# NEWTON’S SECOND LAW.

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NEWTON’S SECOND LAW

The effect of an applied force is to cause the body to accelerate in the direction of the force.

The acceleration is in direct proportion to the force and in inverse proportion to the mass of the body.

F = ma m, mass in kg a, acceleration in m/s2 F, force in Newtons

1 N = 1 kg•m/s2

To find the weight of an object in newtons, use “g” for the acceleration. Fw = mg

NEWTON’S THIRD LAW

When one body exerts a force on another, the second body exerts on the first a force of equal magnitude in the opposite direction.

For every action there is an equal and opposite reaction.

Action and reaction iinvolves two different forces acting on two different objects.

Mass is a measure of inertia in that the larger the mass, the more an object tends to resist a change in motion.

Mass and weight are not equivalent.

Equilibrium is the condition in which all forces on an object add up to zero.

If a person is standing in a room, his weight must be equal to the force with which the floor is pushing upward.

This upward pushing force is called “ the normal force”.

If a person is moving with constant velocity, all forces must be in equilibrium.

Forces are vectors. Forces can be added using vector addition.

Resolution of forces procedure for finding component forces from an original force vector.

A lawn mower has a mass of 130 kg
A lawn mower has a mass of kg. A person tries to push the mower up a hill that is inclined 15° to the horizontal. How much force does the person have to exert along the handle just to keep the mower from the rolling down the hill, assuming that the handle is parallel to the ground?

FRICTION is a force that resists motion
FRICTION is a force that resists motion. It involves objects that are in contact with each other.

CAUSES OF FRICTION Deformation of a surface
CAUSES OF FRICTION Deformation of a surface. Interlocking of irregularities of surfaces. How can friction be decreased?

Polishing surfaces decreases friction
Polishing surfaces decreases friction. However, making surfaces very smooth actually increases friction due to the forces of attraction between the molecules of substances.

Friction is often desirable
Friction is often desirable. Without friction you couldn’t walk across the room, you couldn’t get your car to start moving, nails would spring out of pieces of wood.

Starting friction - Maximum frictional force between stationary objects. (force needed to make an object start moving)

Sliding Friction - frictional force between objects that are sliding with respect to each other.

1. Friction acts parallel to the surfaces that are in contact and in the direction opposite to the motion of the object or to the net force tending to produce such motion.

2. Friction depends on the nature of the materials in contact and the smoothness of their surfaces.

3. Sliding friction is less than or equal to starting friction.

4. Friction is practically independent of the area of contact.

5. Starting or sliding friction is directly proportional to the force pressing the two surfaces together.

Coefficient of Sliding Friction - The ratio of the force of sliding friction to the normal (perpendicular) force pressing the surfaces together.

µ = Ff / FN

Increasing friction sand, tire chains,snow tires, rosin.

Decreasing friction - wax, oil, self-lubricating alloys, rolling friction, Teflon©.

Decreasing friction - wax, oil, self-lubricating alloys, rolling friction, Teflon©. (What makes Teflon© stick to the pan?)

A box is pushed across a level floor at a constant velocity by a force of 100 N. If the box weighs 200 N, what is the coefficient of friction between the box and the floor?

A box is pushed across a level floor at a constant velocity by a force of 100 N. If the box has a mass of kg, what is the coefficient of friction between the box and the floor?

A box with a mass of 175 kg is pulled along a level floor with a constant velocity. If the coefficient of friction between the box and the floor is 0.34, what horizontal force is exerted in pulling the box?

A force of 50 N is directed 30º above horizontal
A force of 50 N is directed 30º above horizontal. What are the vertical and horizontal components of that force?

A 120 N box is placed on a 25º incline
A 120 N box is placed on a 25º incline. What are the perpendicular and parallel components?

A wooden block weighing 130 N rests on an inclined plane
A wooden block weighing 130 N rests on an inclined plane. The coefficient of sliding friction between the block and the plane is Find the angle of the inclined plane at which the block will slide down the plane at constant speed once it has started moving.

A crate is pulled with a constant velocity up an inclined floor that makes an angle of 12° with the horizontal. The crate weighs 950 N and the pulling force parallel to the floor is 460 N. Find the coefficient of friction between the crate and the floor.

A box weighing 200 N is pulled along a level floor at constant speed by a rope that makes an angle of 40.0° with the floor. If the force on the rope is 100 N, (a) what is the horizontal component (Fh) of this force? (b) What is the normal force (FN)? (c) What is the coefficient of sliding friction (µ)?