# FORCE and Newtons Laws of Motion Definitions of

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FORCE and Newton’s Laws of Motion

Definitions of Force is a push or a pull acting on an object. Force is a vector. (magnitude & direction) Force can be contact or long range (aka non-contact or field).

Types of Forces Contact Forces Long Range (Field) Forces Friction (Ff) Gravity or weight Tension (FT) (FW or Fg) Normal (FN) Electrical (FE) Spring (FS) Magnetic (B) Air Resistance (FR) Applied (FA)

More on Force… Forces can change the motion of an object SI unit = newton (N) Net force is the vector sum of all forces acting on ONE object. Since force is a vector, the net force will have magnitude and direction.

More on Net Forces in same direction: add Net force in same direction as forces. See picture 2 below Forces in opposite directions: subtract. Net force in direction of larger force. See picture 3 below Equal forces in opposite directions : subtract. Net force is zero See picture 1

What is the net force acting on each object below? http: //www. slideshare. net/meenng/forces

Balanced forces Net force = 0 Balanced forces do NOT cause any change in an object’s motion.

Balanced forces Net force = 0

Unbalanced force Net force ≠ zero Causes an object to change its motion Many unbalanced forces causing motion: video

Balanced and unbalanced forces, cont. Two tugboats are moving a barge. Tugboat A exerts a force of 3000 Newtons on the barge. Tugboat B exerts a force of 5000 Newtons in the same direction. What is the combined force on the barge? Draw arrows showing the individual and combined forces of the tugboats.

Balanced and unbalanced forces, cont. Now suppose that Tugboat A exerts a force of 2000 Newtons on the barge and Tugboat B exerts a force of 4000 Newtons in the opposite direction. What is the combined force on the barge? Draw arrows showing the individual and combined forces of the tugboats.

Balanced and unbalanced forces, cont. Could there ever be a case when Tugboat A and Tugboat B are both exerting a force on the barge but the barge doesn't move? Draw arrows showing the individual and combined forces in such a situation.

st Newton’s 1 Law n Galileo’s Law of Inertia: An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force. Concerned with the force on ONE object The motion of an object changes only when there are unbalanced forces (Net force is NOT zero)

Newton’s 1 st Law, cont. What two kinds of motion can an object have if the forces are balanced? 1) An object at rest stays at rest 2) An object in motion stays in straight line motion at a constant speed (constant velocity)

Balanced Forces said another way… Equilibrium: Net Force = Zero 1) Static Equilibrium o. Net force = zero o. No motion 2) Kinetic Equilibrium o. Net force = zero o. Constant speed in a straight line (constant velocity)

st Newton’s 1 Law Summary - Also known as the Law of Inertia - Inertia is an object’s resistance to change in motion - More mass = more inertia!

st Newton’s 1 Law & You Don’t let this be you. Wear seat belts! Because of inertia, objects (including you) resist changes in their motion. When the car going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 m/hour.

nd Newton’s 2 Law Net Force equals mass times acceleration. n Fnet = ma Fnet = net force (N) m = mass (kg) a = acceleration (m/s 2)

nd Newton’s 2 Law and Acceleration due to gravity (g) On Earth’s surface: g = 9. 8 m/s 2 On the moon’s surface: g = 1. 6 m/s 2

nd Newton’s 2 Law and Weight Symbol = FW or Fg Weight is the force of gravity on an object. Unit – Newton (N) Formula FW= mg

Examples n n 1. What acceleration will result when a 12 N net force applied to a 3 kg object? 2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s 2. Determine the mass. n 3. How much force is needed to accelerate a 66 kg skier 1 m/sec 2? n 4. What is the force on a 1000 kg elevator that is falling freely at 9. 8 m/sec 2?

Check Your Understanding n 1. What acceleration will result when a 12 N net force applied to a 3 kg object? Given FNet = 12 N m = 3 kg Unknown a=? Equation FNet = ma a = FNet/m Substitute a = 12 N /3 kg Solve 4 m/s 2 2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s 2. Determine the mass. Given Unknown Equation Substitute Solve 3. 2 kg FNet = 16 N m=? FNet = ma m = 16 N /5 m/s 2 n a = 5 m/s 2 n 3. How much force is needed to accelerate a 66 kg skier 1 m/s 2? Given m = 66 kg a = 1 m/s 2 n m = FNet /a Unknown FNet = ? Equation FNet = ma Substitute FNet = 66 kg(1 m/s 2) Solve 66 N 4. What is the force on a 1000 kg elevator that is falling freely at 9. 8 m/s 2? Given Unknown Equation Substitute Solve 2 9800 N m = 1000 kg F =? F = ma F = 1000 kg(9. 8 m/s ) a = 9. 8 m/s 2 Net Net

rd Newton’s 3 Law For every action force there is an equal and opposite reaction force. n ◦Forces are always in balanced pairs ◦TWO objects acting on each other https: //www. youtube. com/watch? v=8 b. Td. Mm. NZm 2 M&index=4&list=PLZHDrmpjw 4 Mx. SWe. F 9 fklm_Agv. M mzk. L 2 I 7

rd Newton’s 3 Law, cont. According to Newton, whenever objects A and B interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body.

Newton’s 3 rd Law, more examples The finger pushes on the wall and the wall pushes on the finger. https: //karlamamporte. wordpress. com/ http: //web. eng. fiu. edu/mathmatters/m 2 ed/newtons 3 rdlaw/index. htm The rocket pushes on the gases and the gases push on the rocket. http: //www. bbc. co. uk/schools/gcsebitesize/science/triple_ocr_gateway/space_for_reflection/action_reaction/revision/1/

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