Physics Unit Test Study Guide
I. Motion, Speed and Velocity and Acceleration
Motion- observed when one object’s distance from another changes.
Speed - equal to the distance an object t travels in a certain amount of time This is the equation to determine velocity and speed.
s = d
t Unit label is km/hr, m/sec
Velocity- is the same as speed but it is a vector quantity. This means it shows direction. For instance- speed would be 20km/hr. Velocity would be
20 km/hr east. Add direction to make speed a velocity.
Acceleration - the rate at which an objects velocity changes. In science we say an object is accelerating if it
a. increases speed
b. decreases speed
c. changes direction
Objects can be accelerating even if they are not changing speed as long as they are changing direction.
To determine the acceleration use Final velocity- Initial Velocity
time
Units for acceleration is m/sec
II. Forces, Newton, Gravity vs Friction, Momentum
Force- is any push or pull.
Forces may be balanced- no movement occurs in object. Forces must be balanced equal and opposite to each other.
Ex- Book resting on table. The force of gravity pulling it down is equal to the force of table pushing it up.
Forces may be unbalanced- object moves in direction that stronger force is directing it. Ex- Box sliding across floor when pushed from behind. Pushing force exerted by person is stronger than force of gravity pulling box down and force of friction pushing back.
Unbalanced forces can cause an object to do three things-
Start moving, stop moving, or change direction
Newton’s Three Laws of Motion
Ex- a person riding in a car at 55mi/hr flies through the windshield when the car stops suddenly. The lack of a seat belt keeps them in motion at the same speed as the car prior to its stopping.
Objects with a large mass have a greater inertia because they are more resistant to change.
Force is equal to a Newton or 1 kg x m/s. to increase an object’s acceleration you can
ex- 1 adult pulling a wagon with three kids in it is harder and goes slower than them pulling a wagon with 1 kid in it.
For instance, the amount of force exerted on a soccer ball when you kick it is equal to the force exerted by the soccer ball back on your foot.
When you walk, the force of your foot pushing down on the ground is equal to the ground pushing back up on your foot.
Momentum- an objects momentum is equal to its mass x velocity.
Momentum is transferred form one object to another equally when they collide.
Remember the marble activity.
Gravity vs Friction
Gravity pulls down on all objects equally – the object’s mass does not matter.
Weight is caused by gravity pull down on objects. Friction opposes gravity.
Air friction will slow an objects fall if the object has a large surface area. Friction is necessary to walk- you fall on ice because there is so little friction.
Acceleration due to gravity is equal to 9.8m/s . This means for every second an object falls, its velocity increases by 9.8m/s.
For example in the first second the objects velocity = 9.8 m/s
2nd second = to 9.8 + 9.8 = 19.6m/s and so on
3 Kinds of Friction
1. Sliding friction- when 2 solid objects move against each other
ex- book on a table
2. Rolling friction- occurs between any object that rolls on a solid surface
ex- skateboard on pavement
3. Fluid friction- the resistance an object encounters when it moves through a gas or liquid
ex- a boat in water, a bike rider