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Newton's First Law of Motion

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Presentation on theme: "Newton's First Law of Motion"— Presentation transcript:

1 Newton's First Law of Motion
An object will remain at rest or move in a straight line at a constant speed if there are no forces acting on it. = Uniform motion with constant velocity is The natural state of an object.

2 Newton’s second Law Fnet = ma [kg m/s2 = 1N] To understand net forces, we need to know free-body diagrams.

3 Normal Force

4 Q1 The weight of the box is 70kg and the pulling force is 500N, the normal force will be about 200N pointing up; 200N pointing down; 1200N pointing down; 0N.

5 Q2 Adam stands in an elevator that is accelerating upward. The normal force acting him is A) larger than 500N; B) smaller than 500N; C) equal to 500N; D) Not enough information.

6 Newton’s third law Forces always come in action/reaction pairs
following the 3rd law: 1) Two members act on different objects; 2) They point in opposite directions; and are equal in magnitude.

7 Weight of a person in an elevator

8 Friction forces

9 Coefficients of friction
Static: S= max(fs)/n fs   S n Kinetic: K= fk /n fk =  K n,  S >  K

10 Q3 See page 114, “Stop to think 4.3”

11 Q4 You are pressing a book (m=1kg) against a wall. The static friction coefficient is 0.4. Which of the following is true for the friction on the book? The friction force is pointing downward; B) The friction force is pointing upward; C) The friction force is pointing away from the wall; D) The friction force is pointing into the wall.

12 Q5 You see that a box is at rest on the top of a car while the car is speeding up. What is the direction of friction force acting on the box? A) forward; B) backward; C) zero; D) depending on the velocity of the car.

13 Interacting objects (textbook, page 121 more on the 3rd law)


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