Using Newton’s Laws

Section 4.2Page 96

Newton’s 2nd Law

• Describes the connection between the cause of a change in an object’s velocity and the resulting displacement.

• Identifies the relationship between the net force exerted on an object and the object’s acceleration.

Newton’s 2nd Law

• Free-falling ball in midair.• Not touching anything

and air resistance can be neglected.

• The only force acting on it is Fg.

• Acceleration = g; therefore Newton’s second law becomes

Fg = mg

• Both force and acceleration are downward.

• The magnitude of an object’s weight is equal to its mass times the acceleration it would have if falling freely.

Mass vs. Weight

• A spring scale measures weight, not mass.

• Weight can change depending on location.

• Mass remains the same.• Mass is measured in

kilograms.• Weight is a force so it is

measured in Newtons.

Apparent Weight

• Fg changes when “g” varies.• On or near the Earth’s surface “g” is

approximately constant.

What if we are in an elevator?

If the elevator accelerates upward…

• The upward force must be greater than the downward force of your weight.

• The scale reading is greater than you weight.

• You feel heavier because the floor would press harder on your feet.

If the elevator accelerates downward…

• You would feel lighter.• The scale would have a

lower reading.

This is referred to as apparent weight!

• Apparent weight – the force an object experiences as a result of all the forces acting on it, giving the object an acceleration.

• Weightlessness – there are no contact forces pushing up on the object, and the object’s apparent weight is zero. The object, however, is not actually weightless.

Strategies for Solving Force and Motion Problems

1. Read the problem carefully, and sketch a pictorial model.2. Circle the system and choose a coordinate system.3. Determine which quantities are known and unknown.4. Create a physical model by drawing a motion diagram

showing the direction of the acceleration, and create a free-body diagram showing the net force.

5. Use Newton’s laws to link acceleration and net force.6. Rearrange the equation to solve for the unknown quantity.7. Substitute known quantities with their units into the

equation and solve.8. Check your results to see if they are reasonable.

Drag Force

• Particles in the air around objects do exert forces on the object, but in most cases it is a balanced force on all sides, and therefore has no net effect.

• HOWEVER…– When an object moves through any fluid, air or

water, the fluid exerts a drag force on the moving object in the direction opposite to its motion.

Drag Force…

• Dependent on the motions of the object, properties of the object, and the properties of the fluid.

• Speed of object increases – magnitude of drag force increases.

• The size and shape of the object affects drag force.

• The properties of the fluid, such as viscosity and temperature also affects drag force.

Terminal Velocity

• As the ball’s velocity increases, so does drag force

• The drag force will soon be equal to force of gravity.

• At that point, there is no net force and acceleration becomes zero.

• The constant velocity that is reached when drag force equals force of gravity is terminal velocity.

Terminal Velocity…

• Light objects with large surface areas are more affected by drag force and quickly reach terminal velocity.

• Heavier, more compact objects are not affected as much by drag force.

• Terminal velocities can be increased by decreasing drag force.– Change body shape (sky diving, skiing) – Wear smooth clothing and streamlined helmets