Force Unit Newton’s 2 nd Law

Objectives Day 4 I will understand Newton’s second Law of Motion. I will be able to describe examples of Newton’s second Law of Motion. I will be able to solve story problems using the 2 nd Law. I will understand the similarities and differences between weight and mass.

New Close Notes Page Newton’s 2 nd Law of Motion

2 nd Law: The acceleration of an object is directly proportional to the net force acting on it... If F a ?

and inversely proportional to its mass. If m a ?

If you double your force, what happens to a? (m constant) Nothing 1/2 doubles

a DOUBLES! (Both F & a go up!)

Sorry!

If you double your mass, what happens to a? (F constant) Nothing 1/2 doubles

“a” is ½! (m & a are inverses!)

Sorry!

2 nd Law equation: F = ma F = force Unit = Newton (N) What does a Newton equal? N = kg m/s 2 m = mass Unit = kilogram (kg) a = acceleration Unit = m/s 2

Graphing F = ma Linear Relationship (force & acceleration)

Graphing F = ma Inverse Relationship (mass and acceleration) acceleration-mass

The dragon’s mass is 725 kg. If you pulled it by its tail with an acceleration of 0.5 m/s 2, what is your force? Group Practice -- Day 4

Givens: m = 725 kg a = 0.5 m/s 2 Equation: F = ma Unknown: F = ? Plug in numbers: F = (725 kg) / (0.5 m/s 2 )

F= ma F = (725kg) / (0.5m/s 2 ) F = ? (A Newton is a kg m/s 2 ) Incorrect (Try again!) back

F = ma F = (725 kg) (0.5 m/s 2 ) (A Newton is a kg m/s 2 ) F = 362 N Correct

Turn to a partner– Day 4 You push a box with a force of 45 N at an acceleration of 3.1 m/s 2. What is the mass of the box? ? kg

Givens: F = 45 N a = 3.1 m/s 2 Equation: F = ma Unknown: m = ? Rearranged equation: m = F / a Plug in numbers: m = (45 N) / (3.1 m/s 2 )

m = F / a m = (45 N) / (3.1m/s 2 ) m = ? (A Newton is a kg m/s 2 ) Incorrect (Try again!) back

m = F / a m = (45 N) / (3.1 m/s 2 ) (A Newton is a kg m/s 2 ) m = 14.5 kg Correct

Weight and Mass How are weight and mass related? Mass and weight are related but are not the same. Mass is the measure of the amount of material an object contains. Weight is the force of gravity acting on an object. Weight is the product mass and acceleration due to gravity.

Mass is measured by a triple beam balance. Weight is measured by a scale.

W =mg (vertical) is a different form of Newton’s Second Law, F = ma (horizontal). The value of g in the formula is 9.8 m/s 2.

If an astronaut has a mass of 112 kilograms, what is his/her weight on Earth where the acceleration due to gravity is 9.8 m/s 2 ? Group Practice – Day 4

Givens: 112 kg = m 9.8 m/s 2 = g Group Practice – Day 4 -- answer Unknown W = ? Equation: W = mg Plug in numbers: W = (112 kg)(9.8 m/s 2 ) W = 1100 kgm/s 2 Answer:W =1100 N

On the moon, the acceleration due to gravity is only about one sixth that on Earth. The astronaut weighsonly about one sixth as much on the moon as on Earth. The mass of the astronaut is the same on the moon and on Earth.

Astronaut on Earth Mass = 88.0 kg, Weight = 863 N Astronaut on Moon Mass = 88.0 kg, Weight = 141 N

Small Group – Day 4 If a 250 N chest is dropped down a cliff, what is its mass? 250 N

Givens: 250 N = W 9.8 m/s 2 = g Unknown: m = ? Equation: W = mg Rearranged equation: m = W/g Plugged in numbers: 250 N = m (9.8 m/s 2 ) Answer: m = 25.5 kg

Exit Question – day 4 A tennis player practices by hitting a ball against a wall, explain which of Newton’s laws are involved in this situation.

Exit Question – day 4 answer 2 nd law – describes the acceleration of the tennis ball when a force from the racquet or the wall is applied. 3 rd law – describes action and reaction forces that occur when the ball is struck by the racquet and when the ball strikes the wall.

Force Unit momentum

Last notes page! Momentum is at the top!

You’ve Heard it Before…. The sports announcer says, "Going into the all-star break, the Chicago White Sox have the momentum they need!” The sports headlines declare "Chicago Bulls Gaining Momentum." The coach pumps up his team at half-time, saying "You have the momentum; it’s critical that you use that momentum and bury them in this third quarter." Momentum is a commonly used term in sports. A team that has momentum is on the move and is going to take some effort to stop. A team that has a lot of momentum is really on the move and is going to be hard to stop! Momentum is also a term in physics; it refers to the quantity of motion that an object has. If an object is in motion (on the move) then it has momentum.

Momentum Momentum is a characteristic of a moving object. Momentum is equal to the product of its mass and velocity. Momentum (p) = mass (kg) X velocity (m/s) The unit for momentum is kg ∙ m/s

Curiosity Question: If a bicycle and a fire engine are both moving at the same speed of m/s, which would be easier to stop? Why?

Mass = 2000 kgMass = 18 kg Both are traveling at m/s 2000 kg x m/s = 4,470.4 kg∙m/s 18 kg x m/s = kg∙m/s Group Practice

Which has more momentum? A kg parakeet flying 80 m/s OR a 0.03 kg pelican flying 3 m/s? Turn to a partner – Day 5

p = ( kg)(80 m/s) = 0.2 kg ∙ m/s p = (0.03 kg)(3 m/s) = 0.09 kg ∙ m/s Turn to a partner – answer Day 5 parakeet has more momentum

Scientists use the term momentum to describe the product of an object’s mass and velocity. Imagine 2 objects moving toward each other. As moving objects, they will continue in a straight line until some force interferes. When they collide, the motion of the object with more momentum will be less affected. As your eyes follow a line in a painting or sculpture, they tend to hold to the same direction until something distracts them. Artists use the term momentum to describe how a viewer’s eyes move along a line. Both the shape and the consistency of a line affect the eye’s momentum. Lines that “pull” the eye quickly and hold it to a direction are said to have strong momentum. Small Group Practice – Momentum and Art – day 5

Look carefully at lines A and B, above. Of the two, on which one does your eye tend to move faster? For most people, your eyes will move faster on line A, because line B has a “distraction” or change in movement of the line, moving your eye to the top line.

On which of the lines C and D (above) do you tend to follow the original line at the curve? For most people, your eyes will follow the curve more completely on D, because line C has a “distraction” where it appears the two lines merge and D has a gentler curve.

What characteristics give a line strong momentum? Lines that are thick or bold, ones without obstacles at the top surface, ones that are relatively straight, or have a gentle transition into a curve. Compare and contrast the meaning of the word momentum in science and the meaning of the word in art. In science, momentum helps describe the tendency of a speeding car to go straight when it comes to a curve. In art, the term is used to describe the line your eye follows and the tendency to go straight when it’s moving fast along a straight line that comes to a sharp curve or angle.

Individual Practice –Day 5 A 75 kg soccer player runs with a velocity of 7 m/s. What is the soccer player’s momentum?

Givens: 75 kg = 525 kg ∙ m/s m 7 m/s = v unknown = p Equation =p = mv Plug in numbers: p = (75 kg) (7m/s)

Summary: Day 5 More than 70% of fatal auto accidents are caused by improper driving. How can you avoid becoming one of those statistics? For one thing, you can understand how the laws of physics influence the movement of vehicles and the people inside them. INERTIA Newton’s First Law of Motion states that an object in motion tends to remain in motion, unless acted upon by an outside force. This tendency is also related to momentum, which is the product of an object’s mass times velocity (speed). Momentum = mass x velocity The greater the momentum, the more difficult it is to bring a vehicle to a stop.

Homework: Driving Safety – Day 5 1.Which of these has the greater momentum: 1,000 kg car moving at 80 km/hr or a 5,000 kg truck moving at 30 km per hour? 2. Why should you drive a car a greater distance ahead of a truck than behind it? 3.What force opposes the motion of your car when you step on the brake pedal? 4. Complete the Review worksheet for force test.

Exit Question: Is there anything you could have done differently this semester during this class?