Laws of Motion Review

Vocabulary Acceleration Coefficient of Friction Equilibrium Force Free Body Diagram Inertia Kinetic Friction Mass Motion Net Force Newton Rolling Friction Static Friction Terminal Velocity Vector

Categories of Forces Contact – direct contact or touch between objects Long Range – acts at a distance

Types of Forces Pushing a crate across the floor – Applied Air Friction - Drag Opposes motion – Friction Support force – Normal Created in a rope, etc when connected to another mass – Tension Force due to gravity – Weight Are the forces above contact or long range?

Free Body Diagrams Pictorial diagram of all the forces acting on an object The length of the arrow is proportional to the magnitude of the force How is the normal force drawn? How is weight drawn?

Free Body Diagram Practice Draw a FBD for the following: A flowerpot falls freely from a windowsill. A skydiver falls at a constant velocity. A cable pulls a box across a table. The box accelerates even though friction is present. A rope lifts a bucket.

Net Force The vector sum of all forces acting on an object Calculated for both the x and the y Represented by ΣF

Net Force Practice Pictorial diagram of all the forces acting on an object The length of the arrow is proportional to the magnitude of the force

1st Law of Motion Law of Inertia An object at rest will stay at rest, an object in motion will stay in motion unless acted upon by an unbalanced force greater than zero Ex. A car rider continues forward when the driver suddenly applies the brakes

2nd Law of Motion ∑F = ma W = mg Ff = µFn What is the net force? What is the unit for mass? Is mass the same quantity as weight? What is g? What is μ? On a flat surface, what is equal to the Fn ?

2nd Law Practice If all of the forces acting on an object balance so that the net force is zero, then A. The object must be at rest B. The object must be at a constant velocity C. The object may be at rest or at a constant velocity

2nd Law Practice Two astronauts tug on opposite sides of a satellite. The first astronaut tugs to the left with a force of 30 N. With what force does the second astronaut tug in order to keep the satellite moving toward him at a constant velocity of 20 m/s? A. 30 N B. – 30 N C. 0 N D. 50 N E. 1.5 N

2nd Law Practice The maximum (or terminal) velocity of a skydiver (m=70kg) with arms and legs extended is about 120 mi/h (= 53.6 m/s). What force does air resistance exert on the skydiver? A. 0 N B. 13 N C. 70 N D. 700 N E. 1000 N

2nd Law Practice An airplane has a mass of 3.1 x 104 kg and takes off under the influence of a constant net force of 3.7 x 104 N. What is the acceleration that acts on the plane?

2nd Law Practice Weight and mass are A. always the same B. inversely proportional C. based on location D. none of the above

2nd Law Practice A large (150 kg) refrigerator needs to be moved across the kitchen. Fred pulls with a force of 100 N and Wilma pulls with 75 N in the same direction. If the force of friction was 40 N, what will be the acceleration of the refrigerator? A 1.17 m/s2 B 0.90 m/s2 C 0.43 m/s2 D 135 m/s2

2nd Law Practice When a parachutist jumps from an airplane, he eventually reaches a constant speed, called the terminal velocity. This means that a. the acceleration is equal to g. b. the force of air resistance is equal to zero. c. the effect of gravity has died down. d. the effect of gravity increases as he becomes closer to the ground. e. the force of air resistance is equal to the weight of the parachutist.

2nd Law Practice The force of kinetic friction is always __________ than the force of static friction. a. less than b. equal to c. greater than d. varies depending on situation

2nd Law Practice The normal force acts ______________ to the surface of the contact object a. perpendicular b. parallel c. at an angle

3rd Law of Motion For every action, there is an equal and opposite reaction

3rd Law Practice A mass is suspended on a spring. The reaction force to the force of the gravity from the Earth acting on the mass is the force exerted by the A. mass on the Earth B. mass on the spring C. spring on the mass D. spring on the Earth E. Earth on the mass