RQ8: A car is driving at a constant speed in a perfect circle on a flat parking lot. The centripetal force acting on the car is: A. Gravity. B. The normal force from the road. C. Friction from the road.

Lecture Quiz 1: When it is directly overhead, why doesn’t the liquid fall in a bucket full of water? Centripetal force pushes it up. Rotating objects have their own gravity. It does fall, but not fast enough to leave the bucket. Because of its high speed, air is pushing it into the glass. Newton’s Third Law.

Quiz 2 You cut the string on a ball that is moving in uniform circular motion. The ball will fly off in a direction a.) along the circle b.) towards the persons arm c.) exactly opposite the persons arm d.) on a line tangent to the circle at the point e.) continue to move along a circular path but in big and big circles to the left.

Fig. 5.3

Fig. 5.4 Centripetal Acceleration: a = v2/r Directed inwards, towards the center. Fig. 5.4

Fig. 5.5 The tension in the string can be divided into two components: Horizontal = centripetal force Vertical = opposes gravity Note: There is no vertical acceleration. Fig. 5.5

Fig. 5.8 Fig. 5.8 Here the NORMAL force from the road has a horizontal component, providing the centripetal force. This ADDS to the frictional force that might be present on a level road.

Lecture Quiz: Question 3 On a Ferris Wheel, when is the NORMAL force on you largest? At the top At the bottom At the sides It must be constant Its magnitude must always equal gravity. Fig. 5.9

Consider again the Ferris Wheel….

Ferris Wheel AT REST N W N N N + W = 0 = ma W W N W There is no acceleration, so no net force on the riders. Weight is exactly cancelled by the Normal force from the seat. N W N N N + W = 0 = ma W W N W

Ferris Wheel IN MOTION Ntop Nbottom Circular acceleration a = v2/r Centripetal force = mv2/r, always directed inward. Weight is constant; Normal force varies. Nbottom

Ntop Nbottom Lecture Quiz: Question 4 What happens if the speed v gets so large that the centripetal acceleration equals g? (a=v2/r = g) The normal force at the top disappears (N=0), and you feel “weightless”. The normal force at the bottom disappears (N=0), and you feel extra heavy. The normal force exactly equals your weight. Ntop Nbottom

Fig. 5.17 Fig. 5.17 Is the Moon falling?

LECTURE QUIZ - Question 5: The Moon is in a circular orbit about the Earth, with a period of about 28 days. The Moon is continuously accelerating towards the Earth. The Moon never falls and crashes into the Earth because no force is pushing or pulling it towards the Earth. The Moon does not fall to Earth because the forces acting on the Earth add up to zero. (They cancel out.)

Fig. 5.19

Fig. 5.p097d

Fig. 5.17 Fig. 5.17 Is the Moon falling?

Fig. 5.19

Fig. 5.p097d

RQ9: The gravitational force exerted by the Earth on the Moon is much larger than the gravitational force exerted by the Moon on the Earth, which is why the Moon revolves around the Earth. A. True B. False