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Transcript
Forces and Newton’s 3 Laws What is a force? • Push or pull • Produce changes in motion or direction Net force: • The net force is a combined total force acting on an object. • Fnet • ΣF • We represent force by using vectorsarrow symbols that represent magnitude and direction by their length and which way they point. Let’s look at forces acting on this box: Stationary: FN – The Normal Force- table pushing up on the box- THIS IS ALWAYS PERPENDICULAR TO THE SURFACE AND EQUAL TO THE Fg. F1 = 20 N F2 = 20 N 10 kg Fg What is the Fnet acting on this box: ΣF = Fnet = F1 – F2 = 20 N - 20 N = 0 FORCES ARE BALANCED A Fnet of zero means no change in movement. The box stays stationary. Let’s look at forces acting on this box: Stationary: FN F1 = 20 N F2 = 40 N 10 kg Fg What is the Fnet acting on this box: ΣF = Fnet = F1 – F2 = 20 N - 40 N = 20 N FORCES ARE UNBALANCED Now the Fnet is not zero which means there is a change in movement. This box is not going to remain stationary. Let’s look at forces acting on this box: Stationary: FN F1 = 20 N F2 = 40 N 10 kg Fg What is the Fnet acting on this box: ΣF = Fnet = F1 + F2 = 20 N + 40 N = 60 N FORCES ARE UNBALANCED Again there is a non-zero Fnet which means there is a change in movement. But what if the box had an initial motion: Moving with a constant velocity: FN F1 = 20 N F2 = 20 N 10 kg Fg What is the Fnet acting on this box: ΣF = Fnet = F1 - F2 = 20 N - 20 N = 0 FORCES ARE BALANCED The Fnet is zero which means there is not a change in movement or direction. This box will continue to move with a constant velocity. What forces are acting on this ball? So what needs to happen to make this box move? 10 kg UNBALANCED FORCES What about when the box is already moving with a constant velocity? What would cause the box to stop? 10 kg UNBALANCED FORCES What would happen if there weren’t unbalanced forces? Would the box ever stop?… …Not according to Newton’s Law of Motion: st 1 An object at rest has a natural tendency to stay at rest, or an object in motion will stay in motion, unless a force is acting upon it. This is also known as the law of INERTIA. INERTIA is an objects resistance to change in motion. Examples of INERTIA: 1. Not wearing your seatbelt- if you get into an accident your body wants to keep moving at the speed you were going. 2. The famous tablecloth trick: http://www.youtube.com/watch?v=D-a3kwvY0WM 3. Sliding Cars: http://www.youtube.com/watch?v=KyfjZlOSq2A The relationship between mass and inertia: MASS IS A MEASURE OF INERTIA- the more massive the object, the more that object tends to resist changes in its state of motion. What would be easier to push a small car or a semi? Adding mass into the mix brings us to Newton’s 2nd Law of Motion: If I push both vehicles with the same amount of force which one would accelerate more? Why? F a m Which turns into Newton’s 2nd equation: F ma Fnet ma kg m/s2 kg • m/s2 = Newton = N A medium-sized apple weighs about one Newton. 1 N = .22 lbs. Let’s find the magnitude of the acceleration for this box if the following forces are applied: FN F1 = 20 N a F2 = 40 N 10 kg Fg a=? Fnet = ΣF = 20 N - 40 N = 20 N m = 10 kg kg m s2 F 20 N 2 a = 10 kg kg m This leads us to the Fg: Fg ma g = 9.81 m/s2 ~ 10 m/s2 What’s mass times the acceleration due to gravity? THIS IS WEIGHT w mg Use the weight equation to find your mass: w mg This needs to be in N. Use 1 N = .22 lbs w m g Let’s go back and talk about our box again this time analyzing all the forces: FN – EQUAL but OPPOSITE to the Fg = 100 N F1 = 20 N F2 = 20 N 10 kg Fg = ma = mg = 100 N Now we are dealing with forces acting in two directions (x and y). ΣFy = Fnety = Fg – FN = 100 N - 100N = 0 ΣFx = Fnetx = F1 – F2 = 20 N - 20N = 0 This box is not moving or changing direction. Practice with Newton’s 2nd: 1. A tractor pulls a loaded wagon with a constant force of 400 N. If the total mass of the wagon is 200 kg, what is the wagon’s acceleration? 2. A broken down car is being pushed to the side of the road with a force of 200 N which is causing it to accelerate at .2 m/s2. What is the mass of the car? 4. A student weighs 600 N. What is his mass? 3. The car below was moving with an initial velocity of 50 m/s until F2 was applied to slow the car down. What is the acceleration of the box? F1 = 300 N F2 = 500 N 10 kg How far did the box travel in 10 s? Remember this guy? He was pushing that car with a force of 200 N. With how much force is the car pushing on him? Let’s go back to our box example… FN 10 kg Fg The force of gravity it pulling the box downward with a force of 100 N. But the table is also pushing back on the box with the same amount of force in the opposite direction. This brings us to Newton’s 3rd Law: For every action there is an equal and opposite reaction (action/reaction law). http://www.youtube.com/watch?v=mNM5tHou4IQ http://www.youtube.com/watch?v=7y7KjanpWOk The air is moving out of the balloon in this direction causing it to move in the opposite direction.