Topic 5: Forces
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What is a force?
A force is a push or pull object that is caused by it interacting with something
What is a non-contact force?
When the objects do not need to be touching for the force to act
What is a contact force?
When two objects have to be touching for the force to act
What is an interaction pair?
• an interaction pair of forces that are equal and opposite and act on two interacting objects • there is a force on both objects when two objects interact (basically Newton’s Third Law)
What are vector quantities? Give some examples
Vector quantities have a magnitude and a direction e.g. force, velocity, displacement, acceleration, momentum etc.
What are scalar quantities? Give some examples
Scalar quantities have only magnitude and no direction e.g. speed, distance, mass, temperature, time, etc.
Describe how vectors are usually presented
Vectors are usually represented by an arrow - the length of the arrow shows the magnitude, and the direction of the arrow shows the direction of the quantity
What does friction do?
If an object has no force propelling it along, it will always slow down and stop because of friction
How does friction work?
Friction always acts in the opposite direction to movement
Why does the driving force need to balance the frictional forces?
To travel at steady speed
When will friction occur?
You get friction between two surfaces in contact, or when an object passes through a fluid (drag)
What does terminal velocity depend on?
Shape and area
What is gravity and how does it affect falling objects?
Gravity is the accelerating force acting on all objects and it would make them all fall at the same rate if it wasn’t for air resistance
How does gravity affect falling objects on the moon?
On the moon where there’s no air, hamsters and feathers dropped simultaneously will hit the ground together
How does air resistance on earth affect falling objects?
Air resistance causes objects to fall at different speeds
What is terminal velocity determined by?
The terminal velocity of any object is determined by its drag in comparison to its weight
What does the frictional force depend on?
Shape and area
What does it mean when objects fall through fluids? What do they reach?
Terminal velocity
What happens to falling objects when they first set off?
The force of gravity is much more than the frictional force slowing it down, so it accelerates
What happens to friction as speed increases?
As speed increases, the friction builds up
As friction builds up when speed increases, how does this affect the acceleration of an object?
This gradually reduces the acceleration until the frictional force is equal to the accelerating force, so the resultant force is zero
What happens to drag when speed increases?
Drag increases as speed increases
What is drag? Give an example
Drag is the resistance you get in a fluid (a gas or a liquid) e.g. air resistance
What is the most important factor for reducing drag?
Keeping the shape of the object streamlined - where the object is designed to allow fluid to flow easily across it, reducing drag (Parachutes work in the opposite way - they want as much drag as they can get)