Motion and Forces

Motion Motion- A change in position in a certain amount of time In order to describe motion, you need a frame of reference to compare it with Motion

Speed Speed = Distance Time Speed- The rate at which an object moves Velocity- Speed in a given direction Since distance is usually measured in meters and time in seconds, the unit for speed is given in meters per second (m/sec) Speed = Distance Time

Measuring Speed If a car travels 500 meters in 20 seconds, what is its speed? 500 meters 20 seconds Speed = distance = 500 meters = 25 m/sec time 20 seconds What is the speed of a jet plane that travels 7200 km in 9 hours? Speed = 7200km / 9 hr = 800 km/hr The speed of a cruise ship is 50km/hr. How far will the ship travel in 14 hours? Distance = Speed x time = 50km/hr x 14 hrs = 700 km

Acceleration Acceleration- The rate in change of velocity If something is accelerating, it is speeding up, slowing down, or changing direction The acceleration of an object is equal to its change in velocity divided by the time in which the change occurs Acceleration = Final velocity – Original Velocity Time

Determining Acceleration A rollercoaster has a velocity of 10 m/sec at the top of a hill. Two seconds later it reaches the bottom of the hill with a velocity of 20 m/sec. What is the acceleration of the rollercoaster? Acceleration = Final velocity – Original Velocity Time Acceleration = 20 m/ sec – 10 m/sec = 10 m/sec = 5 m/sec/sec 2 sec 2 sec A rollercoaster is moving at 25 m/sec at the bottom of a hill. Three seconds later it reaches the top of the next hill, moving at 10 m/sec. What is the deceleration of the rollercoaster? Deceleration = Final velocity – Original Velocity Deceleration = 10 m/ sec – 25 m/sec = -15 m/sec = -5 m/sec/sec 3 sec 3 sec

The Nature of Force Net Force The change in motion of an object is determined by the net force acting on the object. What is the net force for each situation? 0 N

The Nature of Force Which of these three arrows represents the correct amount of net force of the above two arrows?

Friction and Gravity Types of Friction Friction acts opposite the direction of motion. Determine the frictional force at work.

Friction and Gravity Frictions Give five examples for sliding, static, fluid and rolling frictions.

Friction and Gravity Gravitational Attraction Gravitational attraction depends on two factors: mass and distance. Compare the gravitational force between different planets and their sun.

Friction and Gravity Weight and Mass The Mars Phoenix Lander weighs about 3,400 N on Earth. It weighs about 1,300 N on Mars. If the dog weighs 60 N on Earth, what would he weigh on Mars and the moon?

Laws of Motion Newton’s laws of motion describe three states of motion Rest Constant motion Accelerated motion The laws also explain how forces cause all of the states of motion

Newton’s first law of motion The first law states that an object at rest will remain at rest and an object in motion will remain in motion at a constant velocity unless acted upon by an outside force Since constant velocity means the same speed and direction, in order for an object to change velocity, or accelerate, a force must act on it The concept of inertia forms the basis for this law Inertia- the property of matter that tends to resist any change in motion The inertia of an object is related to its mass The more massive an object, the more inertia it has and the more difficult it will be to move You feel the effects of inertia when you are riding in a car that stops suddenly while you keep moving forward

Newton’s second law of motion The second law of motion show how force, mass, and acceleration are related Force = Mass x Acceleration When mass is measured in kilograms and acceleration in meters/sec/sec, force is measured in Newtons (N) Tells us that a greater force is require to accelerate an object with a greater inertia (mass) Explains one reason why smaller cars get better gas mileage than larger ones

Newton’s Laws of Motion Newton’s Second Law Suppose that four dogs pull a sled carrying two people. How can the picture-stories be completed to represent answers to the two different questions?

Newton’s third law of motion States that for every action, there is an equal and opposite reaction According to the third law: Every force must have an equal and opposite force All forces come in pairs You demonstrate the third law when you walk Your feet push against the floor The floor pushes with an equal but opposite force against your feet You move forward The reaction engine utilizes the third law

Newton’s Laws of Motion Action-Reaction Forces How can the illustration of the dogs be completed to represent Newton's Third Law of Motion?

Newton’s Laws of Motion What Makes a Bug Go Splat? Splat! A bug has just flown into the windshield of an oncoming car. The car must have hit the bug much harder than the bug hit the car, right?

Free Fall and Circular Motion Global Positioning Systems

Free Fall and Circular Motion Complete the table below for an object that is dropped from rest.

Free Fall and Circular Motion Satellite Motion A satellite launched from Earth enters orbit because the curve of its path matches the curved surface of Earth. What is the gravitational force on the ball at each point?