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Conceptual Physics Exam 2 – spring 2008 Kinematics
First Law Every material object continues a constant state of rest, or of uniform motion, (velocity) in a straight line, unless it is compelled to change that state of motion by forces acting upon it. Inertial – the property of bodies to resist having their state of motion or velocity changed when acted on by a force. Mass – The quantity of matter in an object. More specifically, mass is the measurement of inertia exhibited by a body. Second Law The acceleration of an object is directly proportional to the net force acting on the object, is in the direction of the net force, and is inversely proportional to the mass of the object.
Force – a push of pull that acts to change a body motion or velocity in either direction or magnitude. These can be caused by gravity, electromagnetism, contact with another object, muscles in our bodies, or other possibilities. Weight – the gravitational force exerted on an object by the Earth when on the Earth’s surface. Net force is the sum, vector sum, of all the forces acting on a body. Two opposing forces of equal magnitude will cancel each other out. Forces applied perpendicularly to the velocity will act to change the direction of motion. Drag – force from the atmosphere that acts to slow objects down and increases with velocity. Friction – force from contact with a surface that acts to slow objects down. Friction does not increase with velocity but it does increase with the contact force (usually weight of the object). Third Law For every action or force acting on body A from B, there is an equal and opposite reaction or force acting back on B from A.
Impulse
Momentum
Impulse and momentum
Work
Kinetic Energy
Gravitational Potential Energy
Work and Energy
Momentum and Energy Momentum – the combination of motion and inertial, or mass
Impulse – force times the time the force is applied Conservation of
momentum Net momentum before the
collision = Net momentum after the collision Work – force time the distance traveled while the force is applied to the body Work = force
* distance Potential Energy – energy associated with the position of a body, like the height for gravitation potential energy. PE = mgh Kinetic Energy – energy associated with the velocity of a body. KE = ½mv2 Work Energy Theorem Work = ΔKE + ΔPE Energy Conservation – energy cannot be created or destroyed; it may only be transformed from on form into another, but the total amount of energy never changes. Power – work done divided by the time involved in completing the work.
Efficiency
Gravity and Satellite Motion Universal Law of Gravitation
The universal Gravitational Constant G = 6.67 X 10-11 Nm2/kg2 Inverse Square Law – The intensity of the gravitational field, or other force fields or energy intensity, fall off with the square of the distance. Weight and weightlessness – a body feels weightless because it is in freefall, not because there is no force of gravity acting on it. Also a body has mass even when it is weightless. Projectile Motion – the motion of projectiles is parabolic. The horizontal motion is constant velocity while the vertical motion is constant downward acceleration. Satellite Motion
– satellites are in constant freefall.
Recall
Descriptions of the orbits of various satellites we will talk about in class. Torque Centripetal Force Angular Momentum Universal Law of
Gravitation Inverse-square Law Kepler’s Laws 1. Planets travel in elliptical orbits 2. Orbital radius sweeps out equal areas in equal times 3. |
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Sample Questions 1. A car crashes into a wall at 25 m/s and is brought to rest in 0.1 seconds. What is the average force exerted on a 75 kg test dummy by the seat belt? 2. If your friend pushes a lawnmower four times as far as you do while exerting only half the force, who does more work? a. You b. Your friend c. You both do the same amount of work d. Cannot be determined 3. * Two people who weigh the same climb a flight of stairs. The first person, Jim, climbs in 30 seconds. The second person, Jan, climbs in 40 seconds. Which person generates more power? (Note, this is asking a different question from the last question.) a. Jim b. Jan c. They both generate the same power d. Cannot be determined 4. Which, if either, has more kinetic energy: a 1 kg ball moving at 2 m/s, or a 2 kg ball moving at 1 m/s? a. The 1 kg ball b. The 2 kg ball c. Both have the same energy d. Cannot be determined 5. The second floor of a house is 6 m above street level. How much work is required to lift a 300 kg refrigerator to the second floor level? 6. When you whirl a can at the end of a string in a circular path, what is the direction of the force you exert on the can? 7. You have two spherical balls that appear identical and have the same mass, but one is hollow while the other is solid. To determine which is hollow you role the two spheres down an incline. Which should reach the bottom first? a. The hollow sphere b. The solid sphere c. They should both reach the bottom at the same time d. Cannot be determined 8. Suppose you have a bicycle wheel with a circumference of 2 meters. What is the linear speed of the bicycle when the wheels rotate at 1 revolution per second? 9. The intensity of light from a central source varies inversely as the square of the distance. If you lived on a planet only half as far from the Sun as our Earth, how would the light intensity compare with that on Earth? a. It would be twice as intense b. It would be half as intense c. It would be four times an intense d. It would be one forth as intense e. I would be 16 times as intense 10. Spring tides are larger in size and occur when a. The sun and moon are lined up with the Earth b. The moon is located 90 degrees away from the sun c. When Jupiter is lined up with the moon d. It is not known what causes neap tides 11. The Earth’s seasons are caused by a. The angle of the Earth’s spin axis with the orbital plane is tilted over b. The Earth is closer to the sun during the summer months c. The tidal currents increase in the summer months bringing warm air north and south d. The moon block solar energy from the Earth during the winter months |
