Presentation on theme: "Reading Quiz A cannonball is dropped from a tower. As it falls,"— Presentation transcript:
1 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.
2 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 DistanceCan work be negative?
3 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?
4 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/s1.0 m/s5.0 m/s10 m/smore than 10 m/s
5 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 J12.5 J-12.5 J25 J
6 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?zero1.25 J2.5 J12.5 J25. J
7 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 m0.8 m4 m5 m8 m
8 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?noneB. 300 JC J
9 Important conceptsWork = force x distanceKinetic energy KE = (1/2)mv2Total work done = change in KEPotential energy from gravity PE = mghEnergy is conserved:E = KE + PE = constant! (Some restrictions apply!!)
10 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.
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