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Newton’s 2nd Law of Motion F=ma • F = force – Measured in newtons (N) • m = mass units – Measured in kilograms (kg) • a = acceleration – Measured in meters per second per second (m/s²) Multiple forces can be exerted on an object at the same time • acceleration = change in velocity over time What is acceleration ? a= 2 − 1 or a = Δ Thinking about the law • If the force acting on an object goes down, and the mass doesn’t change, how does the acceleration change? • Fg = mag (or W=mg or F = mg) Weight is applied Newton’s 2nd – Fg = force due to gravity ( or W=weight) – ag (or g) = acceleration due to gravity • always = 9.8 m/s2 on Earth • Does a big guy or little guy fall faster when bungee jumping? • Fg = mag (or W=mg or F = mg) Weight is applied Newton’s 2nd – Fg = force due to gravity ( or W=weight) – ag (or g) = acceleration due to gravity • always = 9.8 m/s2 on Earth • Does a big guy or little guy fall faster when bungee jumping? • They fall at the same speed, as mass is irrelevant to freefall speed Free-fall utilizes “g” The acceleration of gravity (g) for objects in free fall at the earth's surface is 9.8 m/s2. Galileo found that all things fall at the same rate. Free-fall The rate of falling increases by 9.8 m/s every second. Height = ½ gt2 (Height is abbreviated either “y” or “d”) For example: ½ (9.8 )12 = 4.9 m ½(9.8)22 = 19.6 m ½ (9.8)32 = 44.1 m ½ (9.8)42 = 78.4 m Free-fall A ball thrown horizontally will fall at the same rate as a ball dropped directly. Free-fall A ball thrown into the air will slow down, stop, and then begin to fall with the acceleration due to gravity. When it passes the thrower, it will be traveling at the same rate at which it was thrown. Projectile motion path shape An object thrown upward at an angle to the ground follows a curved path called a parabola. What do you notice about this shape? Air Resistance and free-fall • In air… – A stone falls faster than a feather • Air resistance affects stone less • In a vacuum – A stone and a feather will fall at the same speed. Air Resistance and free-fall – A person in free fall reaches a terminal velocity of around 54 m/s (about 130 mi./hr.) – With a parachute, terminal velocity is only 6.3 m/s Allows for a safe landing -the terminal velocity for a cat is 60 mi./hr. Air Resistance • Ideal angle for a projectile – In a vacuum, maximum distance is at an angle of 45o – With air resistance (real world), angle is less • Baseball will go furthest hit at an angle of around 40o