1.2.3 Chapter 12.2 & 12.3
Sir Isaac Newton (1642-1727) summarized his study of force and motion in several laws of motion.
Newton’s first law of motion – the state of motion of an object does not change as long as the net force acting on the object is zero
· If an object is in motion, it will stay in motion (with the same speed and direction) unless a net force acts on it
· If an object is at rest, it stays at rest unless a net force acts on it
· Sometimes called the law of inertia
· Why we wear seat belts!
· An object’s inertia depends on its mass. The greater the mass means the greater the inertia.
· Example: Soccer ball sits still until you kick it. Friction makes it slow down.
Inertia-tendency of an object to resist any change in its motion
· An object at rest tends to remain at rest
· An object in motion tends to remain in motion with the same direction and speed
· the more mass an object has = the greater its inertia
· the greater the inertia = the greater the force needed to change its velocity
Newton’s Second Law of Motion – the acceleration of an object is equal to the net force acting on it divided by the object’s mass
· Acceleration is always in the direction of the force
· Acceleration is determined by the size of the force and the mass of the object
· Larger force = greater acceleration
· i.e. the harder you throw a ball, the more the ball accelerates
· Larger mass will require greater force to achieve the same acceleration (i.e. softball vs. baseball)
· Expressed in equation: Force = mass x acceleration or Acceleration = Net force/Mass
· Fg = mg same as F=ma just that “a” refers to acceleration due to gravity which is 9.8 m/s/s
F = ma a = F/m m = F/a
Units: F = kg . m/s2 1N = 1 kg . m/s2
m = kg a = m/s2
Falling objects-
· Because of gravitational attraction, an object near Earth’s surface falls with an acceleration of about 9.8 m/s2 in a vacuum (regardless of mass).
· However, the observed acceleration of an object is usually less because of the effects of air resistance.
Newton’s Third Law of Motion – Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object
· Also called action-reaction law
· i.e. bumper cars, balloon on a string, pressing your hand against a wall
· The two forces are equal in size and opposite in direction (do not produce a net force of zero because they act on different objects)
· Not all action-reaction forces produce motion
http://www.youtube.com/watch?v=D9wQVIEdKh8
Samples:
1. Inertia is the tendency of an object to continue its current state of motion. Which of the following best describes the relationship b/w inertia and mass?
A. a greater mass means that something is easier to slow down or speed up.
B. The amount of inertia something has is equal to its mass.
C. The greater the mass of an object, the greater the inertia it has.
D. The greater the mass of an object, the less inertia it has.
2. Newton’s first law of motion describes the tendency of objects in motion to continue in motion and the objects at rest to remain at rest. What term is used to describe this behavior?
A. velocity b. acceleration c. displacement d. inertia
3. What concept is being illustrated in the image to the left?
A. W=PxT b. inertia
c. F=ma d. velocity
4. A passenger in a car that suddenly stops will
a. lean forward b. lean backward
c. lean to the right d. feel no motion.
5. Identify the force needed to accelerate a car from 0m/s to 30m/s in 10s if the mass of the car is 2000kg.
A. 6.7N b. 667N c. 6,000N d. 600,000N
6. Dennis was riding his bike down a hill and he ran straight into a mailbox. Identify the statement that most closely describes Dennis’ motion immediately following his crash.
A. He flies over the handlebars. B. He falls off the back of his bike.
C. He falls sideways off his bike. D. He flies upward into the air.
7. How much force is required to give an object with a mass of 20kg an acceleration of 12 m/s2 ?
8. Jamond is sitting on a raft with a basketball next to his feet. He rows his raft across the river. If he hits the dock head-on to stop, what will happen to the ball?
A. It will roll backwards.
B. It will roll forward toward the front of the raft.
C. It will roll sideways.
D. It will stop along with the raft.
9. The weight of an object is less on the moon b/c
A. objects on the moon experience a greater acceleration due to gravity.
B. objects on the moon experience a lesser acceleration due to gravity
C. there are fewer frictional forces on the moon.
D. there are more frictional forces on the moon.
10. J.R. hits a baseball w/ an acceleration of 40 m/s2 . The mass of the baseball is 0.5kg. What is the force of the swing?
11. Caitlin hits a golf ball that has a mass of 0.45kg. The acceleration of the ball is 41 m/s2 . What is the force of Caitlin’s swing?
12. What is the acceleration of a 500kg crate when it hits a wall w/ a force of 1000N?
13. 4 racing cars are equipped with equally powerful engines. Which one of the racing cars described below will accelerate the faster as their engines provide the same net force?
A. 1000kg car B. 900kg car C. 800kg car D. 700kg car
14. An acorn fell from the tree to the ground. The force of gravity creates a constant acceleration of 9.8 m/s2 . What was the acorn’s final velocity if it fell for 3 seconds? A. 29.4m/s B. 9.8m/s C. 19.6m/s D. 96.0m/s
15. If the force acting on an object equals 150N and the mass of the object is 0.04kg, what is its acceleration?
A. 190 m/s2 B. 110 m/s2 C. 45 m/s2 D. 3,750 m/s2
16. If a 9N force is applied to an object and its acceleration is 3.3m/s/s, what is the mass of the object?
A. 0.35kg B. 2.7kg C. 5.7kg D. 12.3kg
17. If a 5N force acting on an object is 750N and the object is accelerating at 5 m/s2 , what is the mass of the object?
A. 745kg B. 150kg C. 8kg D. 1.5kg
18. A net force of 18N causes a rolling cart to accelerate at a rate of 4.5 m/s2 . What is the mass of the cart?
A. 4kg B. 6.3kg C. 8.1kg D. 13.5kg
19. Using the graphic below, how many Newtons?
20. When I am standing in front of the class, the force of gravity is pulling me towards the ground. The ground is pushing back with an equal and opposite force. This is an example of
A Newton’s 1st law. B. Newton’s 2nd law
C. Newton’s 3rd law D. the law of gravitation.
21. When an ice skater pushes the skate blade back against the ice, the skater glides forward. This is explained by which one of the following:
A. the law of inertia
B. a body in motion tending to stay in motion
C. every action having an equal & opposite reaction
D. negative acceleration being = to positive acceleration.
22. Which statement most correctly describes the diagram to the left?
A. Acceleration is proportional to the net force on an object.
B. For every action, there is an equal and opposite reaction.
C. Acceleration is inversely proportional to the mass of an object.
D. Momentum is proportional to the mass of an object.
23. A box weighing 40N rests on the floor. What is the net force on the box? A 0N B. 1N C. 40N D. 80N
24. A book resting on the table exerts a 10N force. What force does the table exert on the book?