Forces and Motions

Forces and Motions Chapter 13 – The Nature of Forces

Forces 13.1 What is a Force? • Anything that changesthe state of restor motionof an object • It’s what causes ACCELERATION • Has magnitude and direction • Therefore force is a vector

13.1 Force A force can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction.

13.1 Combining Forces 1) Balanced Forces: 5 N 5 N = Equal forces in opposite directions cancel each other to produce no net force and therefore no movement. 0 N

13.1 Combining Forces - Unbalanced Unbalanced Forces Result when the net force acting on an object is NOTequaltozero When an unbalanced force acts on an object, the object acceleratesand moves in the direction of the net force

13.1 Combining Forces 2) Adding Forces: 5 N 5 N 10 N = Two forces, working in the same direction will add together to produce a net force larger than either original force.

13.1 Combining Forces 3) SubtractingForces: 10 N 5 N 5 N = Two forces working in opposite directions will subtract to produce a net force in the direction of the larger force.

13.2 Friction Friction is a force that opposes motion between two surfaces that are in contact. There are two main types of friction: Static - keeps things “static” (i.e. stationary) Force resisting motion of an object at rest. Ex- the force that is keeping this block from sliding downhill Kinetic - friction that occurs with objects in motion. Three types of kinetic friction: Sliding, Rolling and Fluid

13.2 Friction Rolling Friction: happens when an object rolls over a surface. Works opposite to the wheels’ motion. Static Friction: works in the opposite direction to the intended motion. Object is at rest. Static friction Direction of motion Direction of motion Rolling friction

Seating Plan Paul Richard Mia Michel R Sarah Bassel Mikel Chrissana Ingrid Amani Elias Bea Charles Jad Zeina Ryan Jenny Arthur Anne Marie Alain Patricia Michel B Tara Demy

Test • Physics (25 mins, April 4th) • Acceleration • Chapter 13.1 • Chapter 13.2 • Chemistry (25 mins, April 7th) • Acids and Bases • (yes, that’s really it, so be really prepared)

13.2 Friction Fluid Friction: happens when an object pushes fluid aside. The surfboard overcomes the fluid friction of the water. Sliding Friction: acts between the sled and the snow, in the opposite direction to the sled.

FRICTION is defined as is defined as is defined as is defined as & an example is & an example is & an example is & an example is STATIC SLIDING ROLLING FLUID The friction between surfaces that are stationary force that exists when objects slide past each other force that exists when a round object rolls over a flat surface (usually less than sliding friction) force that exists when an object moves through a fluid (air, water) • a book sitting on a table • rocks laying on the side of a mountain -a hockey puck on ice -a child going down a slide -a roller blade on a sidewalk -a bowling ball going down a bowling alley -a plane flying through the air -a swimmer swimming in a pool

13.2 Friction Reducing Friction • A smooth surface does not have as much friction as a rough surface. • A lubricant, ball bearings or even air can be used to reduce friction.

13.3 Gravity Gravity is a natural phenomenon in which objects that havemassareattractedto one another. It is an attractive force thatpulls objects together.

13.3 Gravity Earth’s gravity acts downward toward the center of the Earth. There is an upward force that balances gravity.

Falling Objects Terminal velocity: the constantvelocity of a falling object when the force of airresistanceis equal in magnitude& opposite in direction to the force of gravity.

Question: What other force is not present in a vacuum that would affect acceleration? Answer = air resistance In a vacuum, two objects would accelerate at the same rate, regardless of their shape, because both are in free fall. Free Fall: the motion of a body when only the force of gravity is acting onthe body.

Newton’s 1st Law of Motion According to Newton’s 1st Law, the state of motion of an object does not change as long as the netforce is zero. a) An object at rest will stay at rest unless a force acts on it. b) An object in motion tends to stay in motion unless a force acts on it. Ex: A soccer ball will remain at reston the grass unless a force is acted on it. It will then stay in motion until another force acts on it.

This law is also called the “Law of Inertia” Inertia: the tendency of an object to resist being moved or, if the object is moving, to resist a change in speed or direction until an outside force acts on the object. Ex. In a car crash: You continue forward because of inertia

Newton’s 2nd Law of Motion Force equals mass times acceleration. F = ma

Newton’s 2nd Law of Motion • Force = mass x acceleration • Units: Newtons (kg·m/s2) • Acceleration of gravity on Earth = 9.8 m/s2 • Acceleration of gravity on the Moon =1.6 m/s2 • Weight = mass x acceleration due to gravity • On Earth, weight ≈ mass x 10m/s2 • On the Moon, weight ≈ 1/6 weight on Earth

Newton’s 2nd Law of Motion • Question 1: • If you weigh 540N on Earth, how much would you weigh on the Moon? • Question 2: • A) If your mass is 42.5 kg on Earth, what is your mass on the Moon? • B) What would your weight on Earth be? • C) What would your weight on the Moon be? 90 N 540N x 1/6 = 42.5 kg!!! 425 N 70.8 N

Homework This will help you think about everything you have learned Worksheet Newton’s 2nd Law Questions 1-8 (see, it could be worse!)

Newton’s 3rd Law of Motion For every action there is an equal and opposite reaction.

Action and Reaction on Different Masses Consider you and the Earth Action: Earth pulls on you Reaction: you pull on Earth

Action: tire pushes on road Reaction: road pushes on tire

Reaction: gases push on rocket Action: rocket pushes on gases

Forces and Motions