Newton's First law of Motion

Transcription

1 Newton's First law of Motion Newton's first law of motion is known as the Law of Inertia "Every object continues in a state of rest, or of uniform motion in a straight line at constant speed, unless it is compelled to change that state by forces exerted upon it." N.B. To perform this trick successfully, one must pull the tablecloth down and off the table. To pull the cloth up as in the diagram would be foolish. Inertia is a measure of a body s tendency to resist any changes in its motion. Conceptual Physics 3 rd Edition Paul Hewitt Chapter 4 Newton s first Law of Motion - Inertia Page 1 of 9

2 The inertia of a body depends on the amount of Mass (mass is a measure of inertia). Mass - is the amount of material (matter) present in an object. - is measured in kilograms (kg). Note. Mass is not volume. Mass is not weight. Weight - is a measure of the gravitational force that acts on a body (this also depends on its location). Net Force The net force on an object is the vector sum of all the forces acting on that object. For instance, if you push equally on opposite sides of a book placed on a table will not move, the net force is zero. Conceptual Physics 3 rd Edition Paul Hewitt Chapter 4 Newton s first Law of Motion - Inertia Page of 9

3 Equilibrium All objects on Earth experience the force of gravity (weight) constantly, but they are not all in free fall. This implies that another force must be present keeping these objects from free fall. The force cancelling the force due to gravity is called a normal (support) force. When the net force on an object is zero, the object is said to be in equilibrium. Normal means at right angles to Statics - a close look at the forces that combine to give the Net Force of zero for objects at rest. Conceptual Physics 3 rd Edition Paul Hewitt Chapter 4 Newton s first Law of Motion - Inertia Page 3 of 9

4 Vector Addition When two vectors are acting we need to take both the magnitude and direction into account, for example 1) 10 N 5 N When two parallel forces are acting on the same object in the same direction the result (or resultant) is 15 N East (i.e. simple addition). ) 10 N 5 N When two parallel forces are acting on the same object in the opposite direction the result (or resultant) is 5 N East (i.e. simple subtraction). Note that whenever we represent a vector quantity we need to state both the magnitude and direction to give the complete answer. Conceptual Physics 3 rd Edition Paul Hewitt Chapter 4 Newton s first Law of Motion - Inertia Page 4 of 9

5 Vector Addition Finding the resultant force when two force vectors are not in the same or opposite direction. F = 50 N F 1 = 50 N Scale: 1 cm represents 10 N 45 F 1 (50 N East) F R = F 1 + F F (50 N North) The resultant force is represented by the diagonal of the parallelogram (which in this case is a rectangle). The resultant force has both magnitude and direction. Note that in this case the magnitude of the force (is given by the length of the resultant) can be determined by Pythagoras theorem; F R = F 1 + F F R = 70.7 Newtons F R = 70.7 N at 45 T. Conceptual Physics 3 rd Edition Paul Hewitt Chapter 4 Newton s first Law of Motion - Inertia Page 5 of 9

6 Vector Problems Find the resultant vectors in both magnitude and direction for the following: Resultant (or net) force if the individual forces acting of the object are: a) 5 N East and 5 N East. b) 10 N East and 5 N West. c) 4 N North and 3 N East. d) 3 N West and 4 N South. e) 7 N South and 4 N West. f) What is the resultant force if a force of 6 N West and a force of 8 N South are applied to an object? 6 N 8 N Conceptual Physics 3 rd Edition Paul Hewitt Chapter 4 Newton s first Law of Motion - Inertia Page 6 of 9

7 g) Find the resultant (or net) forces on a body when the forces below are applied. 10 N 3 N Vector Solutions 4 N a) 5 N East and 5 N East. Resultant = 10 N at 90 T b) 10 N East and 5 N West. Resultant = 5 N at 90 T c) 4 N North and 3 N East. b a = 4 N at 0 T b = 3 N at 90 T a c 37 c = a + b c = 5 N at 37 T Conceptual Physics 3 rd Edition Paul Hewitt Chapter 4 Newton s first Law of Motion - Inertia Page 7 of 9

8 d) 3 N West and 4 N South. b a 53 a = 3 N at 70 T b = 4 N at 180 T c c = a + b c = 5 N at 17 T e) 7 N South and 4 N West. c 30 a a = 7 N at 180 T b = 4 N at 70 T c = a + b c = 8.1 N at 10 T b Conceptual Physics 3 rd Edition Paul Hewitt Chapter 4 Newton s first Law of Motion - Inertia Page 8 of 9

9 f) 6 N West and 8 N South. B 6 θ A AC = = 10 N θ = 8 tan 1 = 6 o 53 8 AC = 10 N at 17 T. C 10 N g) 10 N North, 4 N South and 3 N East. 4 N 6 N C B A θ 3 N D AD AB = 10 N North BC = 4 N South CD = 3 N East Conceptual Physics 3 rd Edition Paul Hewitt Chapter 4 Newton s first Law of Motion - Inertia Page 9 of 9 = tan 1 θ = = = 6. 7 N AD = 6.7 N at 6.6 T. o