First Law of MotionSection 3.2

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What do you know? What causes motion?

What causes motion to stop?

What is friction?

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Arm in Arm Suppose two students are arm

wrestling, and neither can bring the other arm down.

What can you infer about the forces being exerted?

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Newton’s First Law of Motion

Newton is recognized as the first person to state the relationship between motion and forces.

He observed that and object at rest stays at rest until an outside forces causes it to move.

He also observed the opposite. An object at rest will remain at rest and

an object in motion will remain in motion unless acted upon by an outside force.

Objects At Rest When you sit in a chair many forces act

on you.• Atmosphere• Chair• Gravity

All of the forces are balanced therefore, you sit comfortably and you don’t move.

Your body is resisting change or experiencing inertia.

To come out of inertia a force must be applied.

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Objects In Motion What happens when you ride a bike? How does the bike start to move? Does it stay in motion even when you’re

not doing anything? Forces can change the speed, direction,

and overall movement of bike. What forces are acting on the bicyclist in

Figure 3.7? What would happen to the bicyclist when

the bike stopped?

Friction If everything stayed in motion until a

collision caused it to stop or change direction, what would the world be like?

Friction acts in the opposite direction of motion.

Friction is an outside force that resists motion when two surfaces come in contact.

Types of Friction Without friction, we couldn’t walk

without slipping. Friction plays an important role in all

aspects of life. There are three main types of

friction:• Sliding Friction• Fluid Friction• Rolling Friction

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Sliding Friction This occurs when two solid surfaces

slide over each other. The friction is determined by the

weight of the object and the type of surface it moves over.• Heavier objects exert more pressure on the

surface therefore the sliding friction will be greater.

Which type of surface would cause less sliding friction?

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Fluid Friction Air, water, and oil are all fluid. Fluid friction occurs in type of fluid. It is very present when an object falls

through the air.

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Rolling Friction Rolling friction- friction produced

when an object rolls over a surface. Force needed to overcome rolling

friction is less than the force needed to overcome sliding friction.

Tread on tires can affect rolling friction. Looking at the tires on page 62, which tire do you think will create more rolling friction?

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Reducing Friction Many machines use techniques to

reduce friction to help save the parts.• Ball bearings• Fluids (air, gas, liquid film)

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What have we learned?1. What is Newton’s first law of motion?2. Explain how a dog overcomes inertia

when it gets up after napping on the floor.

3. Explain how friction works to keep bicycle tires from sliding on the road.

4. Identify two examples each of an object at rest and of an object in motion. Use examples of things in your school. Explain how Newton’s first law applies to each example.https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcTQsvui1Gr27Y87hx2SgrLlFw89VuWr61We9C1LK5kKq8Rgu60w