Newtons Laws The Study of Dynamics Isaac Newton

Newton’s Laws The Study of Dynamics

Isaac Newton Arguably the greatest scientific genius ever. Came up with 3 Laws of Motion to explain the observations and analyses of Galileo and Johannes Kepler. Discovered that white light was composed of many colors all mixed together. Invented new mathematical techniques such as calculus and binomial expansion theorem in his study of physics. Published his Laws in 1687 in the book Mathematical Principles of Natural Philosophy.

What is Force? A force is a push or pull on an object. Forces cause an object to accelerate… n n n To speed up To slow down To change direction

Newton’s First Law A body in motion stays in motion at constant velocity and a body at rest stays at rest unless acted upon by a net external force. This law is commonly referred to as the Law of Inertia.

The First Law is Counterintuitive Aristotle firmly believed this. But Physics B students know better!

Implications of Newton’s 1 st Law If there is zero net force on a body, it cannot accelerate, and therefore must move at constant velocity, which means n n n it cannot turn, it cannot speed up, it cannot slow down.

What is Zero Net Force? The table pushes up on the book. A book rests on a table. FT Physics Gravity pulls down on FG the book. Even though there are forces on the book, they are balanced. Therefore, there is no net force on the book. SF = 0

Diagrams Draw a force diagram and a free body diagram for a book sitting on a table. Force Diagram N Physics W Free Body Diagram N W

Sample Problem a) A monkey hangs by its tail from a tree branch. Draw a force diagram representing all forces on the monkey FT FG

Sample Problem b) Now the monkey hangs by both hands from two vines. Each of the monkey’s arms are at a 45 o from the vertical. Draw a force diagram representing all forces on the monkey. Fa 1 Fa 2 FG

Mass and Inertia Chemists like to define mass as the amount of “stuff” or “matter” a substance has. Physicists define mass as inertia, which is the ability of a body to resist acceleration by a net force.

Sample Problem A heavy block hangs from a string attached to a rod. An identical string hangs down from the bottom of the block. Which string breaks a) when the lower string is pulled with a slowly increasing force? Top string breaks due to its greater force. a) when the lower string is pulled with a quick jerk? Bottom string breaks because block has lots of inertia and resists acceleration. Pulling force doesn’t reach top string.

Newton’s Second Law A body accelerates when acted upon by a net external force. The acceleration is proportional to the net force and is in the direction which the net force acts.

Newton’s Second Law ∑F = ma where ∑F is the net force measured in Newtons (N) n m is mass (kg) n a is acceleration (m/s 2) n

Units of force Newton (SI system) n 1 N = 1 kg m /s 2 1 N is the force required to accelerate a 1 kg mass at a rate of 1 m/s 2 Pound (British system) n 1 lb = 1 slug ft /s 2

Working 2 nd Law Problems 1. Draw a force or free body diagram. 2. Set up 2 nd Law equations in each dimension. SFx = max and/or SFy = may 3. Identify numerical data. x-problem and/or y-problem 4. Substitute numbers into equations. “plug-n-chug” 5. Solve the equations.

Sample Problem In a grocery store, you push a 14. 5 -kg cart with a force of 12. 0 N. If the cart starts at rest, how far does it move in 3. 00 seconds?

Sample Problem A catcher stops a 92 mph pitch in his glove, bringing it to rest in 0. 15 m. If the force exerted by the catcher is 803 N, what is the mass of the ball?

Sample Problem A 747 jetliner lands and begins to slow to a stop as it moves along the runway. If its mass is 3. 50 x 105 kg, its speed is 27. 0 m/s, and the net braking force is 4. 30 x 105 N a) what is its speed 7. 50 s later? b) How far has it traveled in this time?

Newton’s Third Law For every action there exists an equal and opposite reaction. If A exerts a force F on B, then B exerts a force of -F on A.

Examples of Newton’s 3 rd Law

Sample Problem You rest an empty glass on a table. a) How many forces act upon the glass? b) Identify these forces with a free body diagram. c) Are these forces equal and opposite? d) Are these forces an action-reaction pair?

Sample Problem A force of magnitude 7. 50 N pushes three boxes with masses m 1 = 1. 30 kg, m 2 = 3. 20 kg, and m 3 = 4. 90 kg as shown. Find the contact force between (a) boxes 1 and 2 and (b) between boxes 2 and 3.

Newton’s 2 nd Law in 2 -D The situation is more complicated when forces act in more than one dimension. You must still identify all forces and draw your force diagram. You then resolve your problem into an x-problem and a y-problem (remember projectile motion? ? ).

Sample Problem A surfer “hangs ten”, and accelerates down the sloping face of a wave. If the surfer’s acceleration is 3. 50 m/s 2 and friction can be ignored, what is the angle at which the face of the wave is inclined above the horizontal?

Sample Problem How long will it take a 1. 0 kg block initially at rest to slide down a frictionless 20. 0 m long ramp that is at a 15 o angle with the horizontal?

Sample Problem An object acted on by three forces moves with constant velocity. One force acting on the object is in the positive x direction and has a magnitude of 6. 5 N; a second force has a magnitude of 4. 4 N and points in the negative y direction. Find the direction and magnitude of the third force acting on the object.

Mass and Weight Many people think mass and weight are the same thing. They are not. Mass is inertia, or resistance to acceleration. Weight can be defined as the force due to gravitation attraction. W = mg

Sample Problem A man weighs 150 pounds on earth at sea level. Calculate his a) mass in kg. b) weight in Newtons.

Apparent weight If an object subject to gravity is not in free fall, then there must be a reaction force to act in opposition to gravity. We sometimes refer to this reaction force as apparent weight.

Elevator rides When you are in an elevator, your actual weight (mg) never changes. You feel lighter or heavier during the ride because your apparent weight increases when you are accelerating up, decreases when you are accelerating down, and is equal to your weight when you are not accelerating at all.

Going Up? v=0 a=0 v>0 a>0 v>0 a=0 v>0 a<0 Heavy feeling Normal feeling Light feeling Wapp W W Ground floor Just starting up Between floors Arriving at top floor

Going Down? v<0 a<0 v=0 a=0 v<0 a>0 v<0 a=0 Heavy feeling Normal feeling Light feeling Wapp W W Beginning descent Between Arriving at floors Ground floor Top floor

Sample Problem An 85 -kg person is standing on a bathroom scale in an elevator. What is the person’s apparent weight a) when the elevator accelerates upward at 2. 0 m/s 2? b) when the elevator is moving at constant velocity between floors? c) when the elevator begins to slow at the top floor at 2. 0 m/s 2?

Sample Problem A 5 -kg salmon is hanging from a fish scale in an elevator. What is the salmon’s apparent weight when the elevator is a) at rest? b) moving upward and slowing at 3. 2 m/s 2? c) moving downward and speeding up at 3. 2 m/s 2? d) moving upward and speeding up at 3. 2 m/s 2?

Normal force The normal force is a force that keeps one object from penetrating into another object. The normal force is always perpendicular a surface. The normal exactly cancels out the components of all applied forces that are perpendicular to a surface.

Normal force on flat surface The normal force is equal to the weight of an object for objects resting on horizontal surfaces. N = W = mg N mg

Normal force on ramp N = mgcos N mgsin mg The normal force is perpendicular to angled ramps as well. It’s always equal to the component of weight perpendicular to the surface. mgcos

Normal force not associated with weight. A normal force can exist that is totally unrelated to the weight of an object. applied force normal friction weight N = applied force

The normal force most often equals the weight of an object…

…but this is by no means always the case!

Draw a free body diagram for the skier.

Sample problem A 5. 0 -kg bag of potatoes sits on the bottom of a stationary shopping cart. Sketch a free-body diagram for the bag of potatoes. Now suppose the cart moves with a constant velocity. How does this affect the free-body diagram?

Sample problem a) b) Find the normal force exerted on a 2. 5 -kg book resting on a surface inclined at 28 o above the horizontal. If the angle of the incline is reduced, do you expect the normal force to increase, decrease, or stay the same?

Sample problem A gardener mows a lawn with an old-fashioned push mower. The handle of the mower makes an angle of 320 with the surface of the lawn. If a 249 N force is applied along the handle of the 21 -kg mower, what is the normal force exerted by the lawn on the mower?

Sample problem Larry pushes a 200 kg block on a frictionless floor at a 45 o angle below the horizontal with a force of 150 N while Moe pulls the same block horizontally with a force of 120 N. a) Draw a free body diagram. b) What is the acceleration of the block? c) What is the normal force exerted on the block?