3Newton’s 1st Law of Motion An object in motion tends to stay in motion and an object at rest tends to stay at rest, unless the object is acted upon by an outside force.The Law of Inertia-due to an object’s mass
4We feel the effects of Newton's First Law every day, but usually don't notice them because other forces interfere. In space, the First Law is much more obvious. Objects will follow their natural path until they are stopped by an outside force. On Earth, the atmosphere will eventually slow down all moving objects, but in a vacuum (basically an empty space with no air or atmosphere), like space, it will be more obvious that objects obey Newton's Laws.
5Some Examples The person is standing on the floor. The only forces acting on the person are the force due to gravity pulling down & the normal force pushing up. The net force is zero and the person remains still.
6Another ExampleOne of the most common places people feel the First Law is in a fast moving vehicle, such as a car or a bus, that comes to a stop. An outside force stops the vehicle, but the passengers, who have been moving at a high speed, are not stopped and continue to move at the same speed.
9Some ProblemsAn astronaut in outer space away from gravitational or frictional forces throws a rock. The rock will…..
10And now the answer…continue to move in a straight line at a constant speed.The rock’s tendency to do this is called INERTIA.
11A rock is being whirled at the end of a string in a clockwise direction. If the string breaks, the path of the rock is
12It will follow an “inertial path” so it will follow path “C” It will follow an “inertial path” so it will follow path “C”. Once the rock leaves the string, there are no unbalanced forces to affect its motion.
13A Weighty ProblemI spend most Sunday afternoons at rest on the sofa, watching football and consuming large quantities of food. What effect (if any) does this practice have upon my inertia? Explain.My inertia will most definitely increase. My mass will increase because of this practice and if mass increases, then inertia increases.
14How much force?An 2-kg object is moving horizontally with a speed of 4 m/s. How much net force is required to keep the object moving with the same speed and in the same direction?0 NAn object in motion will maintain its state of motion. The presence of an unbalanced force will change the velocity of an object.
15Is a force required?Yes, because even in outer space, an object has mass. If an object has mass then the object is going to resist changes in its motion. A force must be applied to set the object in motion. Newton’s Laws rule—Everywhere!If you were in a weightless environment in space, would it require a force to set an object in motion?
16A Final ProblemWhy isn't the girl hurt when the nail is driven into the block of wood?
17And the answer is….Due to the large mass of the books, the force of the hammer is sufficiently resisted (inertia).This is demonstrated by the fact that the blow of the hammer is not felt by the girl.
20Newtons’ Second Law F = m a The acceleration of an object is directly proportional to the net force acting on the object……and inversely proportional to the mass of the object.
21a = 4 m/s2 Example Questions How much acceleration does a 747 jumbo jet of mass 30,000kg experience in takeoff when the thrust of all of the engines is 120,000N?F= ma a = F/ma = N/30000 kga = 4 m/s2
22Example Questions The same net force is applied to two blocks. If the blue one has a smaller mass than the yellow one, which one will have the LARGER acceleration?
23If the NET FORCE is parallel to the velocity, then the speed of the object increases. If the NET FORCE is anti-parallel (or opposite) to the velocity, then the speed of the object decreases.
24If the net force is perpendicular to the velocity, the direction of the velocity changes.
25Force and acceleration are vector quantities. If v is parallel to F, speed increases.If v is antiparallel to F, speed decreases.If v perpendicular to F, direction of v changes.
26When Acceleration Is Zero... …we say the object is in Mechanical Equilibrium.…the net force is zero.For Static Equilibrium the velocity is zero.For Dynamic Equilibrium the velocity is constant.
27When Acceleration Is Zero - Equilibrium Static EquilibriumVelocity is zeroExamples:Scales pushing upNormal upWeight downComputer setting on a tableWeighing yourself on a set of scalesHanging from a treeTreepulling upWeight downCar parked on an inclineNormalFrictionWeight downWeight down
28Dynamic Equilibrium Velocity is nonzero and constant Examples: Driving at constant velocityNormal upFrictionForce from roadAir resistanceWeight downTerminal velocity in parachutingWeight down
29Newton’s 3rd LawFor every action there is an equal and opposite reaction.Book toearthTable tobook
30Think about it . . .What happens if you are standing on a skateboard or a slippery floor and push against a wall? You slide in the opposite direction (away from the wall), because you pushed on the wall but the wall pushed back on you with equal and opposite force.Why does it hurt so much when you stub your toe? When your toe exerts a force on a rock, the rock exerts an equal force back on your toe. The harder you hit your toe against it, the more force the rock exerts back on your toe (and the more your toe hurts).
31Newton’s Third LawA bug with a mass of 5 grams flies into the windshield of a moving 1000kg bus.Which will have the most force?The bug on the busThe bus on the bug
32Newton’s Third Law The force would be the same. Force (bug)= m x A Force (bus)= M x aThink I look bad?You should seethe other guy!
33Action and Reaction on Different Masses Consider you and the earthAction: earth pulls on youReaction: you pull on earth
34Action: tire pushes on road Reaction: road pushes on tire
35Reaction: gases push on rocket Action: rocket pushes on gases
36Consider hitting a baseball with a bat Consider hitting a baseball with a bat. If we call the force applied to the ball by the bat the action force, identify the reaction force.(a) the force applied to the bat by the hands(b) the force applied to the bat by the ball(c) the force the ball carries with it in flight(d) the centrifugal force in the swing(b) the force applied to the bat by the ball
37Newton’s 3rd LawSuppose you are taking a space walk near the space shuttle, and your safety line breaks. How would you get back to the shuttle?
38Newton’s 3rd LawThe thing to do would be to take one of the tools from your tool belt and throw it is hard as you can directly away from the shuttle. Then, with the help of Newton's second and third laws, you will accelerate back towards the shuttle. As you throw the tool, you push against it, causing it to accelerate. At the same time, by Newton's third law, the tool is pushing back against you in the opposite direction, which causes you to accelerate back towards the shuttle, as desired.
41Review Newton’s First Law: Objects in motion tend to stay in motion and objects at rest tend to stay at rest unless acted upon by an unbalanced force.Newton’s Second Law:Force equals mass times acceleration (F = ma).Newton’s Third Law:For every action there is an equal and opposite reaction.
42Newton's Laws1stlaw: Homer is large and has much mass, therefore he has much inertia. Friction and gravity oppose his motion.2nd law: Homer’s mass x 9.8 m/s/s equals his weight, which is a force.3rd law: Homer pushes against the ground and it pushes back.