2 What is a force?Push or pullProduce changes in motion or direction
3 Net force:The net force is a combined total force acting on an object.FnetΣFWe represent force by using vectors- arrow symbols that represent magnitude and direction by their length and which way they point.
4 Let’s look at forces acting on this box: FN – The Normal Force- table pushing up onthe box- THIS IS ALWAYS PERPENDICULARTO THE SURFACE AND EQUAL TO THE Fg.Stationary:F1 = -20 N westF2 = 20 N east10 kgFgWhat is the Fnet acting on this box:ΣF = Fnet = -20 N + 20 N = 0FORCES ARE BALANCEDA Fnet of zero means no change in movement.The box stays stationary.
5 Let’s look at forces acting on this box: FNStationary:F1 = 20 N westF2 = 40 N east10 kgFgWhat is the Fnet acting on this box:ΣF = Fnet = -20 N + 40 N = 20 N eastFORCES ARE UNBALANCEDNow the Fnet is not zero which means there is a changein movement. This box is not going to remain stationary.
6 Let’s look at forces acting on this box: FNStationary:F1 = 20 N westF2 = 40 N east10 kgFgWhat is the Fnet acting on this box:ΣF = Fnet = 20 N + 40 N = 60 N eastFORCES ARE UNBALANCEDAgain there is a non-zero Fnet which means there is a changein movement.
7 But what if the box had an initial motion: FNMoving with a constant velocity:F1 = -20 N west10 kgF2 = 20 N eastFgWhat is the Fnet acting on this box:ΣF = Fnet = -20 N + 20 N = 0FORCES ARE BALANCEDThe Fnet is zero which means there is not a changein movement or direction. This box is continues to movewith a constant velocity.
8 So what needs to happen to make this box move? 10 kgUNBALANCED FORCES
9 What about when the box is already moving with a constant velocity? What would cause the box to stop?10 kgUNBALANCED FORCESWhat would happen if there weren’t unbalanced forces? Would the box ever stop?…
10 …Not according to Newton’s 1st Law of Motion: 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.
11 Examples of INERTIA: 1. Not wearing your seatbelt- if you get into an accident your body wants to keep moving atthe speed you were going.
13 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?
14 Adding mass into the mix brings us to Newton’s 2nd Law of Motion: If I push both vehicles with the same amount of forcewhich one would accelerate more? Why?Which turns into Newton’s 2nd equation:
17 Let’s find the magnitude of the acceleration for this box if the following forces are applied: FNF1 = -20 NF2 = 40 N10 kgFga = ?Fnet = ΣF = -20 N + 40 N = 20 Nm = 10 kg=
18 This leads us to the Fg: THIS IS WEIGHT g = 9.81 m/s2 ~ 10 m/s2 What’s mass times the acceleration due to gravity?THIS IS WEIGHT
19 Use the weight equation to find your mass: This needs to be in N. Use 1 N = .22 lbs
20 Let’s go back and talk about our box again this time analyzing all the forces: FN – EQUAL but OPPOSITE to the Fg = 100 NF1 = -20 NF2 = 20 N10 kgFg = ma = mg =-100 NNow we are dealing with forces acting in two directions (x and y).ΣFy = Fnety = 100 N N = 0ΣFx = Fnetx = -20 N + -20N = 0This box is not moving or changingdirection.
21 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?
22 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?
23 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 deceleration of the box?F1 = 300 NF2 = -500 N10 kgWhat is the distance the box travels before it comes to a stop?