Chapter 5 Newton’s 2 nd Law of Motion Force, mass and acceleration Newton’s 2 nd Law – Force is directly related to the mass and acceleration of an object (F=ma) Acceleration = change in velocity / time Force causes acceleration – Force ~ acceleration Mass resists acceleration - acceleration is inversely proportional to the mass: acceleration ~ 1/mass Keeping the force the constant, if you triple the acceleration, the mass is: A) TripledB) 1/3C) 1/9D) 1/2

Newton’s 2 nd Law Force = Mass x Acceleration Mass = Force / Acceleration Acceleration = Force / Mass If you double the force on an object, the acceleration will: A) ½B) ¼C) not changeD) Double Acceleration is measured in m/s² Force is measured in Newtons Mass is measured in kg If the man pulls on the box with 400 N, And the box accelerates at.5 m/s², what Is the mass of the box? A) 200 kgB) 400 kgC) 800 kg

Newton’s 2 nd Law More practice If a truck can accelerate at 2 m/s², What acceleration can it attain of it is Towing another truck of equal mass? A).5 m/s ² B) 1 m/s ² C) 2 m/s²D) 4 m/s² How much force is required to move a 4000 kg mass 2 m/s²? A) 2000 NB) 4000 NC) 8000 ND) 16,000 N

Newton’s 2 nd Law Friction Irregularities of surfaces when objects are in contact with each other. This is sliding or rolling friction Fluid friction –resistance that objects undergo as they flow through a fluid (not to be confused with a liquid) What type of friction is air resistance? A) Sliding B) Rolling C) Fluid

Friction When friction is present, an object may move at a constant velocity even when an outside force is applied to it. The frictional force balances out the force applied. While your driving 55 mi/hr in your car, you need to apply a constant Force (gas pedal) to maintain a constant speed - you are overcoming the Frictional force that opposes motion. 500 N 200N A B C 500 N 400 N 300 N

Friction If the forces on an object are balanced – IT DOES NOT MEAN IT IS ALWAYS STOPPED!!!!!! If forces are balanced – it means there is no acceleration! A gas pedal on a car is pressed and the car accelerates to 50 mi/hr at Which point it maintains a constant speed. If the car has to maintain A constant force of 900 N (by pressing the gas), what is the force Applied by friction? A) 0 NB) less than 900 NC) 900 ND) more than 900 N

Friction A skydiver jumps out of an airplane and accelerates to 120 mi/hr. Air resistance (Friction) is equal to the force falling (gravity). This point at which forces are balanced in freefall is known as Terminal Velocity. Is the sky diver accelerating when at Terminal Velocity? If a 70 kg person jumps out of an airplane, how much FORCE is gravity pulling them down? A) 70 kgB) 70 NC) 700 ND) 7000 N When the sky diver reaches terminal velocity, how much force is friction providing? A) Less than 700 NB) 700 NC) more than 700 N

Air Resistance Which sky diver will achieve a greater Terminal Velocity? A)B) A) 8 kg bowling ballB).002 kg pennyC) they will be the same Think about the amount of frictional force needed to balance out the forces. If person A has 2 times the mass of person B and they both pull their identical parachutes out At the same time, who will reach the ground first? Which will reach the ground first if is thrown from a high flying airplane?

Falling and Air Resistance So if there is a cannonball that has a 10 kg mass and a stone that has 1 kg mass and you drop them off a tower – why do they hit the ground at the same time? Cannon ball’s weight = m x a = 10kg x 9.8m/s² = 98 N of force Stone’s weight = m x a = 1 kg x 9.8 m/s² = 9.8 N of force The cannonball has more force towards the ground…. But remember, F =ma rearranged is acceleration = Force / Mass m/s² for the cannonballa = 98 N / 10 kg = 10 m/s² m/s² for the stone a = 9.8 N / 1 kg = 10 m/s² m/s² 10 m/s² is gravity’s acceleration If you add in the effects of friction, the rate of falling changes – the cannonball needs to apply 98 N of force to make balanced forces whereas the stone needs only 9.8 N to balance forces. If you were a friction devil – what would be easier to slow down, 98 N or 9.8 N? By dropping the Objects off a tower, The air friction is Minimal because the Objects can’t reach Fast enough speeds For air friction to Make a noticeable Difference.

Applying Force - Pressure Pressure = Force / area of application P=F/A Think of the air in your car tire, it’s measured in lbs per in². lbs is the force, in² is the area. Metric Unit of pressure is the PASCAL Newtons of force per square meter Newtons of force per square meterN/m²

Pressure and Force The more surface area a force is applied to the more the force is spread out. For example, if you stand on one foot, you are exerting a force in less area than if you are standing on 2 feet. Spreading out a person’s weight (Mass x gravity) is a lot easier when there is more surface area

Importance of Surface Area The force of the hammer is spread out to hundreds of nails 500 N of force from the hammer on 500 nails = 1 N of force on every nail