Chapter 13 The Nature of Forces Forces Friction Inertia Newton’s Laws Gravity Air Resistance

Section 1 - Forces Force  a push or pull that one body exerts on another that can make an object start or stop moving, OR change its speed or direction.  What forces are being exerted on the football? F kick F grav F friction F grav = Force of Gravity F kick = Pushing force of the kicker’s foot F friction = molecules of air “rubbing” against the surface of the football – air resistance!

Balanced Forces  forces acting on an object that are opposite in direction and equal in size  no change in velocity  no movement  the upward force is called the normal force Section 1 - Forces weight (force) normal force

Unbalanced Forces  Forces that cause a change in motion (these forces can be in the same direction OR opposite directions.  velocity changes (object accelerates) Section 1 - Forces F friction F net F pull

Balanced & Unbalanced Forces  Draw the resulting force arrow for each of the 3 force combinations shown to the left!!! Section 1 - Forces = = =

Section 2 - Friction Friction  force that opposes (restricts or ”fights”) motion between 2 surfaces  The amount of friction depends on the: types of surfaces force between the surfaces

Friction is greater...  between rough surfaces  when there’s a greater force between the surfaces (e.g. more weight) Section 2 - Friction

3 main types of friction:  Sliding  Rolling  Fluid What are some Pros and Cons of friction? Section 2 - Friction Decreasing Frictional Force

3 main types of friction:  Sliding – when solid objects slide past or over each other  Rolling – friction produced by solid objects such as wheels or ball bearings.  Fluid – friction produced when an object moves through a fluid such as air or water. Section 2 - Friction

How do people decrease the force of friction?  We use lubricants. Def: any substance that changes sliding friction to fluid friction. Ex: motor oil, grease,

Section 3 - Newton’s First Law Newton’s First Law of Motion  An object at rest will remain at rest and an object in motion will remain in motion at a constant velocity unless acted upon by an outside force.

Section 3 - Newton’s First Law Newton’s First Law of Motion  “Law of Inertia” Inertia  tendency of an object to resist any change in its motion (either resting or in motion)  increases as mass increases

Section 3 - Newton’s First Law Restated: An object will maintain its speed and direction (velocity) or LACK of velocity (remain motionless) unless an outside force changes it.

Test Your Understanding You are a passenger in a car and not wearing your seat belt. Without increasing or decreasing its speed, the car makes a sharp left turn, and you find yourself colliding with the right-hand door. Which is the correct analysis of the situation?

Test Your Understanding 1. Before and after the turn, there is a rightward force pushing you into the door. 2. Starting at the time of collision, the door exerts a leftward force on you. 3. Both of the above 4. Neither of the above 2. Starting at the time of the turn, the door exerts a leftward force on you.

Section 3 - Newton’s Second Law Section 3 - Newton’s Second Law Newton’s Second Law of Motion  Newton’s Second Law shows how force, mass, and acceleration are related. F = ma

Section 3 - Newton’s Second Law Section 3 - Newton’s Second Law F = ma F:force (N) m:mass (kg) a:accel (m/s 2 ) 1 N = 1 kg ·m/s 2 m F a

Force Calculations What force would be required to accelerate a 40 kg mass by 4 m/s 2 ? GIVEN: F = ? m = 40 kg a = 4 m/s 2 WORK : F = ma F = (40 kg)(4 m/s 2 ) F = 160 N m F a

Force Calculations Mr. Miller weighs 557 N. What is his mass? GIVEN: F(W) = 557 N m = ? a( g ) = 9.8 m/s 2 SHOW YOUR WORK : m = F ÷ a m = (557 N) ÷ (9.8 m/s 2 ) m = 56.8 kg m F a

Force Calculations A 4.0 kg shotput is thrown with 30 N of force. What is its acceleration? GIVEN: m = 4.0 kg F = 30 N a = ? SHOW YOUR WORK : a = F ÷ m a = (30 N) ÷ (4.0 kg) a = 7.5 m/s 2 m F a

Section 3 - Newton’s Third Law Section 3 - Newton’s Third Law Newton’s Third Law of Motion  When one object exerts a force on a second object, the second object exerts an equal but opposite force on the first.

Section 3 - Newton’s Third Law Section 3 - Newton’s Third Law Simply put…. Every action has an equal and opposite reaction.

Section 3 - Newton’s Third Law Section 3 - Newton’s Third Law How do the arrows indicate how the forces are acting in the picture?

Section 3 - Newton’s Third Law Section 3 - Newton’s Third Law Explanation: The force of the water (blue arrows) is counterclockwise. The resulting force (red arrows) against the sprinkler head is in the opposite direction – clockwise. This is why the sprinkler spins!!

Section 3 - Newton’s Third Law Section 3 - Newton’s Third Law Action-Reaction Pairs The hammer exerts a force on the nail to the right. The nail exerts an equal but opposite force on the hammer to the left. However, even though the nail exerts as much force as it can, it is not as large as the hammer’s force and the net result is the nail moving toward the right.

Section 4 - Gravity Gravity  force of attraction between any two objects in the universe  increases as... mass increases distance decreases G = m 1 m 2 d 2

Section 4 - Gravity Who experiences more gravity - the astronaut or the politician? less distance more mass Which exerts more gravity - the Earth or the moon?

Section 4 - Gravity Weight  the force of gravity on an object W = mg W:weight (N) m:mass (kg) g:acceleration due to gravity (m/s 2 ) WEIGHT depends on gravity (N or g x m/s 2 ) MASS always the same (kg or g)

Section 4 - Gravity Would you weigh more on Earth or Jupiter? Therefore it has greater gravity And you would have greater weight Jupiter has a greater mass  Jupiter because...

Test Your Understanding Is the following statement true or false?  An astronaut has less mass on the moon since the moon exerts a weaker gravitational force.  False! Mass does not depend on gravity, weight does. The astronaut has less weight on the moon.

Section 4 - Gravity Acceleration due to gravity (g)  In the absence of air resistance, all falling objects have the same acceleration!  On Earth: g = 9.8 m/s 2 elephant feather Animation from “Multimedia Physics Studios.”Multimedia Physics Studios

Section 4 - Air Resistance Air Resistance  “fluid friction” or “drag”  force that air exerts on a moving object to oppose its motion  depends on: speed of object surface area of object shape of object density of fluid

Section 4 - Air Resistance Terminal Velocity  maximum velocity reached by a falling object  reached when… F grav = F air F air F grav  no net force  no acceleration  constant velocity

Section 4 - Terminal Velocity

Section 4 - Air Resistance Terminal Velocity  increasing speed  increasing air resistance until… F air = F grav Animation from “Multimedia Physics Studios.”Multimedia Physics Studios

Section 4 - Air Resistance Falling with air resistance F grav = F air Animation from “Multimedia Physics Studios.”Multimedia Physics Studios  heavier objects fall faster because they accelerate to higher speeds before reaching terminal velocity  larger F grav  need larger F air  need higher speed

JET CAR CHALLENGE CHALLENGE: Construct a car that will travel as far as possible (at least 3 meters) using only the following materials. scissors tape 4 plastic lids 2 skewers 2 straws 1 balloon 1 tray How do each of Newton’s Laws apply?