Newton’s Laws of Motion

By Prof. Liwayway Memije-Cruz

Isaac Newton

English physicist and mathematician Sir Isaac Newton, most famous for his law of gravitation, was instrumental in the scientific revolution of the 17th century.

Force: can set a stationary object in motion can speed up or slow down a moving object can change the direction of an object’s motion causes change in motion can produce a change in velocity (speed/direction)

Newton’s First Law of MotionAn object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

Everyday Applications of Newton's First Law

Blood rushes from your head to your feet while quickly stopping when riding on a descending elevator.The head of a hammer can be tightened onto the wooden handle by banging the bottom of the handle against a hard surface.A brick is painlessly broken over the hand of a physics teacher by slamming it with a hammer. To dislodge ketchup from the bottom of a ketchup bottle, it is often turned upside down and thrusted downward at high speeds and then abruptly halted.Headrests are placed in cars to prevent whiplash injuries during rear-end collisions.While riding a skateboard (or wagon or bicycle), you fly forward off the board when hitting a curb or rock or other object that abruptly halts the motion of the skateboard.

ExerciseAcquire a metal coat hanger for which you have permission to destroy. Pull the coat hanger apart. Using duct tape, attach two tennis balls to opposite ends of the coat hanger as shown in the diagram at the right. Bend the hanger so that there is a flat part that balances on the head of a person. The ends of the hanger with the tennis balls should hang low (below the balancing point). Place the hanger on your head and balance it. Then quickly spin in a circle. What do the tennis balls do?

Newton’s Second Law of Motionstates that the acceleration of an object is

dependent upon two variables - the net force acting upon the object and the mass of the object. The acceleration of an object depends directly upon the net force acting upon the object, and inversely upon the mass of the object. As the force acting upon an object is increased, the acceleration of the object is increased. As the mass of an object is increased, the acceleration of the object is decreased.

The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.

Newton's second law provides the explanation for the behavior of objects upon which the forces do not balance. The law states that unbalanced forces cause objects to accelerate with an acceleration that is directly proportional to the net force and inversely proportional to the mass.

1. Determine the accelerations that result when a 12-N net force is applied to a 3-kg object and then to a 6-kg object.

Answer:A 3-kg object experiences an acceleration of 4 m/s/s. A 6-kg object experiences an acceleration of 2 m/s/s.

2. A net force of 15 N is exerted on an encyclopedia to cause it to accelerate at a rate of 5 m/s2. Determine the mass of the encyclopedia.

Answer:Use Fnet= m * a with Fnet = 15 N and a = 5 m/s/s.So (15 N) = (m)*(5 m/s/s) m = 3.0 kg

3. Suppose that a sled is accelerating at a rate of 2 m/s2. If the net force is tripled and the mass is doubled, then what is the new acceleration of the sled?

4. Suppose that a sled is accelerating at a rate of 2 m/s2. If the net force is tripled and the mass is halved, then what is the new acceleration of the sled?

3. Answer: 3 m/s/sThe original value of 2 m/s/s must be multiplied by 3 (since a and F are directly proportional) and divided by 2 (since a and m are inversely proportional)

4. Answer: 12 m/s/sThe original value of 2 m/s/s must be

multiplied by 3 (since a and F are directly proportional) and divided by 1/2 (since a and m are inversely proportional)

While driving down the road, a firefly strikes the windshield of a bus and makes a quite obvious mess in front of the face of the driver. This is a clear case of Newton's third law of motion. The firefly hit the bus and the bus hits the firefly. Which of the two forces is greater: the force on the firefly or the force on the bus?

Answer:Each force is the same size. For every action, there is an equal ... (equal!). The fact that the firefly splatters only means that with its smaller mass, it is less able to withstand the larger acceleration resulting from the interaction. Besides, fireflies have guts and bug guts have a tendency to be splatterable. Windshields don't have guts. 

Many people are familiar with the fact that a rifle recoils when fired. This recoil is the result of action-reaction force pairs. A gunpowder explosion creates hot gases that expand outward allowing the rifle to push forward on the bullet. Consistent with Newton's third law of motion, the bullet pushes backwards upon the rifle. The acceleration of the recoiling rifle is ...a. greater than the acceleration of the bullet.b. smaller than the acceleration of the bullet.c. the same size as the acceleration of the bullet.

Answer: BThe force on the rifle equals the force on the bullet. Yet, acceleration depends on both force and mass. The bullet has a greater acceleration due to the fact that it has a smaller mass. Remember: acceleration and mass are inversely proportional.

Drew is pulling upon a rope that is attached to a wall. In the bottom picture, Drew is pulling upon a rope that is attached to an elephant. In each case, the force scale reads 500 Newton. Drew is pulling ...a. with more force when the rope is attached to the wall.b. with more force when the rope is attached to the elephant.c. the same force in each case.

Answer: CDrew is pulling with 500 N of force in each case. The rope transmits the force from Drew to the wall (or to the elephant) and vice versa. Since the force of Drew pulling on the wall and the wall pulling on Drew are action-reaction force pairs, they must have equal magnitudes. Inanimate objects such as walls can push and pull.

Reference:

http://www.physicsclassroom.com/Physics-Tutorial/Newton-s-Laws