Chapter 3 Forces Flashcards
What is a force? Is it a vector or a scalar quantity? What is its SI unit?
A force is either a push or a pull that one object exerts on another object. It is a vector and has both magnitude and direction. SI unit of force is Newton (N).
State Newton’s first law of motion.
Every object will continue in its state of rest/ uniform motion in a straight line unless a resultant force acts on it.
State Newton’s second law of motion.
When a resultant force acts on an object of a constant mass, the object will accelerate in the direction of the resultant force. The product of the mass and acceleration of the object gives the resultant force.
Fnet (N) = m (kg) x a (m/s²)
Define Newton.
One newton is defined as the force required to produce an acceleration of 1 m/s² in a body with a mass of 1 kg.
State Newton’s third law of motion.
For every action, there is an equal but opposite reaction (and they act on mutually opposite bodies).
Note: Action and reaction act on different bodies, i.e. equal magnitude but opposite direction
What are the four characteristics of forces that Newton’s third law of motion tells us?
- Forces always occur in pairs. Each pair is made up of an action and a reaction.
- Action and reaction are equal in magnitude.
- Action and reaction act in opposite directions.
- Action and reaction act on mutually opposite bodies.
Define friction.
Friction is the contact force that opposes/ tends to oppose motion between surfaces in contact.
What are the factors that affect (the magnitude of) friction?
- Material in contact with each other.
- Nature of the surface in contact.
- Force pressing the two surfaces together.
- NOT related to the area of contact.*
How does friction help us walk?
In order to move forward, foot pushes ground backwards while ground pushes the foot forward. To walk forward, the foot pushes the ground backwards. The friction acts in the opposite direction (forward) to prevent you from slipping.
Refer to this velocity-time graph showing how a sky-diver reaches terminal velocity. Describe what happens throughout the process at each of the 5 points.
1) When the sky-diver jumps out of plane, the only force acting on him is his weight, so he accelerates downward at 10 m/s².
2) As he starts to fall, air resistance begins to build up. The net downward force decreases, hence his acceleration decreases (a < 10 m/s²).
3) Air resistance increases with speed. Air resistance increases until it is equal to weight of sky-diver. The net force is now zero, hence sky-diver falls at a constant speed (terminal velocity) of about 200 km/h.
4) Parachute opens now. Large surface area of parachute give rise to high air resistance acting upwards which is greater than his weight. The net force is now upwards and so sky-diver decelerates (i.e. his speed decreases)
5) Sky-diver slows down until the air resistance acting upward equals to his weight. By this time, sky-diver reaches a new terminal velocity of around 10 ms-1 and drift safely to the ground at this speed.
