Chapter 10

Forces

Force

When one object pushes or pulls another object, the first object exerts a force on the second object.

A force is described by its strength and by the direction in which it acts.

Strength of a force is measured in Newtons (N)

Arrows

The direction and strength of a force is represented by an arrow.

The arrow points in the direction of the force.

The longer the arrow, the greater the force.

Net Force

The combination of all forces acting on an object is called the Net Force.

Net force determines whether an object moves and also in which direction it moves.

Unbalanced Forces

Whenever there is a net force acting on an object, the forces are unbalanced.

Can cause an object to start moving, stop or change direction.

Unbalanced forces acting on an object results in a net force and cause a change in the object’s motion.

Balanced Forces

Equal forces acting on one object in opposite directions are called Balanced Forces.

Each force is balanced by the other. Balanced forces acting on an object do not

change the object’s motion.

Friction

The force that two surfaces exert on each other when they rub against each other is called Friction.

Cause of Friction

The strength of the force of friction depends on two factors:How hard the surfaces push together.The types of surfaces involved.

Types of Friction

Static FrictionThe friction that acts on objects that are not

moving. Pushing a heavy object.

Sliding FrictionOccurs when two solid surfaces slide over

each otherRubber pads stopping a bicycle tire.

Types of Friction

Rolling FrictionWhen an object rolls across a surface. Ball bearings reduce rolling friction in

skateboards. Fluid Friction

Occurs when a solid object moves through a fluid (water, oil or air).

Air that flows over a cyclist is fluid friction.

Gravity

Gravity is a force that pulls objects toward each other.

Factors affecting Gravity

Two factors affect the gravitational attraction between objects:Mass- The amount of matter in an object.Distance- The further two objects are, the

lesser the gravitational force.

Gravity and Motion

Free Fall- When the only force acting on an object is gravity, the object is said to be in Free Fall.

In free fall, the force of gravity is an unbalanced force, which causes an object to accelerate.

Acceleration due to gravity- 9.8 m/s2

Air Resistance

Fluid friction that objects experience as they fall through the air.

Terminal Velocity- The greatest velocity a falling object reaches. Reached when the upward force of air

resistance cancels the downward force of gravity.

Projectile Motion

An object that is thrown is called a Projectile.A projectile will fall at the same rate as a

dropped object.

Newton’s First Law of Motion

An object at rest will remain at rest, and an object moving at a constant velocity will continue moving at a constant velocity, unless it is acted upon by an unbalanced force.

Inertia

The tendency of an object to resist a change in motion.

The greater the mass of an object, the greater its inertia.

Newton’s Second Law of Motion

Acceleration depends on the object’s mass and on the net force acting on the object.

Net Force (Fnet) = Mass (Kg) x Acceleration (m/s2)

The units for Net Force is Newton (N).

Changes in Force and Mass

How do you increase the acceleration of an object?Change the force on the objectChange the mass of the object

Newton’s Third Law of Motion

If one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object.

For every action there is an equal but opposite reaction.

Action-Reaction

Action- Jumping from the ground with your feet.

Reaction- The ground pushes back on your feet with equal and opposite force.

These forces do not cancel because they are acting on different objects.

Momentum

The product of an object’s mass and velocity.

You can determine the momentum of a moving object by multiplying the object’s mass and velocity.

Momentum = Mass x Velocity The units for momentum is Kg•m/s

Momentum

The more momentum a moving object has, the harder it is to stop.

The greater the mass, the greater an object’s momentum.

The greater the velocity, the greater an object’s momentum.

Conservation of Momentum

In the absence of friction, momentum is conserved when two objects collide.

4 m/s 2 m/s

2 m/s 4 m/s

Before

After

Centripetal Force

Any force that causes an object to move in a circular path.