Download
1 / 29
310 likes | 570 Vues
Force. What is a Force ?. Force can be defined as a push or a pull …. or anything else that has the ability to change motion. Units for Force. kilogram x m/sec 2. kg x m/s 2. Newton is the scientific unit!!! N=kg x m/s 2. newton. 2 Kg Mass. To Understand Force ….
E N D
What is a Force? Force can be defined as a push or a pull… or anything else that has the ability to change motion.
Units for Force kilogram x m/sec2 kg x m/s2 Newton is the scientific unit!!! N=kg x m/s2 newton
2 Kg Mass To Understand Force… You must understand the difference between Mass Weight AND
What is Mass? The amount of “stuff” in an object 1 Kg Mass 1.5 Kg Mass 2 Kg Mass Massis measured in kilograms.
How is Mass Different Than Weight? • Weight is a measurement of force!! • Weight is not measured in kg, it is measured in pounds (lbs) or Newtons. • Gravity has an effect on the weight, but not the mass!!
For example: Mass is the Same:1 kgBut the WEIGHT… 1 Kg Mass 2.2 Pounds 7 On the moon 1.0 kg would weigh 0.3 lbs. On the sun 1.0 kg would weigh 59.5 lbs. On Mars 1.0 kg would weigh 0.8 lbs.
Newton’s First Law: Balanced Forces An object will maintain a constant state of motion (balanced). This means an object at rest tends to stay at rest and an object in motion tends to stay in motion.
Forces that are balanced can be: Not in motion In Motion Stay at rest: a book on a table Stay in motion: body not restrained by a seat belt in car accident
Table pushes up on book IPC BOOK Gravity pulls down on book
Newton’s First Law: Unbalanced Forces An unbalanced force is a force that changes the motion. The book slides and then stops because of a resistant force called friction. Friction is force that opposes motion. In space there is no resistance to cause friction, so a bowling ball would stay in motion ...unless another object got in the way…
Of course “frictional” force happens on earth... but, if you weren’t wearing a safety belt you would continue your motion… Do not let this happen to you; buckle up!
Inertia and Mass • Inertia • an object’s resistance to any change in motion • The amount of laziness • Objects with greater mass have ______ inertia • Greater
Newton’s Second Law:Force = Mass X Acceleration Force and acceleration are related. An object will have greater acceleration if a greater force is applied to it Which will accelerate faster—a car with one person pushing it or the same car with eight people pushing it?
Newton’s Second Law:Force = Mass X Acceleration • The mass of an object also affects acceleration. • Smaller mass=faster acceleration • Which will accelerate faster—a car with two people pushing it or the same two people pushing a fully loaded gravel truck?
How do we calculate force? • Force involves mass and movement so therefore we calculate force by • Mass x acceleration F V a m
Consider the problem… “How much force is needed to accelerate a 1400-kg car 2 m/s2? ” Step 1Read the problem. Draw a picture. 1400-kgcar How much force? 2 m/s2 Formula Plug-in Answer F = m = a = Units, units, units!
“How much force is needed to accelerate a 1400-kg car 2 m/s2? ” Step 2 Write down what you know, What are you trying to find? 1400-kgcar 2 m/s2 Formula Plug-in Answer ? F = 1400kg m = 2 m/s2 a = Units, units, units!
“How much force is needed to accelerate a 1400-kg car 2 m/s2? ” Step 3 Set up the formula. Solve. F = m x a Formula Plug-in Answer ? F = 1400 kg m = m x a 2 m/s2 a = Units, units, units!
“How much force is needed to accelerate a 1400-kg car 2 m/s2? ” Step 4 Plug-in the numbers. Solve. 2800 kg x m/s2 Formula Plug-in Answer ? F = 1400 kg m = m x a 2800 kg x m/s2 1400 kg x 2 m/s2 2 m/s2 a = Units, units, units!
Quick Review • Newton’s first law: Objects at rest stay at rest and objects in motion stay in motion with the same velocity unless acted on by a net force • Newton’s second law: F=ma • So….objects will speed up, change direction or stop only if acted on by a net force
Questions: • Why does my car slow down when I take my foot off the gas? • Why is hard for me to move a desk with a student sitting in it? Friction
Friction • Friction is a force that opposes motion between two surfaces that are touching • It also changes the energy of motion into heat energy---try rubbing your hands together and feel the heat
Types of Friction • Static Friction • The friction between two surfaces that are in contact but not moving • Sliding Friction • The friction that opposes the motion of two surfaces that are in contact and sliding past each other • Rolling Friction • The friction between an object rolling and the surface it is rolling on
Some things that affect motion! • Air resistance • The opposition to something moving through the air • Depends on shape, size, and speed • A good portion of the gas that cars burn is to overcome air resistance • Terminal Velocity • When an object’s weight matches the air resistance—it is the highest velocity that a falling object will reach
Question • What are the three variables that Newton’s Second Law of motion connects?
Newton’s Third Law:Action / Reaction For every action there is an equal and opposite reaction. Look at the picture below: What is the action? What is the reaction? As the gases push downward out of the rocket, The rocket is pushed upward by gases.
Gravity is the force that ALL objects in the UNIVERSE exert on each other... Newton said that gravitational force depends on 2 things: The DISTANCE of the objects The MASS of the objects LESS gravitational force compared to... The blue object has more gravitational force than the green object because the blue object has more MASS. MORE gravitational force here.
