Circular Motion. A car is traveling around a curve at a steady 45 mph. Is the car accelerating? A. Yes B. No

Transcription

1 Circular Motion A car is traveling around a curve at a steady 45 mph. Is the car accelerating? A. Yes B. No

2 Circular Motion A car is traveling around a curve at a steady 45 mph. Which vector shows the direction of the car s acceleration? E. zero

3 Circular Motion A car is slowing down as it drives over a circular hill. Which of these is the acceleration vector at the highest point? Acceleration of changing speed Acceleration of changing direction

4 HW04

5 Chapter 5: Newton s Laws 1 Forces 2 Newton's First Law 3 Newton's Second Law 4 Mass and Weight 5 Newton's Third Law 6 Common Forces 7 Free-Body Diagrams

6 What is Force? A force is a push or a pull. It is always an interaction between two objects. agent & object; force by A on B SI unit of force is the Newton (N) 1.0 lb = N Force is a vector. Field forces act over a distance. gravity, electric & magnetic Contact forces act at the point of contact (all non-field forces).

7 A ball rolls down an incline and off a horizontal ramp. Ignoring air resistance, what force or forces act on the ball as it moves through the air just after leaving the horizontal ramp? A. gravity on the ball acting vertically down. B. A horizontal force that maintains the motion. C. A force whose direction changes as the direction of motion changes. D. gravity on the ball and a horizontal force. E. gravity on the ball and a force in the direction of motion.

8 Force

9 Force When several forces act on an object, they combine to form a net force given by the vector sum of all the forces: Called the superposition of forces.

10 Kinds of Forces Gravitational force acts toward center of the planet

11 Kinds of Forces Gravitational force Spring force any elastic medium

12 Kinds of Forces Gravitational force Spring force Tension force acts along rope (or cable, chain, etc)

13 Kinds of Forces Gravitational force Spring force Tension force Normal force acts perpendicular to a surface

14 Kinds of Forces Gravitational force Spring force Tension force Normal force Friction force opposes relative motion of surfaces

15 Kinds of Forces Gravitational force Spring force Tension force Normal force Friction force Drag force resistive force of a fluid

16 Kinds of Forces Gravitational force Spring force Tension force Normal force Friction force Drag force Pressure force fluid exerts a perpendicular contact force

17 Kinds of Forces Gravitational force Spring force Tension force Normal force Friction force Drag force Pressure force many others... electromagnetic forces nuclear forces dark energy?...

18 Newton s Second Law The net force on an object with a mass m will cause an acceleration in the direction of the force: Sir Isaac Newton (Jan 4, 1643 Mar 31, 1727) July 1687

19 Newton s Second Law forces are balanced net force is zero: acceleration is zero:

20 Newton s Second Law forces are not balanced total force is not zero crate will accelerate

21 Newton s Second Law

22 Newton s First Law If then. or: If zero net force acts on an object, it will continue at the same speed in the same direction. If an object is at rest, it will stay at rest. If it is moving, it will continue moving with the same velocity. Also known as the law of inertia.

23 Inertia the tendency of an object to not change its motion. indistinguishable from mass sometimes called inertial mass measured in kilograms

24 Inertia It takes the same effort to accelerate this mass on earth and in zero gravity:

25 Newton s First Law Every force has an agent which causes the force. Forces exist at the point of contact between the agent and the object (except for the field forces). Forces exist due to interactions happening now, not due to what happened in the past. A pushing force was required to accelerate the arrow as it was shot. No force is needed to keep the arrow moving forward as it flies. It continues to move because of its inertia.

26 Newton s First Law when acceleration is zero, we say a system is in mechanical equilibrium. essential for the engineering analysis of stationary objects: Statics

27 Newton s First Law The figure shows the view looking down onto a sheet of frictionless ice. A puck, tied with a string to point P, slides on the ice in the circular path shown and has made many revolutions. If the string suddenly breaks with the puck in the position shown, which path best represents the puck s subsequent motion?

28 Newton s First Law A ring, seen from above, is pulled on by three forces. The ring is not moving. How big is the force F (in Newtons)? A. 20 B. 10 cosθ C. 10 sinθ D. 20 cosθ E. 20 sinθ

29 Mass & Weight mass is the amount of matter weight is the force of gravity (on the moon, you have less weight, but you have the same mass) free-fall acceleration near the Earth s surface is -g = -9.8 m/s2 so the net force (F = ma) is mg in general the force of gravity can be written as FG = mg

30 Mass & Weight in this context, g is often called the gravitational field strength g = 9.81 N/kg on Earth g = 1.62 N/kg on the moon g = 24.8 N/kg on Jupiter only in free-fall is g the vertical acceleration

31 Mass & Weight g also varies around the earth 0.7% variation shown (beyond the 0.5% smooth pole-to-pole variation)

32 Newton s Second Law An object on a rope is lowered at constant speed. Which is true? A. The rope tension is greater than the object s weight. B. The rope tension equals the object s weight. C. The rope tension is less than the object s weight. D. The rope tension can t be compared to the object s weight.

33 Newton s First Law

34 Newton s First Law

35 Newton s First Law

36 Free Body Diagram body drawn as a point, no environment all forces shown and labeled careful not to double-count approximate magnitudes n show angles, if helpful f 35 F mg

37 Newton s First Law A car is parked on a hill. Which is the correct free-body diagram?

38 Newton s First Law A car is towed to the right at constant speed. Which is the correct free-body diagram?

39 Newton s Second Law