29Newton’s Second Law of Motion Force = Mass x accelerationA force is needed for accelerationAcceleration will occur in the direction of the force.
30Which cars are accelerating? If you watch carefully, the green and blue cars are accelerating. The red car is traveling at constant speed.
31A force is needed for acceleration Will these accelerate?A skier going downhill?What force is acting onthe skier?An astronaut on aspacewalk?Are any forces actingon the astronaut?
32Which of the following will accelerate? A bullet shot froma gun?What forces areacting on thebullet?A hammer andfeather dropped onthe moon?What force is acting?
33Newton’s 2nd Law F_____ M A Force = mass x accelerationF_____M AForce is measured in newtons. A newton is a kg ms2Example: A car with mass 1000kg hits a tree and decelerates at 50m/s/s. What force does it exert on the tree?
34Answer:F_____M A1000kg * 50m/s/s =50,000 kgm or 50,000Ns2
35F = ma A car is traveling at constant speed. Is a force acting on it? Answer: although there are forces acting on the car (gravity, friction, air resistance,etc…) since there is no acceleration, the net force in the direction of motion must be zero.
36More 2nd Law Problems:1. What is the acceleration on a mass of 50kg if a force of 10N is applied?2. An object accelerates due to gravity at a rate of 10m/s/s. If its mass is 15kg, what force is acting on the mass?
37Answers: _F_ M A 1. _10N_ 0.2 n/kg= 0.2 kg m = 0.2 m/s2 50kg A s2/kg 15kg 10m/s2150 kgm/s2 or 150N
38Gravity is a force that causes an acceleration On earth, ALL objects accelerate at 9.8m/s2 (ignoring air resistance) because of gravity.No matter what the mass, ALL objects on earth accelerate at 9.8 m/s2
48How do Newton’s laws describe the motorcycle rider? 1st: Inertia keeps the rider moving forward.2nd: The force with which the rider hits the ground will be equal to his mass times his deceleration.3rd: The action of hitting the ground will equal the reaction of the dirt.