Friction Friction is a force that resists the

Friction • • Friction is a force that resists the motion of objects or surfaces. Many kinds of friction exist.

Friction • • Friction depends on both of the surfaces in contact. When the hockey puck slides on ice, a thin layer of water between the rubber and the ice allows the puck to slide easily.

Identifying friction forces • • Friction is a force, measured in newtons just like any other force. Static friction keeps an object at rest from moving.

Identifying friction forces • Sliding friction is a force that resists the motion of an object moving across a surface.

A model for friction • The greater the force squeezing two surfaces together, the greater the friction force.

Reducing the force of friction • • Unless a force is constantly applied, friction will slow all motion to a stop eventually. It is impossible to completely get rid of friction, but it can be reduced.

Reducing the force of friction • • The friction between a shaft (the long pole in the picture) and an outer part of a machine produces a lot of heat. Friction can be reduced by placing ball bearings between the shaft and the outer part.

Useful friction • • Friction is also important to anyone driving a car. Grooved tire treads allow space for water to be channeled away from the road-tire contac point, allowing for more friction in wet conditions.

Useful friction • Shoes are designed to increase the friction between their soles and the ground. Why do you think these shoes increase friction? • Players wearing cleats can apply much greater forces against the ground to help them move and to keep from slipping.

FRICTIONAL FRICTIO FORCES

Free body diagram: A book lying on a level table

Free body diagram: A person floating in still water

Free body diagram: A wrecking ball hanging vertically from a cable

Free body diagram: A helicopter hovering in place

3 Free Body Diagrams: Move R, accel R Move R, no accel L Move R, accel

Newton’s 3 Laws of Motion

Force changes motion • A force is a push or pull, or any action that is able to change motion.

Law of inertia • • • Newton’s first law says that objects continue the motion they already have unless they are acted on by a net force. If the net force is zero, an object at rest will stay at rest. If an object is acted upon by unbalanced forces, its motion will change.

Force changes motion • Forces can be used to increase or decrease the speed of an object, or to change the direction an object is moving.

Law of inertia • • Inertia is the property of an object that resists changes in motion. Objects with more mass have more inertia and are more resistant to changes in their motion. Which ball has more inertia?

The net force • • Newton’s first law is often written in terms of the net force: “An object at rest will stay at rest and an object in motion will continue in motion at constant velocity UNLESS acted upon by an unbalanced force. ”

In other words : Newton’s 1 st Law a. k. a. law of inertia • Every object continues in a state of rest (or motion in a straight line at a constant speed) unless it is compelled to change by forces exerted on it.

Describe the magic card. Would a coarse sandpaper work too? • How about the table cloth pull? • (what had more resistance to change (inertia)…the full beaker or empty beaker? ) MASS IS A MEASURE OF INERTIA MASS RESISTS ACCELERATION

Crash-test action figure • How do seat belts work? • Why are infant seats rear facing? • What happens? • Do air bags deploy when hit from the front/back or both? MASS RESISTS ACCELERATION (+ OR -)

• How does inertia account for removing the dirt from your shoes before coming into the house? Inertia Ketch-up from a bottle? Flying forward when hit a curb?

Newton’s Second Law links force, acceleration and mass

Newton’s Second Law links force, acceleration and mass • The acceleration produced by a net force on an object – is directly proportional to the net force, – is in the same direction as the net force, and – is inversely proportional to the mass of the object. F = ma acceleration = Force / mass A F M

Newtons are the SI units of Force

Gravity and weight • • A 10 -kilogram rock has a mass of 10 kilograms no matter where it is in the universe. A 10 -kilogram rock’s weight however, can vary greatly depending on where it is.

Newton’s Second Law links force, acceleration and mass • In the picture – cars already in motion… • So will continue in Motion. • Force is NOT needed to keep an object in motion. • Force Causes Acceleration Imagine NO friction – How far will these cars travel?

2 nd Law explains why… • Objects in free fall have equal acceleration • i. e. plastic and steel ball experiment • Less with air drag • When air drag cancels out • weight, speed becomes constant (acceleration = 0) • (i. e. terminal speed) • Directional (downward) motion with constant speed = terminal velocity

Forces always come in matched pairs • Newton’s Third Law (action-reaction) applies when a force is placed on any object, such as a basketball.

The Third Law: Action/Reaction • Newton’s Third Law states that every action force creates a reaction force that is equal in strength and opposite in direction. • There can never be a single force, alone, without its action-reaction partner.

The Third Law: Action/Reaction • • Here, one force acts on the ball, and the other force acts on the hand. It doesn’t matter which force you call the action and which the reaction. The forces do not cancel because we can only cancel forces acting on the same object.

Action and reaction • When sorting out action and reaction forces it is helpful to examine or draw diagrams. Here, one force acts on the ________, and the other force acts on the ________.

Action and reaction forces Below are some guidelines to help you sort out action and reaction forces: 1. Both are always present whenever any force appears. 2. They always have the exact same strength. 3. They always act in opposite directions. 4. They always act on different objects. 5. Both are real forces and can cause changes in motion.

Action Shuttle pushes on gas Woman pulls on spring Tire pushes on road Earth pulls on ball / Reaction Gas pushes on shuttle Spring pulls on woman Road pushes on tire Ball pulls on Earth

Collisions • • Newton’s third law tells us that any time two objects hit each other, they exert equal and opposite forces on each other. However, the effect of the force is not always the same. THE LAW OF MOMENTUM GOVERNS THESE COLLISIONS

Collisions • When a large truck hits a small car, the forces are equal. • However, the small car experiences a much greater change in velocity much more rapidly than the big truck. Which vehicle ends up with more damage?

Momentum – mass in motion • The mass of an object multiplied by its velocity • p=mv p - momentum m - mass v- velocity What has momentum? Slow moving bus? Bullet? Big Elephant eating from a tree?

Combining the 2 nd and 3 rd Laws 2 nd Law F=ma 3 rd Law F 1 = - F 2 Collision – both objects experience forces =/opposite Moving club head strikes a golf ball (at rest). Is the force =/opposite? m 1 v 1 = m 2 v 2