FORCES AND LAWS OF MOTION

FORCE (push) (pull)

Examples of forces: ContactField Pulling the handle of the door Pushing a stroller Hitting a tennis ball with a racquet Bouncing a soccer ball Force of gravity Attraction of negative and positive particles Magnetism

Free body diagram

Aristotle 384 – 322 BC

ISAAC NEWTON

1) A hockey puck is sliding on a frictionless ice. 2) A hockey puck is sliding on a normal ice (with friction)

An object continues in a state of rest or in a state of motion at a constant speed along a straight line, unless compelled to change that state by a net force. The net force is the vector sum of all of the forces acting on an object. Newton ’ s First Law

Inertia is the natural tendency of an object to remain at rest in motion at a constant speed along a straight line. The mass of an object is a quantitative measure of inertia. SI Unit of Mass: kilogram (kg)

An object is resting on a flat surface An object is being pulled on a flat surface, but is not moving The very same diagram would illustrate an object that is already in motion and keeps moving at a constant speed. There are two forces applied to the object: force of gravity, directed downward, and the force from the surface. They are equal in magnitude and opposite in direction, so Force of pull is equal to force of friction.

Net Force

Newton’s II Law When a net external force acts on an object with mass m, the acceleration that results is directly proportional to the net force and has a magnitude that is inversely proportional to the mass. The direction of the acceleration is the same as the direction of the force.

[NEWTON]= [N] [N] = [kg ·m/s 2 ] A net force of 1N exerted on an object with a mass of 1 kg will result in the object moving with an acceleration of 1 m/s 2

Example Space-shuttle astronauts experience accelerations of about 35 m/s 2 during takeoff. What force does a 75 kg astronaut experience during an acceleration of this magnitude?

Example A block with a mass of 5 kg is being pulled across a tabletop by a force of 10 N applied by a string tied to the front end of the block. The table exerts a 2-N frictional force on the block. What is the acceleration of the block?

Example A 7.5 kg bowling ball initially at rest is dropped from the top of an 11m building. It hits the ground 1.5s later. Find the net external force on the falling ball (it’s not free-fall!).

Newton’s Third Law When one body exerts a force on a second body, the second body exerts an oppositely directed force of equal magnitude on the first body. Every action has an equal opposite reaction.

3 rd Law: action-reaction

Mule - Cart If a mule is pulling on a cart with a force F and there is a counter force –F, according to Newton’s third law, why does the cart start moving ?

Educated apprentice

Compare forces ON the object Newton’s III law is about TWO objects and TWO forces that the TWO exert on EACH OTHER. Newton’s I (and II) law is about ONE object and ALL the forces exerted ON IT ALONE. So, when working on problems and drawing FBD, isolate appropriate forces to determine the nature of motion.

Newton’s Law of Gravitation Another example of N’s III law: two bodies, two equal and opposite forces.

Example Two particles are located 1.2m apart. One has a mass of 12 kg, the other has a mass of 25 kg. What is the magnitude of the gravitational force that acts on each particle?

Inverse proportion Magnitude of gravity decreases as the distance to the center of the Earth (or any planet / object) increases. Thus, up in the mountains force of gravity is slightly less than that at or below sea level.

Weight of an object The weight of an object on or above earth is the gravitational force that the earth exerts on the object. On or above another astronomical body, the weight is the gravitational force exerted on the object by that body. Weight = mg, where m- mass of the body, g – acceleration due to gravity (-9.8 m/s 2 )

Relation Between Mass and Weight Weight depends on the strength of gravitational pull and it changes with distance to the center of the Earth; it is different on different planets (depends on the mass and size)

Mass vs. weight Mass depends on the amount of matter (number and mass of particles that make up the object. Mass can change if the number of particles changes. Weight depends on the strength of gravitational pull and it changes with distance to the center of the Earth; it is different on different planets (depends on the mass and size)

Weight can Change:

When can an object be weightless?

Normal Force Normal force is the component of the force that a surface exerts on the object, always to the surface (normal is not ‘natural!’.

Examples of Normal force Normal force = weight only if an object is at rest on a flat surface. Any additional force or motion with acceleration can change (increase or decrease) normal force. Normal force is the reaction of the surface to the force applied to it by the object.

Examples of Normal force

An object is being pulled at a constant speed on a flat surface An object is being pulled with a force that is making an angle with the horizontal. The object is not moving

An object is being pulled with a force that is making an angle with the horizontal. The object is moving with a constant velocity An object is being pulled with a force that is making an angle with the horizontal. The object is not moving

Compare the two normal forces Remember this – you’ll see it again when you study friction!

Examples of Normal force A box with a mass of 10.0 kg is being pulled by a string at an angle of 30.0 degrees to the horizontal with a force of 40.0 N. Neglecting friction find: a) the acceleration of the box b) the magnitude of the normal force

GivenUnitsEquationsSubstitution/Solution

Example A 10.0-kg box is resting on a 20.0 o incline. Determine the normal force acting on the incline. Find the force of friction acting on the incline. If the incline is frictionless, find the acceleration of the box.

Force of friction When an object is in contact with a surface there is a force acting on that object. The component of this force that is parallel to the surface is called the frictional force.

Static friction When the two surfaces are not sliding across one another the friction is called static friction. Static friction is a ‘lazy’ force: it will only be as large as it needs to. Static friction has a maximum value. If pull >>, the object will move

Note that the magnitude of the frictional force does not depend on the contact area of the surfaces. It depends on the roughness / smoothness of the two surfaces in contact.

Static friction opposes the impending relative motion between two objects. Kinetic friction opposes the relative sliding motion motions that actually does occur. is called the coefficient of kinetic friction.

The sled comes to a halt because the kinetic frictional force opposes its motion and causes the sled to slow down. Suppose the coefficient of kinetic friction is 0.05 and the total mass is 40kg. What is the kinetic frictional force? What will be the acceleration (deceleration) of the sled? How far will the sled travel before coming to a halt?

GivenUnitsEquations / SubstitutionSolution