Chapter 3 Forces & Newton’s Laws

Action-at-a-Distance 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 Forces Add forces in the same direction. Subtract forces in opposite directions Forces not in the same directions\ or in opposite directions cannot be directly added together.

3.1- Forces Balanced forces – Unbalanced forces -

Friction 3.1- Forces Friction- What causes friction? Static friction Sliding friction Rolling friction

Gravity 3.1- Forces Gravity- Law of Universal Gravitation Force increases as mass increases or objects move closer Gravity & You The Range of Gravity The Gravitational Field

Gravity 3.1- Forces Weight – Weight (N) = mass(kg) x gravitational strength (N/kg) Fg =mg

Let’s Practice If your mass is 70 kg on Earth, what is your weight? A boy weighs 400 N. What is his mass? An astronaut has a mass of 100 kg and a weight of 370 N on Mars. What is the gravitational strength of Mars?

Gravity 3.1- Forces 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!!! 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? A 10 kg bowling ball would require what force to accelerate it at 4 m/s2? 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 leftwards Reaction: ? 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