Newtons 2 nd Law of Motion In chapters

Newton’s 2 nd Law of Motion

In chapters 1 -3 we have discussed the concept of mechanical equilibrium where all the forces were balanced In this chapter we will consider that happens when forces aren’t balanced

Force Causes Acceleration

Most often, the applied force is not the only force acting on an object Recall the combination of forces acting on an object is the net force. Acceleration depends on the net force

To increase the acceleration of an object, you must increase the net force. If you double the net force then the acceleration doubles

An object’s acceleration is directly proportional to the net force acting on it Acceleration ~ ~ net force = directly proportional to Therefore any change in acceleration is the same amount of change in the net force

Friction

Friction The resistive force that opposes the motion or attempted motion of an object either past another object with which it is in contact or through a fluid

Friction usually reduces the net force and the resulting acceleration.

What Causes Friction? Irregularities in the surface The amount of friction depends on The kinds of materials How much the materials are pressed together

Even surfaces that appear very smooth have microscopic irregularities that obstruct motion Atoms cling together at many points of contact

When one object slides against another, it must either rise over the irregular bumps or else scrape atoms off. In either case it requires force

Direction of Friction Is always in a direction opposing motion.

If an object is to move at a constant velocity, a force equal to the opposing force of friction must be applied so that the two forces exactly cancel each other. The zero net force = zero acceleration resulting in a constant velocity.

A Crate Sitting On The Floor On the floor – No friction exists When you start to push the crate – you’ll disturb the contact surfaces and friction is produced. When enough force is applied then the friction force gives way, which makes it possible to slide the crate.

How is the friction of sliding different from the friction at rest? The friction of sliding is somewhat less than the friction that builds up before sliding takes place.

Static Friction vs Sliding Friction Static friction is somewhat greater than sliding friction

Friction doesn’t depend on speed Friction doesn’t depend on the area of contact Friction isn’t restricted to solids alone

Mass & Weight

The acceleration imparted to an object depends not only on the applied forces and friction forces, but also on the inertia of an object

Mass Resists Acceleration

Mass Resists Acceleration

Newton’s 2 nd Law of Motion

Newton was the first to discover the relationship among three basic physical concepts Acceleration Force Mass

Applied in the direction of the object’s motion, force will increase the object’s speed. Applied in the opposite direction, it will decrease the speed of the object. Acceleration of an object is always in the direction of the net force

Force, Mass, & Acceleration How does force cause motion to change? What is the difference between throwing a baseball and tossing one gently? In both examples they start at rest. One has a faster velocity than the other. Which one has a greater acceleration, the baseball thrown or the baseball gently tossed?

Force, Mass, & Acceleration The greater the force applied to an object, the greater the acceleration will be. AKA – the harder the baseball is thrown the faster it will accelerate.

Force & Mass If you throw a cotton ball and a baseball as hard as you can, why don’t they have the same speed? Because of their different masses. The acceleration of an object depends on its mass as well as the force applied to it. Force, mass, and acceleration are all connected. Newton’s Second Law of motion describes this relationship!

Newton’s Second Law of Motion Net force is the combination of all the forces acting on the object. Newton’s Second Law of Motion states: 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.

Newton’s Second Law of Motion In other words… How much an object accelerates depends on the mass of the object and how much force is applied to it. The equation for this is: Force = mass x acceleration F=ma

Newton’s Second Law of Motion Force is measured in Newtons Force = mass x acceleration 1 Newton = kg x m/s 2

Newton’s Second Law of Motion If a baseball is thrown at 150 m/s 2 and its mass is. 50 kg what force is necessary to change its direction? a = 150 m/s 2 m =. 50 kg F=? F = ma F = (. 50 kg)(150 m/s 2) F = 75 kg m = 75 N s 2

When Acceleration Is g – Free Fall

Galileo discovered many scientific principles, including inertia and acceleration. He wasn’t able to explain why objects with different masses fall at the same rate. Sir Issac Newton’s 2 nd law of motion provides an explaination.

When the force of gravity is the only force (when friction, such as air resistance is negligible) we say that the object is in a state of free fall

The acceleration of an object depends not only on the force, but also on the object’s resistance to motion (its inertia)

The ratio of weight to mass for freely falling objects equals constant – g The ratio of weight to mass is the same both for heavy objects and for light objects, just as the ratio of circumference to diameter is the same both for large circles and for small circles.

When Acceleration Is Less Than g – Nonfree Fall

The most important thing to keep in mind is the idea of net force. The situation of falling objects in a vacuum are very different from the experiences you have on a daily basis.

Air Resistance – or drag The force of air drag experienced by a falling object depends on 2 things 1. The frontal area of the falling object – that is the amount of air the object must plow through as it falls 2. Depends on the speed of the falling object – the greater the speed, the greater the number of air molecules an object encounters per second and the greater the force of molecular impact.

Simply put air resistance depends on the size and speed of the falling object.

Write Board Question How does air drag or resistance affect a falling feather?

When acceleration terminates we say that the object has reached its terminal speed What do you think terminal velocity is then?

Terminal Velocity If we are concerned with the direction of the movement as well as acceleration then we say it has reached terminal velocity

On the White Boards What happens as a sky diver falls from an air plane?