Mr Palmer's AQA GCSE Physics Notes
Forces
Balanced Forces
When two forces act on an object, they are either balanced or unbalanced.
If the two forces are balanced, the object will move at a constant speed. (Remember that 0 meters per second is also a constant speed)
Stationary objects resting on a surface also have balanced forces. The downward force due to gravity is exactly equal to the upward force of the desk.
If the two forces are unbalanced, the object will accelerate in the direction of the larger force.
If you have an object that is already moving, and the larger force points in the direction opposite to the object's motion, the unbalanced forces will cause the object to decelerate.
Unbalanced forces will cause a stationary object to accelerate in the direction of the larger force.
Newton's Law
Mr Palmer visits Japan and gets into a fight with a sumo wrestler. The sumo wrestler knocks Mr Palmer with a force of 500N. If Mr Palmer's mass is 95kg, what is his acceleration?
Mr Palmer retaliates and strikes back at the sumo wrestler with a mighty blow of 200N. The sumo wrestler accelerates backward at 0.8 meters per second squared. What is the mass of the wrestler?
Stopping Distance
Stopping time is the amount of time it takes a car to come to a complete stop.
The reaction time is the amount of time that it takes a driver to realize that he needs to press on the break. The reaction time of a driver will be longer if the driver is tired, intoxicated or on drugs.
The breaking time is the amount of time it takes a car to stop once the driver has pressed the break. The breaking distance of a car will be increased if the road is icy or wet, if the breaks are worn, the speed of the car is greater, or if the mass of the car is greater.
When a car breaks, the friction between the wheel and the break and between the wheel and the ground causes the car to stop. However, when the road conditions are icy or wet the friction between the break and the wheel can be much greater than the friction between the wheel and the ground. When this happens the car will skid.
Conditions that increase breaking time are:
- The faster the car is traveling
- Poor maintenance of the breaks
- adverse road conditions
- adverse weather conditions
Conditions that increase the thinking time are:
- If the driver is under the influence of drugs/alcohol
- If the driver is tired
Dangerous Road Conditions
Friction
Friction occurs whenever two objects are in contact with each other or when objects move through fluids such as air and water.
Friction always acts in the opposite direction to the motion of the object.
Friction causes objects to heat up and wears away their surfaces.
The larger the amount of force pressing two objects together, the greater the frictional force between them will be.
The faster an object moves though a fluid, the greater the friction force that acts on it.
When a car is traveling at a constant speed, because of F=ma we know that the forces acting on it must be balanced. In this case, the frictional forces acting on the care are equal to the driving force from the motor.
Terminal Velocity
When an object falls in an atmosphere, it accelerates due to the force of gravity. As it falls faster, the force of air resistance increases. Eventually the downward force of gravity equals the upward force of the air resistance and the object stops accelerating because the forces are balanced and travels at a constant speed. This is called the terminal velocity.
From 0 seconds until about 6 seconds, the skydiver is accelerating rapidly because the force of air resistance acting on him is small.
From 6 seconds to 10 seconds, his acceleration decreases because the force of air resistance is increasing. Note that even though his acceleration is decreasing, his velocity is still increasing.
From 10 seconds until about 17 seconds, the skydiver's velocity is constant. This is because he has reached terminal velocity. The downward force of his weight and the upward force of air resistance are balanced, so he no longer accelerates.
At 17 seconds, the skydiver opens his parachute and rapidly decelerates. The increased surface area of the parachute causes the air resistance acting on him to be much greater than his weight.
From 21 seconds until 29 seconds the skydiver has reached his new, slower terminal velocity with the parachute open. Again the force of his weight and the air resistance are balanced. At 29 seconds he hits the ground and now his velocity is zero. Hopefully, he is still alive and without broken bones.
Weight
The gravitational field strength on Earth is 10 newtons per kilogram. But this number changes on different objects in the Universe.
Example: The gravitational field strength on Mars is 3.7 newtons per kilogram. Mr Palmer's mass is 95kg. He built a spaceship and is leaving teaching to work on a colony on Mars. What is his weight on Mars?
Jupiter
Mr Palmer takes a holiday to Jupiter later in the year. Unfortunately, on Jupiter his weight is 2,210N and crushes him to death. What is the gravitational field strength of Jupiter?
Physics P1 Topics
Physics P2 Topics
- Motion
- Forces
- Energy
- Static Electricity
- Current Electricity
- Mains Electricity
- Momentum
- Nuclear Physics
Other