# 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 Presentation

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Forces

Section 1: The Nature of ForceWhat is a force?ForceA push or pull on an objectHas both Size & DirectionSize:Measured in SI units called newtons (N) Spring Scale

11 N = about 1/5th pound (lb)How do you combine forces?Direction:Same direction: Add (+)Diff. direction: Subtract (-)Net force = combination of all forces acting on objectUnbalanced ForcesResult in motion

What does balanced forces mean?Balanced Forces Net force = 0No motion

31.Two tugboats are moving a barge. Tugboat A exerts a force of 3000 newtons on the barge. Tugboat B exerts a force of 5000 newtons in the same direction. What is the combined force on the barge? 2.Draw arrows showing the individual and combined forces of the tugboats in #1. 3.Now suppose that Tugboat A exerts a force of 2000 newtons on the barge and Tugboat B exerts a force of 4000 newtons in the opposite direction. What is the combined force on the barge? 4.Draw arrows showing the individual and combined forces of the tugboats in #3.5. Two dogs pulling on a rope (tug of war) 10N in to the right the other dog pulls 12 N to the left. Draw a diagram showing their forces. What is the net force (size & direction)Lab: The Nail Challenge!Objective:Balance nails on single nail headWork in pairs

Section 2: Friction and GravityWhat is friction?FrictionResistance to motion Opposite direction of travelCaused when 2 surfaces rub togetherresistive force (slows down objects)

What does Friction depend on?Friction depends onTypes of surfacesHow hard surfaces push together

6The amount of friction depends on the force pushing the surfaces together. If this force increases, the hills and valleys of the surfaces can come into closer contact.

The close contact increases the friction between the surfaces. Objects that weigh less exert less downward force than objects that weigh more, as shown on the next slide.

What are sliding and rolling friction?Types of FrictionSliding Friction: solid surfaces slide over each otherRolling Friction: object rolls over surface

What are fluid and static friction?Types of FrictionFluid 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 frictionWays to increase friction

What is a gravitational force?Gravitational ForceForce of attraction between 2 objectsPulls things toward each otherDepends on: Mass Distance

10What is the difference between mass and weight?MassAmount of matterSame no matter where you areSI 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 FallOnly force acting on an object is gravityObjects in free fall accelerate as they fallAll objects free fall at the same rate (9.8 m/s2)

12If you drop an elephant and a piece of paper at the same time law of gravity says they should hit the ground at the same time.why dont they? They arent in free fall other forces are acting on them (air friction)- But the elephant has more surface area therefore there will be more air friction acting on it. yes, but this is offset by the much greater weight (more force of gravity acting on it) so it falls faster even if it experiences more air resistance!- DEMONSTRATE drop 2 sheets of paper 1 flat, 1 on its side- Parachutes create lots of air resistance slowing object down.Gravity & Freefall

13Recall Newtons 2nd law of motion what is the formula for calculating the relationship between acceleration, mass and force? Remember that gravity depends on mass (more mass = more gravity force)What is Air resistance?Air resistance Type of fluid frictionOpposes motion of objects through airDepends on: Size, Shape, Speed

What is Terminal Velocity?Terminal VelocityAs an object falls it picks up speedIncreased speed increased air resistanceEventually force of air resistance = force of gravity TERMINAL VELOCITYObject stops accelerating!15If you drop a marble and a piece of paper at the same time law of gravity says they should hit the ground at the same time.why dont they? They arent in free fall other forces are acting on them (air friction)

- Parachutes create lots of air resistance slowing object down. Terminal Velocity

Section 3: Newtons First and Second LawsWhat is Inertia?InertiaTendency of object to resist a change in its motion

17When treadmill stops (you keep going), blood rushes to your head when in elevatorWhat does inertia depend on?Inertia depends on MassAmount of inertia depends on objects mass

18When treadmill stops (you keep going), blood rushes to your head when in elevatorWhat is Newtons 1st Law?Newtons 1st Law of MotionObject at rest will remain at restObject in motion will remain in motion unless acted on by an unbalanced force.

19** Force is NOT needed to keep an object in motion (assuming no friction is resisting motion)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. Newtons 1st Law ReviewUnbalanced force from another car changes your CARs motionYou continue as before until your seatbelt changes YOUR motion

What is Newtons 2nd law of motion? Newtons 2nd law of MotionForce, Mass & Acceleration are related

Force = Mass X Acceleration OR Acceleration = Force Mass

FYI, 1 N = 1kg X 1 m/s2Force, 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 N26 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 N27Section 4: Newtons Third LawWhat is Newtons 3rd Law? Newtons 3rd law of MotionFor every action, there is an equal and opposite reactionAction & Reaction are names of forces

How do forces always occur?Forces ALWAYS occur in pairs.Single forces NEVER happen2 objects are involved in every forceAction force: A pushes BReaction force: B pushes A

29 For every action force, there is a reaction force means: Forces ALWAYS occur in pairs.Single forces NEVER happen

Since a force is an interaction between objects, two objects are involved in every force. Call the objects A and B:Action force: A pushes BReaction force: B pushes A

What do equal and opposite mean?In Newtons Third Law, equal means: Equal in sizeEqual in time

Opposite Means:Opposite in direction

30The action & reaction forces are EXACTLY the same size.

The action & reaction forces occur at EXACTLY the same time.

In Newtons Third Law, opposite means:Opposite in directionThe action and reaction forces are EXACTLY 180o apart in direction. Dont Action & Reaction forces cancel each other?Action & Reaction forces act on DIFFERENT objectsIn Net force problems, we are talking about opposing forces acting on the SAME object

31 Two Logical DifficultiesIf Newtons Third Law action & reaction forces are equal and opposite, how come they dont always cancel, making net force and acceleration impossible?

If the Newtons Third LaOnly forces pushing or pulling on an object affect the objects motion. Only forces that act on the same object can cancel.Newtons Third Law action and reaction forces act on different objects, so they dont cancel. w action and reaction forces are always equal and opposite, how do two objects of different sizes get different accelerations in the same interaction?

(When a bug hits a windshield, different things happen to the bug and windshield.) 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.

When a small bug is splattered across a fast moving windshield what experiences more force- the bug or the windshield?Some what of a tricky question, they both experience the SAME force.Why does the force have a greater effect on the bug?Because the bug's mass is much much smaller than the car's, it will experience a much greater change in acceleration than the car. This change in acceleration over a very small fraction of time is why the bug experiences a greater effect.

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