PHY 231 February 1, 2016
Newton’s second law 5.3 -5.4
The Kentucky Department of Motor Vehicles recommends that when riding in a car your position should
not differ from that of the car seat by more than a meter at any time.
1. Is your average velocity the same as the average velocity of the car seat?
2. Can your instantaneous velocity be different from that of the car seat?
3. Harold is riding in a car when Margo stomps on the gas. The speed of the car changes from 0 m/s to
16 m/s (35 mph) in 4 seconds. A PHY 231 student standing on the sidewalk (observes from an inertial
frame!).
*Does Harold accelerate? In what direction?
*What forces are acting on Harold? In what directions?
*How large is the net force acting on Harold?
4. Margo sees a turtle on the road and slams on the brakes. The car’s speed decreases to 0 in 2
seconds. What is the net force acting on Harold? In what direction?
What are seat belts for?
5. If a car travelling 16 m/s encounters a solid object, it will come to a stop. The passengers in the car
cannot move more than 1 m before they come to a stop, too. The best case is constant (negative)
acceleration. What is their acceleration?
What is the net force on a passenger?
6. Suppose the net force on an object is constant and not zero. Describe the motion of the object, if it
starts with zero velocity.
8-1
7. Suppose the net force on an object is constant and not zero. Describe the motion of the object, if it
starts with an arbitrary velocity (in an arbitrary direction).
8, What do Q6 and Q7 tell us about a projectile?
9. All falling objects have the same acceleration. What does this tell us about the force on a falling
object?
10. Here is a cart with excellent wheels on an incline. There is a downward force (called “the weight of
the object) of 100 N. The object is held in place by the force T. What does Newton’s second law tell us
about the forces on the object?
11. Suppose the string breaks in the previous example. Now the force T is absent.
the other forces?
Does this change
12. What will happen when the string breaks?
13. Jerry is standing in a bus when it begins to move. What would happen if he didn’t hold on?
14. How does holding on change the story in Q 13? What does Newton’s second law have to say about
this?
Next Class: Force, weight, and Newton’s Third Law
8-2
Please read 5.5-5.6