NOT necessarily mean there is no motion! The second law is introduced.

Acceleration and force are vectors. Direction of force and acceleration Acceleration and force are vectors. If the student knows the direction of the net force on an object, he/she knows that the object must also have an acceleration IN THAT DIRECTION.

Units The second law can help you remember the definition of a newton. Always use mass in kilograms and acceleration in m/s2 when applying Newton’s second law.

Applying Newton’s second law If you know the force on an object, you can predict changes in its motion. If you know the acceleration of an object, you can determine the net force on it. Point out that knowing the forces on an object allows you to predict its motion. AND the reverse is also true: the motion of an object can help you figure out the forces acting on it. The next set of slides will show students how to go in both directions.

Finding motion from forces If you know the force on an object, you can predict changes in its motion. A 0.25 kg ball is traveling 40 m/s to the right when it is hit with a force of 3,000 N for 0.005 seconds. What is its final velocity? If you know the force on an object, then you can predict changes in its motion.

Steps A 0.25 kg ball is traveling 40 m/s to the right when it is hit with a force of 3,000 N for 0.005 seconds. What is its final velocity? Use force and mass to find acceleration through the second law. Use the acceleration to find the change in velocity or position.

Solution Impacts can cause very large accelerations for short times! A 0.25 kg ball is traveling 40 m/s to the right when it is hit with a force of 3,000 N for 0.005 seconds. What is its final velocity? Use force and mass to find acceleration through the second law. Impacts can cause very large accelerations for short times!

Solution The ball reverses direction! A 0.25 kg ball is traveling 40 m/s to the right when it is hit with a force of 3,000 N for 0.005 seconds. What is its final velocity? Use force and mass to find acceleration through the second law. Use the acceleration to find the change in velocity or position. Point out to the students that acceleration is the key variable that lets them link the force on the ball to its velocity. The ball reverses direction!

Action and reaction forces Forces always come in pairs. If one object puts a force on a second object, the second object always puts an equal and opposite force back on the first object. Newton’s third law: For every action (force) there is always an equal and opposite reaction (force). Newton’s third law is a law of interactions between objects. Reaction Point out that to understand the motion of a SINGLE object, the student doesn’t need the third law. We need the third law to understand how objects put forces on each other. We need the third law when we need to understand more than one object.

There are always two objects Action: racquet pushes on ball. Reaction: ball pushes back on racquet. Action-reaction forces always act on different objects: One force acts on the racquet. Its partner force acts on the ball.

Free-body diagrams Action: racquet pushes on ball. Reaction: ball pushes back on racquet. Only one of these forces appears on an object’s free-body diagram: the force that acts ON the object. Additional forces may act on each of these objects. Understanding and properly sketching these action/reaction forces is just PART of the process of creating the free-body diagram for the racket or the ball.

Force pairs don’t cancel out Action-reaction pairs don’t cancel out because they always act on different objects: One force acts on the racquet. Its partner force acts on the ball. Only forces that act on the same object can cancel each other out. Racquet pushes on ball. Ball pushes back on racquet.