Forces and the Laws of Motion

Forces and the Laws of Motion - Transcript

Mass VS Weight
Mass is the quantity of matter in an object Mass is measured in kilograms kg



Mass is different from weight Weight is the force of gravity on an object It is an object s gravitational pull to the earth

Force






Force is any interaction between an object and its environment Measured in Newtons 1 kg 9 8N If you know the mass of an object measured in kilograms and you want to know its weight in Newtons just multiply the mass by 9 8 N kg Weight mass acceleration due to gravity g W mg Weight N mass kg x g N kg g is always 9 8 N kg

Find the weight in Newtons
1 A can of soup with a mass of 2 kg 2 A pillow with a mass of kg 3 A person with a mass of 45 kg 4 A car with a mass of 230 kg If a 1 kg bag of nails weighs 9 8 N at the earth s surface is it true that 1 kg of orange juice weighs 9 8 N

1 lb 4 448 N
If you know the weight of an object in lbs and want to find the weight in Newtons just multiply the weight in lbs by 4 448 Find the weight of the following objects in Newtons 1 1 lb of lunch meat 2 A person who weighs 125 lbs 3 An 8 lb bowling ball


Force of gravity






The force of gravity increases as you near earth s center The force of gravity can be defined as g GME r2 G is the universal gravitational constant 6 67 x 10 11 Nm2 kg2 not to be confused with little g r is the distance from the object to earth s center At high altitudes the force of gravity is a bit smaller so objects at high altitudes will weigh a bit less than they normally do at sea level

Contact Forces and Field Forces


Contact forces result from physical contact between two objects Field forces do not involve physical contact between two objects An example is gravitational force when an object is in free fall Earth exerts some force on the object pulling it to the ground but there is no direct contact between the object and earth when it is in air

Do Now 1 27 09
1 Find the weight in Newtons of the following objects
1 kg of yogurt 5 kg book bag A person who weighs 176 lbs A dog with a mass of 10 kg

2 Give an example of a contact force Give an example of a field force 3 List an example of each of the following
A force causing an object to start moving A force causing an object to stop moving A force causing an object to change its direction of

motion

Net Force and Newton s First Law




Net force is the combination of all of the forces acting on an object It is the vector sum of the forces acting on an object Newton s First Law of Motion An object at rest remains at rest and an object in motion continues in motion with constant velocity that is constant speed in a straight line unless the object experiences a net external force

Newton s First Law of Motion
When an object does not experience a net external force that means that its net external force is zero When the net force is zero the object does not have an acceleration which means that it continues at a constant velocity or remains at rest Newton s First Law is also called the law of inertia because inertia is the tendency of an object not to accelerate it is an object s resistance to a change in its motion Mass is a measure of the inertia of an object An object that has a large mass has a large inertia The greater the mass of an object the less the object accelerates under an applied force


Do Now 1 28 09
1 Explain the difference between mass and weight 2 What is Newton s First Law of Motion Give the definition and then describe it in your own words 3 What is the definition of net force 4 If the net force acting on an object is zero is the object at rest or in motion or possibly both

Do Now 1 29 09


In packet p 129 1 3 You do not have to rewrite the question

1 Mary leaves her suitcase on a hill while she unloads the rest of her car The hill is at a 55 angle with the horizontal The following forces act on her suitcase Fgravity on suitcase Fg 34 N Ffriction Ff 15 N Fground on suitcase Fground 20 N Fnet Draw a free body diagram of the situation Then find the net force acting on the suitcase

2 A traffic light weighing 100 N hangs from a vertical cable tied to two other cables that are fastened to a support The upper cables make angles of 37 and 53 with the horizontal The following is a free body diagram for the knot the joins the cables
T2
37 53

T1

T3 100 N
Find T1 and T2 Hint The traffic light is in equilibrium so Fx Fy 0

Do Now 2 4 09

1 In your own words define Newton s First Law of Motion 2 In your own words define Newton s Second Law of Motion 3 A 0 8 kg football is thrown across the field with a net force of 85 N What is the acceleration of the football 4 An 20 kg crate is pushed across the floor with a force of 50 N If the crate maintains a constant velocity what is the acceleration of the crate What is the net force acting on the crate

1 Define Newton s Third Law of Motion in your own words 2 The forces acting on a sailboat are 400 N north and 160 N west If the boat has a mass of 320 kg what are the magnitude and direction of the boat s acceleration Hint Find the magnitude and direction of the net force then plug your answer into the equation a Fnet mass 3 If a small sports car collides head on with a massive truck which vehicle experiences the greater impact force Which vehicle experiences the greater acceleration Explain your answers

Do Now 2 5 09

Fn

TV
Fg
The normal force is a force that acts on a surface in a direction perpendicular to the surface

Fn

Fn mg cos

Fg



The normal force is always perpendicular to the contact surface but is not always opposite in direction to the force due to gravity

1 A car is parked on a hill that makes an angle of 30 with the horizontal If the car has a mass of 320 kg find the magnitude of the normal force acting on the car First draw a free body diagram of the situation 2 A loading ramp makes a 45 angle with the ground If a 132 kg refrigerator sits on the loading ramp find the magnitude of the normal force acting on the refrigerator

Do Now 2 6 09
1 Define Newton s first law of motion 2 Define Newton s second law of motion 3 Which object has a greater inertia a raw egg or a hardboiled egg Why 4 A dog pulls on a pillow with a force of 4 N at an angle of 27 above the horizontal Find the x and y components of this force

Coefficient of Friction








The coefficient of friction is defined as the ratio of the force of friction to the normal force between two surfaces The coefficient of kinetic friction k is the ratio of the force of kinetic friction to the normal force The coefficient of static friction s is the ratio of the maximum value of the force of static friction to the normal force In a word problem the maximum value of the force of static friction is usually described as the force that sets an object in motion The force of kinetic friction is described as the force that keeps an object in motion