Physics Exam #1 review

Transcription

1 Physics 1010 Exam #1 review

2 General Test Information 7:30 tonight in this room (G1B20). Closed book. Single 3x5 note card with own notes written on it allowed. Calculators are allowed (no memory usage). Worth 40 points total. Multiple choice plus one long hand question.

3 Concept Overview Kinematics - velocity, acceleration, etc. Dynamics Newton s laws Forces of interest gravity, friction, and springs If you are comfortable with these concepts, you can handle any problem on the test!

4 Velocity Velocity is the change of an object s position with respect to time. x is position, v is velocity. v =! x! t Position s 1 s 2 s 3 s 4 s Time Velocity is the slope!

5 Acceleration Acceleration is the change of an object s velocity with respect to time. v is velocity, a is acceleration. Velocity a =! v! t 0 0 s 1 s 2 s 3 s 4 s Time Acceleration is the slope!

6 Kinematic Equations of Motion Velocity as a function of time: v = v 0 + at Similarly for position: x = x 0 + v 0 t + (1/2) at 2

7 Newton s Second Law Fundamental equation of dynamics: F = m a A quantitative generalization of Newton s first law. Acceleration is in the same direction as net force.

8 Newton s First Law An object that is not subject to any outside forces moves at a constant velocity. When an object is not accelerating, it can still be subject to external forces, but the net force vanishes. In the object s frame of reference, there s no way to distinguish uniform motion from sitting still.

9 Newton s Third Law For every force that one objects exerts on a second object, there is an equal and opposite force that the second object exerts on the first. This does not apply to two forces acting on the same object.

10 Gravity Objects of different masses fall at the same rate. The gravitational force on an object must be proportional to its mass. F gravity = mg g! 9.8 m / s 2

11 Friction Frictional forces oppose motion. Two types: static and sliding. Sliding friction force is generally smaller than weight force. Friction forces on an object are related to the weight force on the object.

12 Springs Hooke s law: F =!kx spring The spring force is a restoring force proportional to the displacement of the object. Spring constant k depends on the type of spring.

13 Clicker Question 1 At t=0 s, a 0.5 kg ball is thrown straight up with an initial velocity of 20 m/s. At what time will it reach its maximum height? A. 0.0 s B s C. 1.0 s D s Answer: (+ is up) v f =v i +at v f =0, v i =20m/s, a=-9.8m/s 2 : D (+ is down) v f =v i +at v f =0, v i =-20m/s, a=9.8m/s 2 : D

14 Clicker Question 2 At t=0 s, a 0.5 kg ball is thrown straight up with an initial velocity of 20 m/s. What is the maximum hight it reaches (+ is up)? A m B m C m D m Answer: x=x i +v i t+(1/2)at 2 x i =0, v i =20m/s, a=-9.8m/s 2, t=2.04s C

15 Clicker Question 3 At t=0 s, a 0.5 kg ball is thrown straight up with an initial velocity of 20 m/s. At what height is the ball when its velocity is half its initial value? A m B m C. 4.9 m D m Answer: v f =v i +at v f =10m/s, v i =20m/s, a=-9.8m/s 2, t=1.02s x=x i +v i t+(1/2)at 2 x i =0, v i =20m/s, a=-9.8m/s 2, t=1.02s B

16 Kinematics Tips Pick a positive direction Make sure all your values have the correct sign If question is composite FIND TIME FIRST

17 Clicker Question 4 A woman hangs a 1 kg mass on the end of a spring with k = 200 N/m. How far does the spring stretch from its position before the mass was suspended? (disregard the sign) A. About 2 cm. B. About 5 cm. C. About 10 cm. D. With such a large spring constant, it would hardly move. Answer: mg=-kx, x=-mg/k=-0.049m=-4.9cm B

18 Clicker Question 5 M = 1 kg m = 0.1 kg F =mg A cart of mass of 1kg is attached to a hanging weight of mass 0.1kg. They accelerate at: A. 9.8 m/s 2. B. 0 m/s 2. C. 8.9 m/s 2. D m/s 2. Answer: mg=(m+m)a, a=mg/(m+m) D

19 Clicker Question 6 You are pushing a cart up a ramp. Which vector best represents: The direction of the force of gravity on the cart. Answer: A

20 Clicker Question 7 You are pushing a cart up a ramp. Which vector best represents: The direction of the force the surface of the ramp exerts on the cart. Answer: D

21 Clicker Question 8 You are pushing a cart up a ramp. Which vector best represents: The direction of the force of gravity and the force that the surface of the ramp exerts on the cart, added together. Answer: E