Chapter 3 Forces & Newton’s Laws

3.1- Forces• Forces

– What is a Force?

– Force –

– Measured in Newtons

– Changing Motion-Contact Forces Action-at-a-Distance Forces

Frictional Force Gravitational Force

Tension Force Electrical Force

Normal Force Magnetic Force

Air Resistance Force

Applied Force

Spring Force

3.1- Forces

• Forces–Net force –

Rules for Adding Forces1.Add forces in the same direction.2.Subtract forces in opposite directions3.Forces not in the same directions\ or in opposite directions cannot be directly added together.

• Balanced forces –

• Unbalanced forces -

3.1- Forces

3.1- Forces

• Friction–Friction-

–What causes friction?

• Static friction

• Sliding friction

• Rolling friction

3.1- Forces

• Gravity–Gravity-–Law of Universal Gravitation

• Force increases as mass increases or objects move closer

• Gravity & You• The Range of Gravity• The Gravitational Field

3.1- Forces

• Gravity–Weight –

–Weight (N) = mass(kg) x gravitational strength (N/kg)• Fg =mg

Let’s Practice1. If your mass is 70 kg on Earth, what

is your weight?

2. A boy weighs 400 N. What is his mass?

3. An astronaut has a mass of 100 kg and a weight of 370 N on Mars. What is the gravitational strength of Mars?

3.1- Forces

• Gravity–Weight and Mass

–Weight on Earth

–Finding other Planets

3.2- Newton’s Laws of Motion•Newton’s 1st Law of Motion

•Newton’s 1st Law – •Break it Down – Things want to keep doing what they are doing

•Inertia – •Inertia and Mass

•Mass corresponds to an object’s inertia •More massive objects have more inertia than less massive objects

3.2- Newton’s Laws of Motion•Newton’s 2nd Law of Motion

•Newton’s 2nd Law•Break It Down – F=ma•Acceleration is caused by net force•Acceleration is directly proportional to net force

•Push an object, object accelerates•Push an object with twice as much force, then it accelerates twice as much

•Direction of acceleration is always the direction of the net force

3.2- Newton’s Laws of Motion•Newton’s 2nd Law of Motion

•Mass and acceleration•More massive objects are harder to accelerate•Acceleration is inversely proportional to mass

•Push a brick, brick accelerates•Push 2 bricks with same amount of force, accelerates ½ as much •3 bricks accelerates 1/3 as much

3.2- Newton’s Laws of Motion•Newton’s 2nd Law of Motion

•Mass and acceleration•More massive objects are harder to accelerate•Acceleration is inversely proportional to mass

•Push a brick, brick accelerates•Push 2 bricks with same amount of force, accelerates ½ as much •3 bricks accelerates 1/3 as much

Let’s Practice!!!1. With what force will a car hit a tree if

the car has a mass of 3,000 kg and it is accelerating at a rate of 2 m/s2?

2. A 10 kg bowling ball would require what force to accelerate it at 4 m/s2?

3. If a helicopter’s mass is 4,500 kg and the net force on it is 18,000 N upward, what is it’s acceleration?

3.2- Newton’s Laws of Motion•Newton’s 3rd Law of Motion

•Newton’s 3rd Law•Break It Down - forces always occur in pairs

A Simple Rule Helps to Identify Action and Reaction

• Action: Object A exerts a force on Object B Reaction: Object B exerts a force on Object A

Identifying Force Pairs• Identify the following Force Pairs

Action: Enclosed air particles push balloon wall outwards.Reaction: ?

Action : Baseball pushes glove leftwardsReaction: ?

Action: Shuttle pushes exhaust gases down

Reaction: ?

3.2- Newton’s Laws of Motion

• Action and reaction forces do not cancel each other out

• Forces only cancel when they act on the same body

• Action-Reaction forces work on different objects

3.3- Using Newton’s Laws• What happens in a crash?

– Newton’s 1st Law and Safety Belts– Air Bags in Cars

3.3- Using Newton’s Laws• Newton’s 2nd Law and Gravitational

Acceleration– Air Resistance– Depends on size and shape– Depends on speed of object– More speed, more air

resistance– Terminal velocity -

3.3- Using Newton’s Laws• Newton’s 2nd Law and Gravitational

Acceleration– Free Fall –

• Weightlessness• What does weightless mean?

3.3- Using Newton’s Laws• Centripetal Forces

– Centripetal force-– Force acting toward the center of circle

3.3- Using Newton’s Laws• Force and Momentum

– P = mv– Conservation of Momentum

• Law of Conservation of momentum• Momentum conserved