SP2 Motion and Forces Flashcards
What is the difference between the speed of an object and its velocity?
Speed can be constant, but velocity could be changing, e.g,. Roundabout speed is constant,t, but direction is ever-changing so velocity is changing
How do we represent all the forces acting on an object?
Using arrows of different sizes starting from the object’s center and moving away represents the direction the force affecting the object, e.g, thrust from a plane starts from the middle and goes up.
How do we calculate resultant forces?
arrows same direction = add
arrows in opposite = subtract
What are Balanced Forces?
if the resultant of all the forces is zero, then they are classified as balanced
What are Unbalanced Forces?
If the resultant of all the forces is Non-Zero, then they are classified as unbalanced
What is the centripetal force?
An object moving in a circle has a changing velocity, even though its speed remains the same. The resultant force that causes the change in direction is called the centripetal force, and acts towards the centre of the circle.
What is the difference between mass and weight?
Mass is the quantity of matter there is in an object, and only changes if the object itself changes. For example, your mass increases when you eat a meal. Weight is a measure of the pull of gravity on an object and depends on the strength of gravity. The units for mass are kilograms. Weight is a force, so it is measured in Newtons. Weight can be measured using a force meter, which has a scale in newtons.
What are the factors that determine the weight of an object?
mass
gravitational field strength
How do you calculate weight?
weight = mass x gravitational field strength
What is inertial mass and how is it defined?
The size of the force (for the same mass, the bigger the force the bigger the acceleration)
The mass of the object (for the same force, the more massive the object the smaller the acceleration).
What are the factors that affect the acceleration of an object?
The more massive an object is, the more force is needed to change its I velocity (either to make it start moving or to change the velocity of a moving object). We define the inertial mass of an object as the force on it divided by the acceleration that force produces.
What does Newton’s Third Law tell us?
There is a pair of forces acting on the two interactive objects, often called action-reaction forces. The two forces are always the same size and in opposite directions. They are also the same type of force. In photo A the rope and the dog are both exerting pulling forces on each other. In photo B the two forces are both gravitational forces.
Photo A shows an equilibrium situation, because nothing is moving. A force in the rope is pulling on the dog but the dog is also pulling on the rope.
How does Newton’s Third Law apply to stationary objects?
an object sitting on the ground has two forces occurring.The weight of a objecton the ground is equal to the force pushing up on the object from the ground
How are action-reaction forces and balanced forces different?
action-reaction forces act on different objects.
balanced forces all act on the same object
How do objects affect each other when they collide?
We can apply the idea of action-reaction forces to what happens when things collide. In photo D, the ball will bounce off the footballer’s head. His head exerts a force on the ball, but the ball also exerts a force on his head, as you can feel if you have ever tried heading a ball!
The action and reaction forces that occur during the collision are the same size, but they do not necessarily have the same effects on the two objects, because the objects have different masses.
What is momentum?
Momentum is a measure of the tendency of an object to keep moving - or of how hard it is to stop it moving. The momentum of an object depends on its mass and its velocity. Momentum depends on a vector quantity (velocity), and is also a vector.
How is momentum calculated?
momentum = mass x velocity
How is momentum related to force and acceleration?
m x (v-u) / t
What happens to momentum in collisions?
When moving objects collide the total momentum of both objects is the same before the collision as it is after the collision, as long as there are no external forces acting. This is known as conservation of momentum. Remember, momentum is a vector so you need to consider direction when you add the quantities together. If two objects are moving in opposite directions, we give the momentum of one object a positive sign and the other a negative sign.
How are human reaction times measured?
A reaction time is the time between a person detecting a stimulus (such as a flashing light or a sound) and their response (such as pressing a button or applying the brakes in a car). Response times can be measured using computers or electric circuits that measure the time between a stimulus and a response.
What are typical human reaction times?
The typical reaction time to a visual stimulus, such as a computer screen changing colour, is about 0.25 seconds. However this time can be much longer if the person is tired, ill or has been taking drugs or drinking alcohol. Distractions, such as using a mobile phone, can also increase reaction times.
What are the factors that affect the stopping distance of a vehicle?
thinking distance – if the person is tired, ill or has been taking drugs or drinking alcohol. Distractions, such as using a mobile phone, can also increase reaction times.
Braking distance – Car brakes use friction to slow the car down. If the brakes are worn, they create less friction and do not slow the vehicle as effectively. Friction between the tyres and road is also important. If the road is wet or has loose gravel on it, or if the tyres are worn, there is less friction and the braking distance is increased. If a vehicle has more mass, more force is needed to decelerate it. So if the same amount of friction is used to stop a vehicle, a heavier vehicle will travel further than a lighter one (it has a greater braking distance).
What is work done, and how is it calculated?
The energy transferred by a force acting over a distance is called work done. We calculate the work done using this equation:
work done = force x distance moved in the direction of the force
What is kinetic energy, and how is it calculated?
The energy stored in a moving object is called kinetic energy. The amount of kinetic energy depends on the mass of an object and its velocity.
kinetic energy = 1/2 x mass x speed 2
How are work done and kinetic energy related to braking distances?
When a vehicle stops, the kinetic energy is transferred to other energy stores by the braking force. We can use this idea to calculate the braking distance of a vehicle.
What are the dangers caused by large decelerations?
A The amount of damage caused by a collision depends on the mass of the lorry and on how fast it was travelling. In a car crash, the vehicles involved come to a stop very quickly. Slowing down is a deceleration (or a negative acceleration). The force needed for any kind of acceleration depends on the size of the acceleration and on the mass of the object.
How can the hazards of large decelerations be reduced?
Modern cars have lots of safety features built into them to help to reduce the forces on the occupants in a collision. Crumple zones are built into the front (and sometimes the back) of cars. If the car hits something it takes a little time for this crumpling to happen, so the deceleration of the car is less and the force on the car is also less than if it had a more solid front.
Photo C also shows that the passengers do not stop moving when the car stops! Seat belts hold the passengers into the car, so the effect of the crumple zone reduces the forces on the passengers as well as on the car. Airbags increase the time it takes for a person’s head to stop in a collision
