Let’s Recall!

• Net force• Mass• Acceleration

Suppose you hit these balls with the same amount of force, which would accelerate faster? Why?

Newton’s Second Law of Motion

2nd Law

Force and Acceleration

• Force is a push or a pull.

•Acceleration is when the motion of an object changes.

Examples:Speed upSlow downChanges direction

•Mass is the amount of matter in an object.

Force and Acceleration

• In this equation, a is the acceleration, m is the mass, and Fnet is the net force.

• If both sides of the above equation are multiplied by the mass, the equation can be written this way:

Formula Practice

A book with a mass of 2.0kg is pushed along a table. If the net force on the book is 1.0N, what is the book’s acceleration?

Answer: .5m/s2

Force and Acceleration

• Units of Force = Newtons (N)SI units for mass is kgSI units for acceleration is m/s2

1 Newton (N) = 1kg * m/s2

Gravity & Weight• Gravity is the force of attraction that exists

between any two objects that have mass.

• The force of gravity depends on the mass of the objects and the distance between them.

Gravity & Weight

• Weight is a force, like the push of your hand is a force, and is measured in Newtons.

• The force of gravity causes all objects near Earth’s surface to fall with an acceleration of 9.8 m/s².

• Your weight on Earth is the gravitational force between you and Earth.

Gravity and Weight• How are weight and mass different?

• Weight is a force, like the push of your hand is a force, and is measured in newtons.

• Mass is the amount of matter in an object, and doesn’t depend on location.

• Weight will vary with location, but mass will remain constant.

Newton’s 2nd Law• Newton’s Second Law states: an object

acted upon by an unbalanced force will accelerate in the direction of the force.

• If you kick the ball, it starts moving.

• The ball accelerates only while your foot is in contact with the ball.

2nd Law

The net force of an object is equal to the product of its mass and acceleration, or F=ma.

Newton’s 2nd Law• Newton’s second law of motion can be used

to calculate acceleration.• For example, suppose you pull a 10-kg sled

so that the net force on the sled is 5 N. • The acceleration can be found as follows:

If mass remains constant, doubling the acceleration, doubles the force. If force remains constant, doubling the mass, halves the acceleration.

Newton’s 2nd Law proves that different masses accelerate to the earth at the same rate, but with different forces.

• We know that objects with different masses accelerate to the ground at the same rate.

• However, because of the 2nd Law we know that they don’t hit the ground with the same force. F = maF = ma

98 N = 10 kg x 9.8 m/s/s98 N = 10 kg x 9.8 m/s/s

F = maF = ma

9.8 N = 1 kg x 9.8 9.8 N = 1 kg x 9.8 m/s/sm/s/s

Problems:

1. How much force is needed to accelerate a 1400 kilogram car at 2 m/s2?

2. How much force is needed to accelerate a 1000-kg car at a rate of 3 m/s2?

3. A dancer lifts his partner above his head with an acceleration of 2.5 m/s2. The dancer exerts a force of 200 N. What is the mass of the partner?

Section Check

Question 1Explain Newton’s second law of motion.

AnswerAn object acted upon by an unbalanced force will accelerate in the direction of that force. Newton’s Second Law says that the acceleration of an object is equal to the net force on it divided by its mass.

Section CheckQuestion 2

Your _______ on Earth is the gravitational force between you and Earth.

A. accelerationB. inertiaC. massD. weight

Section CheckAnswer

The answer is D. Since weight is dependant on gravity, your weight would be different if you were standing on a planet other than Earth.

Section CheckExplain

A thin old man moves slowly despite his little mass. Why?

Section CheckValue Focus

How is the law applied in accomplishing a task in relation to a person’s ability and the effort exerted?

Assignment: ½ crosswise

A. Solve the following word problems. Show your solution.1.What net force is required to accelerate a car at a rate of 2m/s2 if the car has a mass of 3,000 kg?2. A 10 kg bowling ball would require what force to accelerate down an alley at a rate of 3 m/s2?

B. Explain your answer. The net force on an object and its resulting acceleration are always in the same direction. To demonstrate this relationship, a spool of thread is pulled horizontally to the right. In which direction will it roll? Will it make a difference whether the string is on top or at the bottom when it was pulled?

C. by group: Bring the following materials on July 4, Monday:•5 pcs. Party balloon (unblown)•Adhesive tape/scotch tape•2 m string/yarn •Scissors•2 pcs. Drinking straw