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. How do you combine forces?. Direction: Same direction: Add (+) Diff. direction: Subtract (-) - PowerPoint PPT PresentationTRANSCRIPT
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
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
What does balanced forces mean?
Balanced Forces Net force = 0No motion
Lab: The Nail Challenge!Objective:
Balance nails on single nail head
Work in pairs
Section 2: Friction and GravityWhat is friction?
FrictionResistance to motion Opposite direction of travelCaused when 2 surfaces rub together
resistive force (slows down objects)
What does Friction depend on?Friction depends on…
Types of surfacesHow hard surfaces push together
What are sliding and rolling friction?Types of Friction
Sliding Friction: solid surfaces slide over each other
Rolling Friction: object rolls over surface
What are fluid and static friction?Types of Friction
Fluid Friction: object moves through fluid (or air)
Static Friction: objects not moving
What are some uses for friction?Is Friction harmful or helpful?
Ways to reduce friction
Ways to increase friction
What is a gravitational force?
Gravitational ForceForce of attraction between 2 objects
Pulls things toward each other
Depends on: Mass Distance
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
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)
Gravity & Freefall
What is Air resistance?Air resistance
Type of fluid frictionOpposes motion of objects through air
Depends on: Size, Shape, Speed
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!
Terminal Velocity
Section 3: Newton’s First and Second LawsWhat is Inertia?
InertiaTendency of object to resist a change in it’s motion
What does inertia depend on?Inertia depends on Mass“Amount” of inertia depends on objects’ mass
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.
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.
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)
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.
Newton’s 1st Law ReviewUnbalanced
force from another car changes your CAR’s motion
You continue as before until your seatbelt changes YOUR motion
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
Force, Mass & Acceleration
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
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
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
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”
What do equal and opposite mean?In Newton’s Third Law, “equal” means: Equal in sizeEqual in time
“Opposite” Means:Opposite in direction
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
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.
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.