Class Physics I 111010

Common errors from major quiz

Units calculations

What do you get if you divide 14 kg by 7 (m^2 / s^3)?

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Find (5 m/s + 9 m/s) * 14 kg.  Does this result have units of energy?

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Find ( (3 m/s) / 5 s ) * 12 kg.  What quantity has the same units as the result?

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Are the units of vf^2 the same as the units of v_0 * `dt?

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Are the units of `ds the same as the units of v0 * `dt * a?

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Applying definitions

Define the following:

average rate of change

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average velocity

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average acceleration

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force

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work

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kinetic energy

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potential energy 

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net force

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Identifying quantities

Give possible units for each of the quantities v0, vf, `ds, a, `dt, vAve, `dv.

Identify which of the quantities v0, vf, `ds, a, `dt, vAve, `dv are given in each of the following situations:

A car travels 40 meters down a hill, starting at velocity 5 m/s and increasing velocity by 12 m/s.

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A ball rolls 60 cm down an incline, starting from rest and changing velocity at a constant rate of 10 cm/s^2.

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A runner starts at the 50-yard line.  8 seconds later, after having traveled 70 yards, he arrives at the goal line.

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An arrow leaves the bow traveling at 60 meters / second.  3 second later it strikes its target, traveling at 55 meters / second.

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A swimmer pushes off the wall at 3 meters / second, and begins her stroke after traveling 5 meters, at which point her velocity is 2 meters / second.

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Two masses are suspended from a pulley.  The lesser mass is given an upward velocity of 20 cm/s, and its acceleration is -6 m/s^2.  When does it reach the point 50 meters above its original position?

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A ball is released from rest at a height of 3 meters above the floor, and falls freely to the floor.

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Projectiles

A 70 gram ball is shot from a spring gun resting on a level tabletop, clears the edge of the table and falls to the floor.  Its velocity when leaving the gun is 5 meters / second. 

Is that initial velocity in the horizontal or the vertical direction?

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What is the nature of the net force on the ball after losing contact with the gun?  Specify the source of this force.

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What is the ball's acceleration after leaving the gun?  Specify the magnitude and direction of this acceleration.  Assume air resistance to be negligible. 

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Is the acceleration of the ball in the same direction as its initial velocity?

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Is the acceleration of the ball in the same direction as its horizontal displacement?

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Is the acceleration of the ball in the same direction as its vertical displacement?

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Knowing equations

List the four equations of uniformly accelerated motion, and state which two come directly from the v0, vf, `dt trapezoid.

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List the four equations of uniformly accelerated motion, and state which two come directly from the definitions of average velocity and average acceleration.

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Solving equations

Solve the first equation of motion for vf.

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Solve the second equation of motion for a.

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Solve the third equation of motion for a.

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Solve the fourth equation of motion for `ds.

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Force and work\energy

What is the speed of a 60 gram arrow whose kinetic energy is 3 million gram cm / sec^2?

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If a 60 kg runner speeds up from 3 m/s to 5 m/s, how much work is done by the net force acting on her?

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Masses of 5 kg and 6 kg are suspended from a pulley.  How much force does gravity exert on each, and what is the net force on this 11-kg system?

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How much work does gravity do on a 15 kg block as it drops from a height of 140 cm to a height of 20 cm?

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How much work does gravity do on the same block if it rises from 20 cm to a height of 80 cm?

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To load a spring gun I exert an average force of 50 Newtons through a distance of 20 cm.  How much work do I do?

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A 70 gram ball is shot from the spring gun in the preceding question.  How fast will that ball be traveling as it leaves the gun, if the energy imparted to it by the spring is equal to the work I did to compress the spring?

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