Reading Quiz A cannonball is dropped from a tower. As it falls,

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Presentation transcript:

Reading Quiz A cannonball is dropped from a tower. As it falls, the kinetic energy increases, while the potential energy decreases. the kinetic energy and potential energy increase. the kinetic energy and potential energy decrease. the kinetic energy decreases, while the potential energy increases.

WORK = Force times Distance A crate is moved a distance d across a concrete floor under the influence of a constant horizontal force F. WORK = Force times Distance Can work be negative?

The work done on an object by the net force acting on the object results in an increase in the object’s kinetic energy. What about friction?

Lecture Quiz: Question 1 A ball of mass m=1/4 kg is initially at rest at a distance of h=5 meters above the ground. It is let go and falls (g=10 m/s2). What was the ball’s speed just before hitting the ground? 0.5 m/s 1.0 m/s 5.0 m/s 10 m/s more than 10 m/s

Lecture Quiz: Question 2 A ball of mass m=1/4 kg is initially at rest at a distance of h=5 meters above the ground. It is let go and falls (g=10 m/s2). Just before hitting the ground, what is its kinetic energy? 1.25 J -1.25 J 12.5 J -12.5 J 25 J

Lecture Quiz: Question 3 A ball of mass m=1/4 kg is initially at rest at a distance of h=5 meters above the ground. It is let go and falls (g=10 m/s2). How much work was done by gravity on the ball during the 5 meter fall? zero 1.25 J 2.5 J 12.5 J 25. J

Lecture Quiz: Question 4 A ball of mass m=1/4 kg is thrown straight up with an initial speed of 4 m/s. How high does it rise? (Hint: use conservation of energy.) 0.4 m 0.8 m 4 m 5 m 8 m

Lecture Quiz: Question 8 A box of books with mass m = 50 kg is at rest on the floor. You push on it horizontally with a force of 200 N for 2 seconds. After that time the box is at rest again, but has moved d= 1.5 meters. How much work was done on the box? none B. 300 J C. -300 J

Important concepts Work = force x distance Kinetic energy KE = (1/2)mv2 Total work done = change in KE Potential energy from gravity PE = mgh Energy is conserved: E = KE + PE = constant! (Some restrictions apply!!)

Conservation of mechanical energy: If there are no forces doing work on a system, the total mechanical energy of the system remains constant. If W=0, E = PE + KE = constant.