Science Chapter 6 Lesson 3

Science Chapter 6 Lesson 3

Newton's Laws of Motion

Newton's First Law of Motion

Newton's First Law of Motion states that an object at rest will stay at rest unless acted upon by a nonzero net force. An object moving at a constant velocity will continue moving at a constant velocity unless acted upon by a nonzero net force.

What does that mean?

- An object at rest will stay at rest and an object in motion will stay in motion UNLESS a nonzero net force acts on it

Inertia

• Resistance to change in motion is called inertia

• Newton's first law is also called the law of inertia

• Some objects have more inertia than others

• The greater the mass of an object, the greater the inertia, and the greater the force required to change its motion

Newton's Second Law of Motion

Newton's Second Law of Motion states that an object's acceleration depends on its mass and on the net force acting on it

What does this mean?

- The speed and direction (velocity) of an object depends on its mass and on how strong a force is acting on it

Acceleration

Newton found a way to represent the second law of motion mathematically.

Acceleration = Net Force/Mass

Therefore,

Net Force = Mass x Acceleration

Acceleration

- Acceleration is measured in meters per second per second

- Mass is measured in kilograms (kg)

Force is thus measured in kilograms times meters per second per second (kg)(m/s²)

- This unit is also called the newton (N), which is the SI unit of force

Multiple Choice

Using the formula for Acceleration (A=NF/M), find the acceleration for the problem: A 100N net force acts on a 50kg car. What is its acceleration?

• 2 m/s²

• 5000 m/s²

• 1/2 m/s²

Newton's Third Law of Motion

Newton's Third Law of Motion states that if one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object

What does this mean?

- For every action, there is an equal but opposite reaction

Example of Newton's Third Law

When you walk, you push backward on the ground with your feet. Think of this as an action force. The ground pushes forward on your feet with an equal and opposite force. This is the reaction force.

You can only walk because the ground pushes you forward.

Detecting Motion

• Since Earth has such a large mass, its acceleration is so small that you don't notice it

• If you drop a pen, for example, the pen pulls the earth upward with an equal and opposite reaction force. You see the pen fall. You don't see Earth accelerate toward the pen.

Do Action-Reaction Forces Cancel?

Action and reaction forces do not cancel out because they act on different objects

Example: A swimmer exerts a backward action force on the water. The water exerts an equal but opposite forward reaction force on her hands. The action force acts on the water and reaction force acts on the swimmer's hands. Therefore, the action and reaction forces are acting on different objects and do not cancel out.