Newton’s Second Law of Motion

Bell Work

• Identify three forces acting on a speeding bullet.

• Why do cars sometimes skid going around corners on icy roads?

Objectives

• Define and apply Newton’s second law of motion to real life situations.

• Relate the acceleration of an object to the mass of the object and the force applied to the object.

Newton’s Second Law of Motion

• This law states that an unbalanced force on an object causes the object to accelerate in the direction of the force.

Force of wind

Air/Water Resistance

These forces areunbalanced so theboat accelerates in the direction of theunbalanced force.

20-30 mph

Gravity

Table

If gravity becomes greater than the upward force of the table, the papers will accelerate downward.

UnbalancedForces

Wagon movesthis way Force Applied

Unbalanced

Force, Acceleration, and Mass

• The greater the force applied to a given object, the greater its acceleration.

Force, Acceleration, and Mass

• For a given force, the greater the mass of an object, the smaller its acceleration and vice-versa.

Newton’s Second Law in Real Life

• Why would a truck take longer to accelerate when the driver hits the gas if it were loaded with bricks than if it were empty?

• How might you design a car to achieve maximum acceleration?

Newton’s Second Law in Real Life

• Using Newton’s second law of motion, explain why larger trucks usually need more distance to stop than do smaller trucks.

Science, Technology, & Sports

• Friction is the main force that slows the speed of athletes in sports such has skiing, swimming, cycling, speed skating, and running. How can these athletes combat the effects of friction?

Formula for Newton’s Second Law of Motion

• Newton’s second law of motion can be expressed by the formula F = ma.

• F stands for force, m stands for mass, and a stands for acceleration.

Formula for Newton’s Second Law of Motion

• According to this formula, if the force is constant and the mass increases, what will happen to acceleration?

Question

• Identical twins are riding in identical wagons. A friend gives the first wagon a stronger push than the second. Which twin will experience greater acceleration?

Question

• Would a car towing a boat take less, the same, or more time to accelerate than it would without the boat? Explain.

Question

• What will happen to the acceleration of an object if the mass decreases and the force is constant?

Question

• If two identical objects accelerate at the same rate, what must be true of the force applied to each object?

Review

• Use Newton’s second law of motion to explain why a free-falling object accelerates.

• How is the acceleration of an object affected by the force applied to it?

Challenge

• Using the formula a = f/m, explain why all objects in free fall have the same acceleration, regardless of mass.