Chapter 11: ForcesChapter 11: Forces

11.1: Forces change motion11.1: Forces change motion11.2: Force and mass determine acceleration11.2: Force and mass determine acceleration

11.3 Forces act in pairs11.3 Forces act in pairs11.4 Forces transfer momentum11.4 Forces transfer momentum

11.2 Forces and mass determine 11.2 Forces and mass determine accelerationacceleration

Newton’s second law relates to force, Newton’s second law relates to force, mass, and accelerationmass, and acceleration Slide a milk container: use more force if fullSlide a milk container: use more force if full Force = mass x acceleration Force = mass x acceleration

F = m x a F = maF = m x a F = ma

To give two objects with different mass (m) the To give two objects with different mass (m) the same acceleration (a), must apply a different same acceleration (a), must apply a different force (F).force (F).

Combining ForcesCombining Forces

In the same direction (think tailwind)In the same direction (think tailwind)

In opposite directions (headwind)In opposite directions (headwind)

Newton’s Second LawNewton’s Second Law

The The acceleration of an object increasesacceleration of an object increases with increased forcewith increased force and and decreases with decreases with increased massincreased mass F = maF = ma Also: the direction in which an object Also: the direction in which an object

accelerates is the same as the direction of the accelerates is the same as the direction of the forceforce

Calculating AccelerationCalculating AccelerationIf a team pulls with a combined force of 9000 N on an airplane with a If a team pulls with a combined force of 9000 N on an airplane with a

mass of 30,000 kg, what is the acceleration of the airplane?mass of 30,000 kg, what is the acceleration of the airplane?What do you know?What do you know?

What do you want to find out?What do you want to find out?

Formula?Formula?

Substitute:Substitute:

Calculate:Calculate:

Check units:Check units:

Mass and AccelerationMass and Acceleration If the same force acts on two objects, the If the same force acts on two objects, the

object with less mass will have the greater object with less mass will have the greater acceleration F = maacceleration F = ma

Calculating Mass: model rocket it accelerating at 2 m/sCalculating Mass: model rocket it accelerating at 2 m/s22. . Force on it is 1 N. What is the mass of the rocket?Force on it is 1 N. What is the mass of the rocket?

Forces can change the direction of Forces can change the direction of motionmotion

Force can either speed up or slow down Force can either speed up or slow down the motion of an object, and can also the motion of an object, and can also make an object change directionmake an object change direction Recall: if an object changes direction, it is Recall: if an object changes direction, it is

acceleratingaccelerating Can change the direction of an object without Can change the direction of an object without

changin its speedchangin its speed

Forces can change the direction of Forces can change the direction of motionmotion

Centripetal ForceCentripetal Force Think spinning in a circle with a friend: arms get tired Think spinning in a circle with a friend: arms get tired

– force used to change the direction of your motion to – force used to change the direction of your motion to keep in a circlekeep in a circle

Any force that keeps an object moving in a circleAny force that keeps an object moving in a circle Points toward the center if the circlePoints toward the center if the circle Ball on a string stays in a circle due to the force on Ball on a string stays in a circle due to the force on

the string changing direction, yet always along the the string changing direction, yet always along the string toward the centerstring toward the center

Centrifugal ForceCentrifugal Force Points towards the outside of a circlePoints towards the outside of a circle

The centrifugal force does not act on the body in The centrifugal force does not act on the body in motion; the only force acting on the body in motion is motion; the only force acting on the body in motion is the centripetal force the centripetal force

The centrifugal force acts on the source of the The centrifugal force acts on the source of the centripetal force to displace it radially from the center centripetal force to displace it radially from the center of the pathof the path

Thus, in twirling a mass on a string, the centripetal Thus, in twirling a mass on a string, the centripetal force transmitted by the string pulls in on the mass to force transmitted by the string pulls in on the mass to keep it in its circular path, while the centrifugal force keep it in its circular path, while the centrifugal force transmitted by the string pulls outward on its point of transmitted by the string pulls outward on its point of attachment at the center of the pathattachment at the center of the path

The centrifugal force is often mistakenly thought to The centrifugal force is often mistakenly thought to cause a body to fly out of its circular path when it is cause a body to fly out of its circular path when it is released; rather, it is the removal of the centripetal released; rather, it is the removal of the centripetal force that allows the body to travel in a straight line as force that allows the body to travel in a straight line as required by Newton's first law required by Newton's first law

Circular Motion and Newton’s Circular Motion and Newton’s Second LawSecond Law

Skater in photo p55: must apply more force for a faster Skater in photo p55: must apply more force for a faster direction changedirection change

If you know the size of the centripetal force acting upon If you know the size of the centripetal force acting upon the object, you can find its accelerationthe object, you can find its acceleration

A greater acceleration requires a greater centripetal A greater acceleration requires a greater centripetal forceforce

A more massive object requires a greater centripetal A more massive object requires a greater centripetal force to have the same circular speed as the less force to have the same circular speed as the less massive objectmassive object

Force and acceleration are always directed towards the Force and acceleration are always directed towards the center of the circlecenter of the circle

Newton's second law can describe the exact effects of what happens  Newton's second law can describe the exact effects of what happens  when things like the air bag and crumple zone are put to use in an when things like the air bag and crumple zone are put to use in an automobile. Newton's second law can be easily expressed by one automobile. Newton's second law can be easily expressed by one equation alone. Acceleration - Force / Mass. This law governs equation alone. Acceleration - Force / Mass. This law governs acceleration and is simple to understand. You can lower the acceleration and is simple to understand. You can lower the acceleration which is produced by your body in a collision by using acceleration which is produced by your body in a collision by using safety features like crumple zones and air bags. "The acceleration safety features like crumple zones and air bags. "The acceleration aa of a mass of a mass mm by an unbalanced force by an unbalanced force FF is directly proportional to the is directly proportional to the force and inversely proportional to the mass, or force and inversely proportional to the mass, or a-a- FF / / m." m."  This is This is the second law of motion.the second law of motion.

                                                                          

                                                                           

 

                                                     

                                                                                          

 

        "Every action has an equal and opposite re-action." This is Newton's third "Every action has an equal and opposite re-action." This is Newton's third law and it also has plays a role in exactly how both airbags and crumple law and it also has plays a role in exactly how both airbags and crumple zones work. Newton's third law would probably be the most important of all zones work. Newton's third law would probably be the most important of all of them. When you have an impact with your car, not only does your car hit of them. When you have an impact with your car, not only does your car hit another object, but the object pushes back on your car. Since the crumple another object, but the object pushes back on your car. Since the crumple zone on your car is made soft to absorb impacts, it will take the brunt of the zone on your car is made soft to absorb impacts, it will take the brunt of the collision so that the time it takes the car to come to a complete stop is collision so that the time it takes the car to come to a complete stop is spread out over a longer period of time. Not only may this save other spread out over a longer period of time. Not only may this save other people's lives it may also save money in low speed collisions.people's lives it may also save money in low speed collisions.