Physics C-5. Objects at rest tend to stay at rest. Objects in motion tend to stay in motion. Also called the Law of Inertia.

Newtons Laws: Force and Motion

Physics C-5

Objects at rest tend to stay at rest.

Objects in motion tend to stay in motion.

Also called the Law of Inertia.

5.1 Newtons First Law

5.1 Force Force is an action that can change

motion.◦ A force is a push or a pull.

Change is not required, just the ability. (push on car)

◦ Forces can be used to increase the speed, decrease the speed, or change the direction in which an object is moving.

5.1 Inertia Inertia is a term used to measure the

ability of an object to resist a change in its state of motion.

Large inertia = large force req. to move Small inertia = small force req. to move

5.1 Newton’s law of inertia Because inertia is a key idea in Newton’s

first law, the first law is sometimes referred to as the law of inertia.

The word inertia comes from the Latin word inertus, which can be translated to mean “lazy.”

Force Force is the cause of acceleration of an

object or a change in motion. We typically imagine a force as a “push”

or “pull. SI unit of Force: Newton (N)

◦Newton = mass x acceleration◦1N = 1kg • 1m/s2

External Force – a single force that acts on an object as a result of an interaction between the object and its environment.

Types of Force Contact Force – force that arises from the

physical contact of two objects.◦ you push a cart to move it forward◦ pull on a wagon◦ Friction◦ Air resistance

Field Force – force that can exist between objects, even in the absence of physical contact between objects. (force at a distance)◦ Gravity◦ Attraction or repulsion between electrical or magnetic

charges

Force Symbol Type/Definition Direction

Friction Ff Contact force – acts to oppose sliding motion between surfaces

Parallel to the surface and opposite the direction of sliding

Normal FN Contact force – exerted by a surface on an object

Perpendicular to and away from the surface

Spring Fsp Contact force – push or pull a spring exerts on an object

Opposite the displacement of the object at the end of the spring

AirResistance

Fair Contact force – frictional force which acts upon objects as they travel through the air.

Opposite to the force of gravity. * frequently neglected due to its negligible magnitude (and due to the fact that it is mathematically difficult to predict its value).

Force Symbol

Type/Definition Direction

Tension FT Contact force – pull exerted by a string, rope or cable when attached to a body and pulled taut.

Away from the object and parallel to the string, rope, or cable at the point of attachment

Thrust Fthrust Contact force – forces that move objects such as rockets, planes, cars, and people

In the same direction as the acceleration of the object barring any resistive forces

Weight Fg Field Force – the magnitude of the force of gravity acting on an object

Straight down toward the center of the Earth

5.1 Newton's First Law

Can you explain why the long table would make the trick hard to do?

The engine◦ supplies force that allows you to change motion

by pressing the gas pedal. The brake system

◦ is designed to help you change your motion by slowing down.

The steering wheel and steering system◦ is designed to help you change your motion by

changing your direction.

How do these systems in a car overcome the law of inertia?

Newton’s First Law• An object at rest remains at rest and an object in

motion continues in motion with constant velocity unless it is acted upon by an unbalanced force.

• The tendency of an object to not accelerate or resist change is inertia.oInertia is directly proportional to an

object’s mass. • greater mass = less acceleration• smaller mass = more acceleration

• The unbalanced force is when there is an individual force which is not being balanced by a force of equal magnitude and in the opposite direction.

1st Law Cont…

Net External Force – the total force resulting from a combination of external forces on an object.

If the net external force on an object is zero, then the acceleration (or change in its motion) is zero and the object is in equilibrium.

Equilibrium – the state of a body in which there is no change in its motion.

5.2 Newton's Second Law

If you apply more force to an object, it accelerates at a higher rate.

5.2 Newton's Second Law

If the same force is applied to an object with greater mass, the object accelerates at a slower rate because mass adds inertia.

5.2 The definition of force

The simplest concept of force is a push or a pull.

On a deeper level, force is the action that has the ability to create or change motion.

5.2 The definition of force

In the English system, the unit of force, the pound, was originally defined by gravity.

The metric definition of force depends on the acceleration per unit of mass.

5.2 Newton's Second Law

A force of one newton is exactly the amount of force needed to cause a mass of one kilogram to accelerate at one m/s2.

We call the unit of force the newton (N).

5.2 Using the second law of motion The force F that appears in the second law

is the net force. There are often many forces acting on the

same object. Acceleration results from the combined

action of all the forces that act on an object. When used this way, the word net means

“total.”

5.2 Converting newtons and pounds

A force of one pound is equal to about 4.448 newtons.

5.2 Using the second law of motion To solve problems with multiple forces, you

have to add up all the forces to get a single net force before you can calculate any resulting acceleration.

1. You are asked for the acceleration (a).2. You are given mass (m) and force (F).3. Newton’s second law applies: a = F ÷ m4. Plug in numbers. (Remember: 1 N = 1 kg·m/s2)

Calculating acceleration

A cart rolls down a ramp. Using a spring scale, you measure a net force of 2 newtons pulling the car down. The cart has a mass of 500 grams (0.5 kg). Calculate the acceleration of the cart.

5.2 Newton's Second Law

Three forms of the second law:

5.2 Finding the acceleration of moving objects

The word dynamics refers to problems involving motion.

In dynamics problems, the second law is often used to calculate the acceleration of an object when you know the force and mass.

5.2 Direction of acceleration

Speed increases when the net force is in the same direction as the motion.

Speed decreases when the net force is in the opposite direction as the motion.

5.2 Positive and negative acceleration

We often use positive and negative numbers to show the direction of force and acceleration.

A common choice is to make velocity, force, and acceleration positive when they point to the right.

1. You are asked for the acceleration (a) and direction

2. You are given the forces (F) and mass (m).

3. The second law relates acceleration to force and mass: a = F ÷ m

4. Assign positive and negative directions. Calculate the net force then use the second law to determine the acceleration from the net force and the mass.

Acceleration from multiple forces

Three people are pulling on a wagon applying forces of 100 N, 150 N, and 200 N. Determine the acceleration and the direction the wagon moves. The wagon has a mass of 25 kilograms.

5.2 Finding force from acceleration Wherever there is acceleration there must

also be force. Any change in the motion of an object

results from acceleration. Therefore, any change in motion must be

caused by force.

1. You asked for the force (F).2. You are given the mass (m) and acceleration (a).3. The second law applies: a = F ÷ m4. Substitute. (Remember: 1 N = 1 kg·m/s2.)5. F = 5,000kg x 5 m/s2 = 25,000 N

Calculating force

An airplane needs to accelerate at 5 m/sec2 to reach take-off speed before reaching the end of the runway. The mass of the airplane is 5,000 kilograms. How much force is needed from the engine?

Calculating force

A tennis ball contacts the racquet for much less than one second. High-speed photographs show that the speed of the ball changes from -30 to +30 m/sec in 0.006 seconds. If the mass of the ball is 0.2 kg, how much force is applied by the racquet? (hint: Calc acceleration first ---(a=vf-vo / t)

Mass and weight are not the same thing.

Mass is measured in kg and is the same everywhere in the universe.

Weight is a force which depends on gravity (9.8 N/kg on earth)

F = ma or W = mg

Weight is still a force

5.2 Equilibrium

The condition of zero acceleration is called equilibrium.

In equilibrium, all forces cancel out leaving zero net force.

Objects that are standing still are in equilibrium because their acceleration is zero.

5.2 Equilibrium Objects that are moving at

constant speed and direction are also in equilibrium.

A statics problem usually means there is no motion.

Suppose you are walking two dogs which pull on the leash with 80 N each. How much force do you have to use to keep the dogs from moving?____________

5.3 The Third Law: Action and Reaction

“For every action there is an equal and opposite reaction.”

This statement is known as Newton’s third law of motion.

Newton’s third law discusses pairs of objects and the interactions between them.

5.3 Forces occur in pairs

The astronauts working on the space station have a serious problem when they need to move around in space: There is nothing to push on.

One solution is to throw something opposite the direction you want to move.

5.3 Forces occur in pairs

The two forces in a pair are called action and reaction.

Anytime you have one, you also have the other.

If you know the strength of one you also know the strength of the other since both forces are always equal.

5.3 Newton's Third Law

Newton’s third law states that for every action force there has to be a reaction force that is equal in strength and opposite in direction.

Action and reaction forces act on different objects, not on the same object.

5.3 Newton's Third Law

Newton’s third law states that for every action force there has to be a reaction force that is equal in strength and opposite in direction.

Action and reaction forces act on different objects, not on the same object.

The forces cannot cancel because they act on different objects.

Calculating force

Three people are each applying 250 newtons of force to try to move a heavy cart. The people are standing on a rug. Someone nearby notices that the rug is slipping. How much force must be applied to the rug to keep it from slipping? Sketch the action and reaction forces acting between the people and the cart and between the people and the rug.

5.3 Locomotion

The act of moving or the ability to move from one place to another is called locomotion.

Any animal or machine that moves depends on Newton’s third law to get around.

When we walk, we push off the ground and move forward because of the ground pushing back on us in the opposite direction.

5.3 Locomotion

Jets, planes, and helicopters push air.

In a helicopter, the blades of the propeller are angled such that when they spin, they push the air molecules down.