Unit 2: Chapter 2 - Forces Flashcards
Forces Between Objects
What are equal and opposite forces?
How can
A FORCE CAN CHANGE THE SHAPE OF AN OBJECT OR CHANGE ITS STATE OF REST OR ITS MOTION.
EQUAL AND OPPOSITE FORCES
When two subjects pull or push on each other, they exert equal and opposite forces on each other.
To move an object, one needs to exert of force greater than the object it is attached to.
For example, a tractor is pulling a car through the mud to get it out. The tractor is exerting a larger force than the car and so it moves towards the tractor. The forces are not equal because the objects aren’t the same. The resultant force is the pull of rope on the car rather that pull of car on the rope.
FRICTION IN ACTION
The driving force of a car makes it move. This is sometimes called the engine force or MOTIVE FORCE. This force is due to friction between the ground and the tyre in each wheel.
When the car moves forward:
- The force of friction of the ground on the tyre is in the forwards direction.
- The force of friction of the tyre on the ground is in the reverse direction.
The two forces are equal and opposite to each other.
Resultant Force
IF THE RESULTANT FORCE IS 0, THIS MEANS:
- OBJECT IS STATIONARY
- OBJECT IS MOVING AT SAME SPEED AND IN SAME DIRECTION AS IT WAS MOVING. (EQUAL FORCES)
When the Resultant Force Isn’t 0
- Depends on size and direction of resultant force.
E.g. When a jet sets off. The resultant force is the difference between the force of the engine and air resistance (draft). The force of the engine is much greater and therefore the resultant force is non-zero. The bigger the resultant force, the faster the take-off.
Force and Acceleration
Resultant Force (N) = Mass (Kg) X Acceleration (m/s2)
The acceleration is always in the same direction as the resultant force.
If the velocity increases, we say the resultant force is in the same direction as the velocity. We say its acceleration is positive because it is in the same direction as its velocity.
The velocity of an object decreases if the resultant force is in the opposite direction to the velocity.
On the Road
FOR ANY OBJECT THAT IS MOVING AT A CONSTANT VELOCITY, THE RESULTANT FORCE IS 0.
The driving force and resistive forces is balanced.
To stop the vehicle, we need to BRAKE.
The greater the SPEED or MASS, the greater the deceleration needed to stop the vehicle at a certain distance.
STOPPING DISTANCES
The THINKING DISTANCE is the time taken for a DRIVER to REACT.
The BRAKING distance is the time taken for the CAR to STOP AS SOON AS THE BRAKING FORCE ACTS.
Stopping distance = thinking distance + braking distance
FACTORS THAT AFFECT STOPPING DISTANCE
THINKING DISTANCE:
- Alcohol, drugs and tiredness increase reaction times.
- Distraction of mobile phone increases reaction time.
Thinking distance = speed x reaction time
BRAKING DISTANCE:
- The faster a vehicle is travelling
- Adverse road conditions
- Poorly maintained vehicles (Worn brakes, tyres) as it is less effective
Falling Objects
Weight (N) = Gravitational Field Strength (N/Kg) x Mass (Kg)
Gravitational Field Strength is approximately 10N/Kg
Force / Mass = Acceleration
E.g. 10N/Kg / 1KG = 10m/s2
When an object falls in a fluid, the drag force increases with speed. So the resultant force slowly decreases.
Eventually, the object reaches constant velocity when the drag force is equal to the weight. We call this velocity TERMINAL VELOCITY.
Stretching and Squashing
AN ELASTIC OBJECT REGAINS ITS ORIGINAL SHAPE WHEN FORCES DEFORMING IT ARE REMOVED.
Increase from the original length is called EXTENSION.
The extension of material = Length at stretched stage - Original length
The extension of the spring is DIRECTLY PROPORTIONAL. For e.g. the original weight is 2.0N. The extension of the spring will be 4.0N.
ELASTIC ENERGY
When an elastic object is stretched, the elastic potential energy is stored in the object.
When the object is released, some of the energy is converted to kinetic energy, making the atoms vibrate and become warmer.
HOOKE’S LAW
The extension of a steel spring is directly proportional to the force applied, provided its limit of proportionality is not exceeded.
Rubber and polythene have a low limit of proportionality.. Beyond this limit, they do not obey Hooke’s law.
Force applied (N) = Spring Constant (N/m) x Extension (M)
The spring constant is equal to the force per unit extension needed to extend the spring, assuming its limit of proportionality is not exceeded. The stiffer the spring, the greater the spring constant is.
Force and Speed Issues
SPEED COSTS
Reducing speed reduces fuel cost. This is because air resistance is high if speed is high.
This effect can be reduced for lorries by installing wind deflectors. This requires less force to maintain a speed against the wind and so costs are reduced.
SPEED KILLS
The force of the car increases if the speed increases. It also requires more distance to stop. Schools are now enforcing 20mph maximum speeds.
SPEED CAMERAS
A driver may be fined if he exceeds speed limits and can even lose their driving license. This is controversial as some people argue that local council is siphoning money from drivers.
ANTI-SKID SURFACES
Rough surfaces increase friction between surface and tyre so it reduces chance of skidding when a driver applies brakes. Surface is marked out clearly by its lighter surface.
