Newton’s Laws of Motion 1st, 2nd, 3rd Law mass vs weight coefficient of friction

Newton’s 1st Law Law of inertia “An object at rest stays at rest, and an object in motion stays in motion at the same speed in the same direction, unless acted upon by an unbalanced outside force.”

What is Inertia? Inertia – tendency of an object to continue doing the same thing When your car stops suddenly, you keep going until the seatbelt or windshield stops you.

Law of inertia

What is Inertia? An 18-wheeler, stopped at a light is harder to get going than a little VW Bug. The truck has more inertia because it has more mass. An 18-wheeler, driving 60 mph is harder to get stopped than a little VW Bug. The truck has more inertia because it has more mass.

Newton’s 1st Law

Newton’s 1st Law What will happen to the probe as it continues through space? https://www.youtube.com/watch?v=hHXx8AmBwXg

Newton’s 2nd Law Law of Acceleration Net force is directly proportional to mass and acceleration. In equation form Fnet = m a

Newton’s 2nd Law Acceleration means “change in motion” all accelerations require a Net Force to create the change in motion this acceleration may include a change in direction or speed

Mass vs Weight mass: quantity of matter measured in kg. weight: force (Fg) caused by acceleration due to gravity measured in N. **mass of an object is constant, but weight is dependent upon gravity.

Now you try: GIZMO – Weight and Mass Mass vs Weight Newton’s 2nd Law says F = m a but we can say Fg = m ag or F = m g Calculate the weight of a 3.87 kg box on Earth (ag = -9.8m/s2). Fg = (3.87)(-9.8) = -37.926  F = -37.9 N or Fg = 37.9 N Calculate the weight of a 3.87 kg box on the Moon (ag = -1.6m/s2). Fg = (3.87)(-1.6) = -6.192  F = -6.19 N or Fg = 6.19 N Now you try: GIZMO – Weight and Mass

Newton’s 2nd Law Fnet = m a Acceleration means “change in motion” all accelerations require a Net Force to create the change in motion this acceleration may include a change in direction or speed Fnet = m a

Newton’s 2nd Law At equal accelerations: Bigger mass means bigger force. F= m a F= m a

Newton’s 2nd Law At equal masses: Bigger acceleration means bigger force. F= m a F= ma Compare easing away from a stop sign slowly increasing speed to 30 mph and peeling out to quickly reach 30 mph.

G.U.E.S.S. method You will use this method to work all math problems in this class. It is a 5-step process to organize and solve math questions in physics. G: given U: unknown E: equation S: substitution S: solution

Ex1:How much force is needed to accelerate a 68 Ex1:How much force is needed to accelerate a 68.0 kg skier at a rate of 1.20 m/s2? G m=68.0 kg a=1.20m/s2 U F=? E F=ma S F=(68)*(1.2) S F=81.6N

F=ma divide both sides by mass a = m F Ex2: What is the acceleration of a 0.35kg ball that is hit with a force of 25N? G m=0.35 kg F=25 N U a=? E F=ma S a=(25)/(0.35) S a=71m/s2 Algebra is required to isolate the unknown variable. So … F=ma divide both sides by mass to isolate acceleration. a = m F use: m m Acceleration is directly proportional to Force Acceleration is inversely (opposite) proportional to Mass

Newton’s 3rd Law Law of Action/ Reaction Every action (force) has an equal and opposite reaction (force) .

Newton’s Third Law If an object A puts a force on object B, Then object B exerts an equal force Back on object A. A B Amazing Race – Watermelon Smash

Newton’s 3rd Law What are the two forces that will occur in this picture? Action – Reaction – the hammer hits the nail the nail hits the hammer

What force pairs are in this picture? Newton’s 3rd Law What force pairs are in this picture? Action: the ball pushes on the net Reaction: the net pushes on the ball

What force pairs are in this picture? Newton’s 3rd Law What force pairs are in this picture? Action: foot pushes down on the ground Reaction: ground pushes up on the foot

How does each of Newton’s Laws apply?

How does each of Newton’s Laws apply?

How does each of Newton’s Laws apply?

because of 3rd Law….. Ff is always opposite motion! FN is opposite Fg FN = 25 N Fg = 25 N Ff is always opposite motion! FN is opposite Fg Fnet = -10 + 40 = 30N acutal Force effecting the object is 30N (right) FA = 40 N Ff = 10 N 5.0 kg If the object’s mass is 5.0 kg, what acceleration will it experience? Vertically Fnet = 0 N …… no vertical acceleration Horizontally Fnet = 30 N … acceleration ???? Fnet = ma using the Net Force acting horizontally. to solve for a ……. a = F / m a = (30) / (5) = 6.0 m/s2 Horizontally Fnet = 30N (right) … 6.0m/s2 (right) acceleration

Coefficient of Friction friction is caused by contact between rubbing surfaces (“rough” tiny bumps – even smooth surfaces have them on the microscopic level). static friction – initial friction to overcome for a “body at rest” kinetic friction – friction while an object is “moving”

Coefficient of Friction Ff = μ m g Force of friction coefficient of friction acceleration due to gravity mass remember …. “m g” represents the same thing as “m a” if given mass …. you must multiply by gravity if given force or “weight” ….. do not multiply by gravity 1. 2. A 2.1 N ball rolls across the yard. A force of friction of 3.5 N brings the ball to a stop. What is the coefficient of friction between the ball and the yard? Billy pushes a 28 kg box across the floor. If the floor has a static coefficient of friction of 0.075, what force is needed to move the box? Ff = μ m g Ff = (.075)(28)(-9.8) Ff = -20.58  Ff = -21N Ff = μ m g ….. Ff = μFg 3.5 = μ (2.1) … ÷ (2.1) μ = (3.5)/(2.1)  1.66… μ = 1.7