Download Do Now - Hicksville Public Schools

Aim: What did Isaac Newton teach us about motion?
Do Now:
1. A 2009 Ford Mustang convertible is travelling at constant velocity on Interstate 95 south from Philadelphia to Wilmington Delaware. It passes the 120 mile marker at exactly 1:00pm and the 130 mile marker at 1:08pm. What is a force? What is the acceleration of the car?
Isaac Newton
Home Entertainment
• Inclined Plane AND Marble Launcher lab due Monday
> Abstract only for inclined plane
> Be sure procedure is detailed for marble launcher
"If I have seen further it is by standing on the shoulders of giants"
­Isaac Newton to Robert Hooke
Newton’s Three Laws of Motion
Newton’s First Law of Motion – The Law of Inertia
An object at rest will remain at rest, and an object in straight line motion will remain in straight line motion, unless Newton’s 1st law – What is inertia?
Inertia is Newton’s Second Law of Motion – Net Force
When an unbalanced (net) force acts on an object, the object will undergo an acceleration
a =
also written as Units:
Newton’s Third Law of Motion
For every action/force, there is an equal and opposite reaction/force.
2. Which person has the greatest inertia?
(1) a 110­kg wrestler resting on a mat
(2) a 90­kg man walking at 2 m/s
(3) a 70­kg long­distance runner traveling at 5 m/s
(4) a 50­kg girl sprinting at 10 m/s
Newton’s 2nd law – What is net force?
A force is Net force is Newton’s 3rd law
What does that mean? I push down on the desk with a force of 10 N
The desk pushes up on me with a force of ____________________
3. Mr. Vigneaux pushes on a box with a force of 10 N, while Mr. Evans pushes the other direction with a force of 7 N. What is the net force on the box?
How does an airplane propeller push an airplane forward?
4. A 1200kg car is accelerated at a rate of 4 m/s/s. What is the net force acting on the car?
5. A train engine with a mass of 5000 kg is pushed with a force of 12,000 N to the East. Friction provides a force of 2,000 N to the West. What is the net force on the train?
What is the acceleration of the train?
6. The following forces act on a box of mass m. The box accelerates at a rate of 6 m/s/s. What is the mass, m, of the box?
7. A woman is pushing a baby stroller. Compared to the magnitude of the force exerted on the stroller by the woman, the magnitude of the force exerted on the woman by the stroller is
1. zero
3. larger
2. smaller, but greater than zero
4. the same
8. A 100 kg ice skater pushes on a 50 kg ice skater with a force of 200 N.
a. What is the force on the 100 kg ice skater?
b. What is the acceleration of the 50 kg ice skater?
c. What is the acceleration of the 100 kg ice skater? Even though the forces are the same, the effect of the forces is not the same
9. Mary has a mass of 55 kilograms. The Earth pulls down on her (gravity) with a force of 539 N.
a. What is her acceleration?
b. Mary pulls up on the Earth with the same force. Calculate the acceleration of the Earth due to Mary’s upward force.
Forces
Four fundamental forces ­ all interactions come from these forces
Normal Force
What is the force due to gravity acting on a 2 kg mass, sitting on the top of a table?
Force due to Gravity
Weight vs. Mass
What is its acceleration?
Explain this behavior
Normal Force: Weight Normally, FN = Fg = mg when an object is on a surface. Why?
10. What is the weight of a 2.5 kg textbook?
If FN > Fg: If FN < Fg 11. A student weighs 600 N. What is his mass?
Usually, Exceptions:
On an elevator
12. An object has a weight of 100 N on Earth. What is the weight of the same object on the Moon, where acceleration due to gravity is 1.6 m/s/s?
On an incline
In free­fall
Tips:
13. A 10 kg dog sits on the floor. What is the normal force pushing up on him?
• When you go to another planet or to space, your weight changes because it depends on the force of gravity from the Earth (or other planet), but your mass does not change.
• When an object is described in terms of Newtons, rather than a force, those Newtons refer to the weight of the object.
14. A 600 N student sits at his desk. What is the normal force pushing up on him?
Ex: A 72 N force acts on a 120 N box
72 N is the force acting on (accelerating) the box, 120 N is the weight of the box
15. A spring with a spring constant of 2.0 N/m is stretched 0.40 m from its equilibrium position. What is the force stretching the spring?
Force on a Spring ­ Hooke's Law
Hooke’s Law – The restoring force exerted by an object (FS) is proportional to the amount it is stretched or compressed (x) and its ‘spring constant’ (k)
Formula: Fs =
(on reference table) 16. A spring with spring constant 200 N/m has a force of 5 N pushing on it. Find the distance the spring is compressed.
Spring constant: k
17. A 2.0 kg rock is attached to the lower end of a vertical spring that has a spring constant of 150 N/m. How far will the rock stretch the spring?
18. A force of 18 Newtons is required to compress a spring a distance of 0.08 m. Find the spring constant of the spring.
Force of Friction
Friction occurs because all surfaces are rough
(rough is a key word used on the regents implying a friction problem)
Friction is a force that occurs when surfaces slide or tend to slide across each other
Force of friction depends on:
The direction of the frictional force is always Friction occurs because glass window under a microscope
Graphite zoomed in to the molecular level
Static Friction Kinetic Friction Some coefficients for friction can be found on the reference table
19. What is the static force of friction for a 3 kg rubber tire on dry concrete?
Static friction is said to be variable, meaning
20. You push a stationary 10 kg wooden block on a wooden tabletop. What is the force you must apply in order for it to begin moving? Is it static or kinetic friction?
Friction is FµN
(on reference table) Max Static Friction Kinetic Friction
Equilibrium
When an object is in equilibrium:
What does unbalanced force mean?
Equilibrant
Concurrently
21. Are the following objects in equilibrium? If not, sketch and calculate the equilibrant.
Forces Quick Review
Fnet means 2 things:
i.e. 2N West + 3N East, Fnet =
i.e. m = 1 kg, a = 2 m/s2, Fnet =
22. A 75 kg skydiver falls with constant velocity. What is his weight?
What does Fg mean?
How do you calculate Fg?
What is the net force on him?
What does FN mean?
Fs, Ff
Is he in equilibrium?
FN usually is equal to Fg because
What is the force from air resistance?
23. A 5 kg block is being dragged across the floor in a straight line with a constant speed of 10 m/s. If the pulling force is 25 N, what is the force of friction acting on the block?
What does equilibrium mean?
Forces at an Angle
To calculate horizontal and vertical components of force, use the equations:
10. A little boy pulls a wagon with a force of 10 N at an angle of 30 degrees.
What is the horizontal component of the applied force? Vertical component?
12. Molly the 5 kg dog doesn’t want to go on a walk. You pull on the leash with a force of 15 N at an angle of 20 degrees from the horizontal. How much force of friction does Molly have to apply so that you can’t move her? (Hint: Think of the last problem)
11. You walk an unwilling 10 kg dog by pulling on its leash with a force of 20 N at an angle of 50 degrees from the horizontal.
a. Draw a free body diagram to the right.
b. What is the horizontal component of the pulling force?
13. You push a 7 kilogram desk with a 30 N force pointing downward at 15 degrees.
What is the horizontal component of your pushing force?
c. What is vertical component of the pulling force?
What is the acceleration of the desk? (Assume no friction)
Which force accelerates the dog, vertical or horizontal?
d. Calculate the dog’s acceleration.
14. A 12 kg lawnmower is pushed with a force of 40 N at an angle of 15 degrees below the horizontal.
a. What is the horizontal component of the push?
b. What is the acceleration of the lawnmower (neglect friction)?
14. A 10 N block is accelerating down a frictionless ramp inclined at 20 degrees. Calculate the magnitude of the net force causing this acceleration
•
To get the force down an incline, use sin. •
When going down an incline, think of a road sign (use sin) 15. A 0.5 kg marble is rolling down a 12 degree frictionless incline. Calculate the magnitude of the net force causing it to accelerate.
16. A 600 N sled and sledder slide down an 18 degree hill with constant velocity. Calculate the magnitude of the force of friction acting on the sled