Chapter 5 – Newton’s Second Law of Motion Sections 5.1 to 5.4 Physics 1 Honors Chapter 5 – Newton’s Second Law of Motion Sections 5.1 to 5.4

What causes acceleration? An object at rest stays at rest until a force acts on it. Once in motion it will continue to move with the same velocity until a force acts upon it. A force must be applied to cause an acceleration.

Net Force Often there is more than one force acting on an object. Therefore, acceleration is caused by a net force. Acceleration ~ net force ~ means “is directly proportional to”

Direct Proportions When two values are directly proportional, as one value increases, so does the other value. Example – mass and volume – as the mass increases, so does the volume. The proportionality between the two is called density. For a given material, density is constant no matter how large the sample is.

Direct Proportions

Mass Resists Acceleration Push on a chair Push on a chair with Joe in it. Which accelerates more? Why?

Mass Resists Acceleration For a given force, the mass is inversely proportional to the acceleration When two values are inversely proportional, as one value increases, the other value decreases. Or as mass increase (for the same force) acceleration decreases.

Inverse Proportion

Newton’s Second Law The acceleration produced by a net force on an object is directly proportional to the magnitude of the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object.

Units Force – Newtons Mass – kilograms Newton= kg x m/sec2 Force/mass = m/sec2 (acceleration)

Newton’s Second Law What happens if you triple the force, with the same mass? What happens if you triple the mass, with the same force? What happens if you triple the force and triple the mass?

Example What would be the force necessary to accelerate a 1700 kg car to an acceleration of 20 m/sec2?

Friction Affects motion Must have 2 surfaces in contact. Always acts in the direction opposite motion. Depends on the kind of materials in contact.

Friction Give an example of 2 surfaces with high friction. Give an example of 2 surfaces with low friction.

Friction Not limited to just motion of solids. Also occurs in liquids and gases. Both liquids and gases are fluids – they flow.

Fluid Friction Occurs when an object pushes aside (displaces) the fluid it is moving through Try running in a pool.

Air Resistance This is friction acting on something moving through air. Put your hand out the window of a moving car.

Friction If an object is moving at constant velocity and there is friction, what needs to happen to make this occur? What is the net force in this situation?

Free-body Diagram Shows all the forces acting on an object