Units. Example Problem 1. Time (s) Distance (m)

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1 Time (s) Distance (m) Units Speed: average speed and Instantaneous speed (m/s) Velocity (m/s) +direction Acceleration (m/s 2 ) Force (N) or lbs Inertia (Kg) Density (g/ml) Example Problem 1 Which of the following is NOT an appropriate unit of measurement for speed? A. cm/s B. km/hr C. ft/year D. kg/s E. miles/month 1 2 1

2 Example Problem 2 A car accelerates along a straight road, which is 15 km long. The car reaches a speed of 75 km/h in 10 s. The average acceleration of the car is A. 2.1 m/s 2. B. 7.5 m/s 2. C. 1.5 m/s 2. D. 5.0 m/s 2. Example Problem 3 The speed of light in space is 3.00 x 10 8 m/s. The time for light from the Sun to reach Earth is 8 minutes 20 seconds. Therefore, the distance from the Sun to Earth is A x 10 9 m. B x m. C x m. D x 10 9 m

3 Force simply a push or a pull Net force combination of all forces that act on an object changes an object s motion Net Force Motion: free fall d = Δv 2 t Where v = gt d= ½ g t

4 Example Problem 5 Example Problem 6 A bowling ball of mass 7 kg is dropped from the top of a tall building. It safely lands on the ground 3.0 seconds later. Neglecting air friction, the height of the building is A. 44 m. B. 24 m. C. 15 m. D. 29 m. A bowling ball of mass 7 kg is dropped from the top of a tall building. Neglecting air drag, the speed of the ball when it strikes the ground is A. 29 m/s. B. 10 m/s. C. 44 m/s. D. 88 m/s

5 Galileo s Concept of Inertia Galileo s Concept of Inertia Experiment: Balls rolling down inclined planes and then up others tend to roll back up to their original heights. The tendency of a moving body to keep moving or to remain at rest is natural every material object resists change in its state of motion. This property of things to resist change is called inertia

6 Galileo s Concept of Inertia Italian scientist Galileo demolished Aristotle s assertions in early 1500s. In the absence of a force, objects once set in motion tend to continue moving indefinitely. Inertia Inertia of non-moving objects Inertia of moving objects

7 Inertia How do we measure inertia? All things are made of atoms And all atoms have Inertia Mass Mass A Measure of Inertia Quantity of matter in an object Measure of inertia or sluggishness that an object exhibits in response to any effort made to start it, stop it, or change its state of motion in any way Ex: Temperature Degrees (ᵒC, K, ᵒF) The units of Inertia are: Kilograms (Kg)

8 Mass A Measure of Inertia The amount of inertia possessed by an object depends on the amount of matter the amount of material that composes it its mass: greater mass greater inertia smaller mass smaller inertia Mass A Measure of Inertia Mass versus volume: Mass involves how much matter an object contains Volume involves how much space an object occupies

9 Mass A Measure of Inertia CHECK YOUR NEIGHBOR The concept of inertia mostly involves A. mass. B. weight. C. volume. D. density. Explanation : Anybody get this wrong? Check the title of this slide! :-) Physical Science, 6e Chapter 2 Newton s laws

10 Newton s Laws of Motion The law of inertia: (originating with Galileo) Every object continues in a state of rest or of uniform speed in a straight line unless acted on by a nonzero force. F net = 0 Newton s First Law (law of inertia) The law of acceleration: The acceleration produced by a net force on an object is directly proportional to the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object. F net 0 acc = net force mass An object at rest tends to stay at rest and an object in motion tends to stay in motion unless acted upon by an unbalanced force

11 Newton s First Law (law of inertia) INERTIA is a property of an object that describes how much it will resist change to the motion of the object more mass means more inertia 1 st Law (Inertia) Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever

12 Examples of Inertia Inertia

13 Newton s Newton s Second Law 2 nd Law Force equals mass times acceleration. F = ma

14 Balanced Force Unbalanced Forces Unequal opposing forces produce an unbalanced force causing motion Equal forces in opposite directions produce no motion

15 Newton: 2 nd law Example 1: What resultant force will give a 3 kg mass an acceleration of 4 m/s 2? F (N) = m (kg) a (m/s 2 ) Example 2: A cart and driver have a mass of 120 kg. What force F is required to give an acceleration of 6 m/s 2 with no friction? 3 kg a = 4 m/s 2 F =? Remember F = m a 2 F = (3 kg)(4 m/s ) F = 12 N

16 Example 3: What is the mass of a block that has an acceleration of 2 m/s 2 and a resulting force of 12 N?? kg a = 2 m/s 2 F = 12N Newton's Second Law of Motion When acceleration is g free fall. Twice the force on twice the mass same acceleration as half the force on half the mass. Remember consistent units for force, mass, and acceleration in all problems

17 Weight and mass are not the same WEIGHT is a measure of the force of gravity on the mass of an object measured in Newtons or lbs Mass vs Weight Mass is not the same as Weight Inertia Atoms/matter Force of gravity on an object Gravity: Earth Moon 9.8m/s m/s 2 Weight= _Kg(9.8m/s 2 ) _Kg(1.6 m/s 2) Weight = mass (kg) gravity (m/s 2 ) The units of weight are Newtown (N) or pounds (lbs) Newtons /4.4 lbs Ex: 70 kg what my weight in N, lbs.? 17

18 Mass and weight are proportional, as one increases the other increases too or vice versa Weight = mass (kg) gravity (m/s 2 ) The units of weight are Newtowns (N) or pounds (lbs) 4.4N= 1 lbs Ex: 70 kg what my weight in N, lbs.? x9.8 /4.4 Kg N lbs Example Problem 7 An astronaut (with equipment) has a mass of 125 kg on Earth. On the moon (g = 1.67 m/s 2 ), her mass and weight would be A. 125 kg and 1225 N. B. 125 kg and 209 N. C. 21 kg and 209 N. D. 125 kg and 125 N. 150lbs how much Inertia?

19 Weight and Mass: Examples Example 4: What is the weight of a 10-kg block? 10 kg m 9.8 m/s 2 W W = mg = (10 kg)(9.8 m/s 2 ) Newton's Second Law of Motion CHECK YOUR NEIGHBOR A 5-kg iron ball and a 10-kg iron ball are dropped from rest. For negligible air resistance, the acceleration of the heavier ball will be A. less. B. the same. C. more. D. undetermined

20 Always Remember!! In Physics, the use of Newton s second law and many other applications makes it absolutely necessary to distinguish between mass and weight. Use the correct units! Metric SI units: Mass is in kg; weight is in N. Summary Newton s First Law: An object at rest or an object in motion at constant speed will remain at rest or at constant speed in the absence of a resultant force. Newton s Second Law: A resultant force produces an acceleration in the direction of the force that is directly proportional to the force and inversely proportional to the mass. N = (kg)(m/s 2 )