Presentation on theme: "Forces and Motion Book Chapter 12"— Presentation transcript:
1 Forces and Motion Book Chapter 12 Newton’s 1st and 2nd LawsNewton’s 3rd Law & MomentumUniversal Forces
2 Forces A force is a push or pull that acts on an object. A force can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction.
3 Measuring ForceForces are measured in Newtons (named for Sir Issac Newton).One Newton is the force that causes a 1 kilogram mass to accelerate at 1 m/s2.
4 Combining ForcesForces are vectors, so you can use an arrow to represent it. The arrow has to be proportional to the force and in the same direction.When you combine forces by adding or subtracting, you are finding the Net Force.The net force is the overall force acting on an object after all forces have been combined.
5 Balanced and Unbalanced Forces When the forces on an object are balanced, the net force is zero and there is no change in the object’s motion.When an unbalanced force acts on an object, the object accelerates.
6 FrictionFriction is a force that affects motion by slowing an object down.It is always opposite to the direction of motion and affects how much an object can accelerate.Friction is present because of irregularities in the surfaces of objects that are in contact with one another.Fluids like water and air also exert a friction force on a moving object.
7 Four Types of FrictionStatic Friction – friction force that acts on objects that are not movingSliding Friction – Force that opposes the motion of an object as it slides over a surfaceRolling Friction-friction that acts on rolling objectsFluid Friction-opposes motion as an object moves through a fluid
8 Friction Direction of motion FRICTION BETWEEN TIRES AND ROAD Return to Home Page
9 GravityGravity is an attractive force that acts between any two masses.Gravity causes objects to accelerate downward towards the center of the earth.
11 Falling and Air Resistance As an object falls faster, the force of air resistance increases until it equals the weight of the object. At this time, the net force equals zero and the object stops accelerating.This final speed is called terminal velocity.
20 Mass Mass determines the amount of inertia possessed by an object. Mass is the amount of matter in an object.
21 Mass is NOT Volume Volume is the amount of space an object takes up. EXAMPLE:A pillow will have a volume larger than a car battery but less mass.
22 Mass is NOT weight Weight is the force of gravity acting on a body Objects in space will still have mass but will not have weight.Metric Units: mass - kilogramweight - NewtonOn the surface of the earth, mass and weight are proportional.1 kg = 9.8 Newtons
23 Calculating the weight of an object Since weight depends on the mass of an object AND the gravitational pull of the earth, we can say thatWeight = mass x gravityOrWeight = mgThe weight of an object is measured in Newtons.Because weight depends on gravity, it is also a force.
24 What is the weight of a 500 gram rock? 1. Convert to standard units500 grams = .5 kg2. Remember the constant for gravity is 9.8 m/s2.3. Plug into the equation and solve.Weight = mgWeight = (.5 kg)(9.8 m/s2)Weight = 4.9 N (Newtons)
25 Questions1. Your empty hand is not harmed if it bangs against the wall, but it is harmed if you are carrying a heavy load. Why?2. Does a person diet to lose mass or to lose weight?3. Can the force of gravity on a 1 kg mass ever be greater than on a 2 kg mass? Explain how.4. A car at a junk yard is compressed until its volume is less than 1 cubic meter. Has its mass changed? Has its weight changed? Has its volume changed? Explain.5. If you jump up in a bus that is moving at a constant velocity, will you land farther back in the bus? Explain.Return to Home Page
26 Newtons 2nd Law of Motion Law of AccelerationWhat causes an object to accelerate?-or change its state of motion?NET FORCE
27 What is a Net Force?A net force is the combination of all forces acting on an objectApplied Forces Net Force15 N5 N10 N5 N5 N10 N
28 Equilibrium Applied Forces Net Force 0 N If the net force is zero, the motion of the object will not change.This is a condition called EQUILIBRIUM.If an object is in equilibrium, we have to go back to the Law of Inertia.
29 Acceleration Acceleration is directly related to the net force. Acceleration ~ Net ForceThe larger the net force is acting on an object, the greater the acceleration of the object.
30 AccelerationAcceleration is inversely proportional to the mass of an objectAcceleration ~Try to push a 250 pound crate. Now try to push a 25 pound crate. Which could you move faster?1mass
31 Newton’s 2nd Law of Motion The acceleration of an object is directly proportional to the net force acting on it, is in the same direction as the net force, and inversely proportional to the mass of the object.a =FmOR F = ma
33 Newton’s 3rd Law of Motion Action / ReactionLaw of InteractionNewton realized that force is not a thing in itself, but part of an interaction between one thing and another.
34 Forces always occur in pairs One force is called the action force and the other the reaction force.Action: Object A exerts a force on Object B.Reaction: Object B exerts an equal and opposite force on Object A.
35 Newton’s 3rd LawFor each action force, there is an equal and opposite reaction force.Whenever one object exerts a force on a second object, the second object exerts anequal and opposite force on the first object.
37 Action / Reaction Forces Consider the apple at rest on the table. If we call the gravitational force exerted on the apple action, what is the reaction force? Are there any other action/reaction forces present?
38 Action / Reaction Forces If a cement truck and a car have a head-on collision, which vehicle will experience the greater impact force?A. The cement truckB. The carC. Both the sameD. ….it depends on other factors
39 Momentum Momentum is inertia in motion. Momentum = (mass)(velocity) It is harder to stop a large truck than a small car when both are moving at the same speed because the truck has a larger mass and larger momentum
41 ExampleWhich would be harder to stop….a .25 kg ball thrown at you at 40 m/s or a 7 kg bowling ball rolled at 1 m/s?It would be harder to stop the baseball; it has more momentum.
42 Law of Conservation of Momentum In the absence of an external force, the total momentum of a system remains unchanged.This is an extension of Newton’s 3rd Law(Mass)(Velocity before) = (Mass)(Velocity after)See an example of the Law of Conservation of Momentum (and Energy)
45 Universal ForcesElectromagnetic forces, Nuclear forces, and Gravitational forces are all universal forces.Electromagnetic force is associated with charged particles.Electric force and magnetic force are the only forces that can both attract and repel.
46 Nuclear ForcesTwo forces, the strong nuclear force and the weak nuclear force, act within the nucleus to hold it together.The strong nuclear force acts only on the neutrons and protons in the nucleus.The weak nuclear force acts over a short range and affects all particles, not just protons and neutrons.
47 Gravitational ForceGravitational force is an attractive force that acts between two masses.The bigger the masses and the closer they are, the more the attraction force.Newton’s Law of Universal Gravitation states that every object in the universe attracts every other object.
48 Gravitational ForceGravity is the weakest universal force, but it is the most effective over long distances.Earth’s gravitational force keeps the moon in a nearly circular orbit.The gravitational pull of the moon on the Earth causes ocean tides.
49 SatellitesWhen an artificial satellite is put into orbit, its inertia and the pull of the Earth keep it in orbit. If the satellite is too close to the atmosphere, friction slows it down and it crashes into the Earth.