section 1: the nature of force what is a force?

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Section 1: The Nature of Force What is a force? Force A push or pull on an object Has both Size & Direction Size: Measured in SI units called newtons (N) Spring Scale

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Section 1: The Nature of Force What is a force?. Force A push or pull on an object Has both Size & Direction Size: Measured in SI units called newtons (N) Spring Scale. How do you combine forces?. Direction: Same direction: Add (+) Diff. direction: Subtract (-) - PowerPoint PPT Presentation

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Page 1: Section 1: The Nature of Force What is a force?

Section 1: The Nature of ForceWhat is a force?

ForceA push or pull on an objectHas both Size & Direction

Size:Measured in SI units called newtons (N)

Spring Scale

Page 2: Section 1: The Nature of Force What is a force?

How do you combine forces?

Direction:Same direction: Add (+)Diff. direction: Subtract (-)Net force = combination of all forces acting on object

Unbalanced ForcesResult in motion

Page 3: Section 1: The Nature of Force What is a force?

What does balanced forces mean?

Balanced Forces Net force = 0No motion

Page 4: Section 1: The Nature of Force What is a force?

Lab: The Nail Challenge!Objective:

Balance nails on single nail head

Work in pairs

Page 5: Section 1: The Nature of Force What is a force?

Section 2: Friction and GravityWhat is friction?

FrictionResistance to motion Opposite direction of travelCaused when 2 surfaces rub together

resistive force (slows down objects)

Page 6: Section 1: The Nature of Force What is a force?

What does Friction depend on?Friction depends on…

Types of surfacesHow hard surfaces push together

Page 7: Section 1: The Nature of Force What is a force?

What are sliding and rolling friction?Types of Friction

Sliding Friction: solid surfaces slide over each other

Rolling Friction: object rolls over surface

Page 8: Section 1: The Nature of Force What is a force?

What are fluid and static friction?Types of Friction

Fluid Friction: object moves through fluid (or air)

Static Friction: objects not moving

Page 9: Section 1: The Nature of Force What is a force?

What are some uses for friction?Is Friction harmful or helpful?

Ways to reduce friction

Ways to increase friction

Page 10: Section 1: The Nature of Force What is a force?

What is a gravitational force?

Gravitational ForceForce of attraction between 2 objects

Pulls things toward each other

Depends on: Mass Distance

Page 11: Section 1: The Nature of Force What is a force?

What is the difference between mass and weight?Mass

Amount of matterSame no matter where you are

SI units = kilograms (kg)1 kg = 1000 grams (g)

Weight Force of gravitySI units = newtons (N)Depends on where you are

Page 12: Section 1: The Nature of Force What is a force?

What is Free fall?Free Fall

Only force acting on an object is gravity

Objects in free fall accelerate as they fall

All objects free fall at the same rate (9.8 m/s2)

Page 13: Section 1: The Nature of Force What is a force?

Gravity & Freefall

Page 14: Section 1: The Nature of Force What is a force?

What is Air resistance?Air resistance

Type of fluid frictionOpposes motion of objects through air

Depends on: Size, Shape, Speed

Page 15: Section 1: The Nature of Force What is a force?

What is Terminal Velocity?Terminal Velocity

As an object falls it picks up speed

Increased speed increased air resistance

Eventually force of air resistance = force of gravity TERMINAL VELOCITYObject stops accelerating!

Page 16: Section 1: The Nature of Force What is a force?

Terminal Velocity

Page 17: Section 1: The Nature of Force What is a force?

Section 3: Newton’s First and Second LawsWhat is Inertia?

InertiaTendency of object to resist a change in it’s motion

Page 18: Section 1: The Nature of Force What is a force?

What does inertia depend on?Inertia depends on Mass“Amount” of inertia depends on objects’ mass

Page 19: Section 1: The Nature of Force What is a force?

What is Newton’s 1st Law?Newton’s 1st Law of Motion

Object at rest will remain at rest

Object in motion will remain in motion unless acted on by an unbalanced force.

Page 20: Section 1: The Nature of Force What is a force?

Practice Problem 1 Imagine a place in the cosmos far from all gravitational and frictional influences. Suppose an astronaut in that place throws a rock. The rock will:a) gradually stop.b) continue in motion in the same direction at constant speed.

Page 21: Section 1: The Nature of Force What is a force?

Practice Problem 2 An 2-kg object is moving horizontally with a speed of 4 m/s. How much net force is required to keep the object moving with the same speed and in the same direction?

0 N (no force)

Page 22: Section 1: The Nature of Force What is a force?

Practice Problem 3 Ben Tooclose is being chased

through the woods by a bull moose which he was attempting to photograph. The enormous mass of the bull moose is extremely intimidating. Yet, if Ben makes a zigzag pattern through the woods, he will be able to use the large mass of the moose to his own advantage. Explain this in terms of inertia and Newton's first law of motion.

Page 23: Section 1: The Nature of Force What is a force?

Newton’s 1st Law ReviewUnbalanced

force from another car changes your CAR’s motion

You continue as before until your seatbelt changes YOUR motion

Page 24: Section 1: The Nature of Force What is a force?

What is Newton’s 2nd law of motion? Newton’s 2nd law of Motion

Force, Mass & Acceleration are related

Force = Mass X Acceleration OR Acceleration = Force ÷Mass

FYI, 1 N = 1kg X 1 m/s2

Page 25: Section 1: The Nature of Force What is a force?

Force, Mass & Acceleration

Page 26: Section 1: The Nature of Force What is a force?

Force, Mass & AccelerationA 52 kg water skier is being pulled by a speedboat. The force causes her to accelerate @ 2 m/s2. Calculate the FORCE that causes this acceleration.F = 52 kg x 2 m/s2

= 104 kg x m/s2 = 104 kg*m/s2

= 104 N

Page 27: Section 1: The Nature of Force What is a force?

Force, Mass & AccelerationWhat is the force on a 1000 kg elevator accelerating at 2 m/s2?1000 kg X 2 m/s2 = 2000 N

How much force is needed to accelerate a 55 kg cart at 15 m/s2?55 kg X 15m/s2 = 825 N

Page 28: Section 1: The Nature of Force What is a force?

Section 4: Newton’s Third LawWhat is Newton’s 3rd Law?

Newton’s 3rd law of MotionFor every action, there is an equal and opposite reaction

“Action” & “Reaction” are names of forces

Page 29: Section 1: The Nature of Force What is a force?

How do forces always occur?Forces ALWAYS occur in

pairs.Single forces NEVER happen

2 objects are involved in every forceAction force: “A pushes B”Reaction force: “B pushes A”

Page 30: Section 1: The Nature of Force What is a force?

What do equal and opposite mean?In Newton’s Third Law, “equal” means: Equal in sizeEqual in time

“Opposite” Means:Opposite in direction

Page 31: Section 1: The Nature of Force What is a force?

Don’t Action & Reaction forces cancel each other?

Action & Reaction forces act on DIFFERENT objects

In “Net force” problems, we are talking about opposing forces acting on the SAME object

Page 32: Section 1: The Nature of Force What is a force?

Try These!!

If forces are equal and opposite why don't they cancel each other out?

They occur on two different objects.  Forces can only cancel out when the forces are acting on the same object.

Page 33: Section 1: The Nature of Force What is a force?

If the forces are equal and opposite how do two different objects obtain different accelerations in the same interaction? (Remember F=ma)

Different accelerations are obtained when the objects have different masses.