Physical Science Unit 9 force and motion

Forces, Gravity, and Projectile motion

What is a force?

• Force is a push or a pull that causes an object to change speed, change direction, or to change size or shape.

• It is measured in Newtons (N). 1 Newtons = 1 kg*m/s²

Examples of Force

• Change in speed

  • Hitting a golf ball with a club changes that speed of the ball.

• Change in direction

  • Kicking a soccer ball as it flies towards you and changes its direction.

Change in shape or size

• Pushing on a spring exerts a force that changes its shape.

Net force

• Net force is the combination of all the forces acting on an object.

• If the forces acting on an object are in the same direction, they add together to form the net force.

• If forces are in opposite directions, the net force is the difference between two forces.

Forces

• Balanced Forces

  • Forces are said to be “balanced” if their effects cancel each other and there is no change in the object's speed or direction.

• Unbalanced Forces

  • Forces are said to be “unbalanced” if their effects do not cancel each other and there is a change in the object’s speed or direction.

Representing Force (Fill in the Blanks)

• Arrows can represent a force. 

• The lengths of the arrows show relative amounts of force.

Combining Forces

The net (total) force is the overall force acting on an object after all the forces are combined.

• Forces in the same direction add together.

• Forces in opposite directions subtract from one another. 

Balanced Forces

• Balanced forces are forces that combine to produce a net force of zero

Unbalanced Forces

• An unbalanced force is a force that results when the net force acting on an object is not equal to zero.

Gravity

• Gravity is a force that acts between any two masses

• If an object is falling, both gravity and air resistance affect its motion

• As it falls to the ground, it accelerates in speed

• Terminal velocity occurs when the force of air resistance equals the force of gravity

Projectile motion

• A thrown ball follows a curved path. 

• Projectile motion is the motion of a falling object (projectile) after it is given an initial forward velocity. 

• Air resistance and gravity are the only forces acting on a projectile.

Falling motion

• Ignoring air resistance, any two objects will land at the same time!

• It doesn’t matter if they have different masses (A) or are given an initial forward velocity (B) , they will hit the ground at the same time! 

Newton’s Laws

1st Law of Motion - Law of Inertia

• An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.

• Inertia is the  tendency of an object to resist changes in its velocity: whether in motion or motionless.

• Example: Once airborne, unless acted on by an unbalanced force (gravity and air – fluid friction), it would never stop!

2nd Law of Motion - Law of Momentum

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

• When mass is in kilograms and acceleration is in m/s/s, the unit of force is in newtons (N).

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

• One newton is equal to the force required to accelerate one kilogram of mass at one meter/second/second.

3rd Law of Motion - Law of Reactions

• For every action, there is an equal and opposite reaction.

• According to Newton, whenever objects A and B interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body. 

• There are two forces resulting from this interaction - a force on the chair and a force on your body. These two forces are called action and reaction forces.

Weight vs. Mass

Mass and Weight are related but they are not the same

• Mass is the measure of the amount of material an object contains.

• Weight is the force of gravity acting on an object. Weight is the product of mass and acceleration due to gravity.

How to calculate weight

• Weight = mass x acceleration due to gravity

• acceleration due to gravity = 9.8 m/s²

Gravity on the moon versus on earth

On the moon, the acceleration due to gravity is only about one sixth that on Earth. 

• The astronaut’s weight is only about one sixth as much on the moon as on Earth. 

• The mass of the astronaut is the same on the moon and on Earth.

Momentum

• Momentum is the quantity of motion of a moving body, aka “Mass in motion” 

• An object with large momentum is harder to stop than an object with small momentum. 

• The momentum for any object at rest is zero. 

Law of conservation of momentum

• The law of conservation of momentum: if no net force acts on a system, then the total momentum of the system does not change.

• Mass is measured in kilograms.

• Velocity is measured in meters per second.

• Momentum is measured kilogram x meters/second.

• Formula= mass x velocity