3.1-3.3 & 3.5 Forces Flashcards
Define displacement.
Displacement is the distance travelled in a particular direction.
Define instantaneous speed.
The rate of change of displacement at one particular instant in time.
Define average speed.
Average speed = total distance travelled / total time taken
Define velocity
Velocity is the rate of change of displacement.
Define acceleration.
Acceleration is the rate of change of velocity.
What does the gradient on a displacement-time graph represent?
The object’s velocity.
What does the area under a velocity-time graph represent?
The object’s displacement.
What does the gradient on a velocity-time graph represent?
The object’s acceleration.
What does a horizontal line on an acceleration-time graph represent?
Constant acceleration.
Which two quantities in the equations of constant acceleration have the same unit?
u and v (both are measured in ms^-1)
Why can’t we apply the equations of constant acceleration to the motion of a skydiver during a skydive?
Because a skydiver’s acceleration isn’t constant.
In a rebound question, why must u and v have opposite signs?
Because they are in opposite directions.
What is the acceleration g of free fall?
The force acting per unit mass
What is the value of g on and near the surface of the earth?
9.81 m s^-1
What is thinking distance proportional to?
Initial speed.
What is breaking distance proportional to?
The square of the speed.
Define 1N.
The force required to accelerate a 1kg object at 1ms^-1
1N = 1kg X 1ms^-2
Define the net (resultant) force.
The sum of all the forces acting on an object.
What is a projectile?
An object in freefall / an object that is only acted on by weight / an object whose acceleration is g
If you have a two-dimensional motion problem with constant velocity in one direction and constant acceleration in a perpendicular direction, how do you solve it?
- Split the page into vertical and horizontal - break the initial motion down into vertical and horizontal components of velocity
- Use positive and negative to indicate direction (e.g. positive = up, negative = down)
- Use SUVAT to solve vertically - generally for the time.
- Use s = v t to solve horizontally
If you are given a SUVAT question and a force starts off at an angle, how should you tackle it?
Resolve it into horizontal and vertical components.
If a projectile is launched and lands at the same horizontal level, which two sets of values are the same?
Time up = Time down
Vertical take off speed = Vertical landing speed
What is a free body diagram?
A diagram that represents the forces or the net (resultant) force acting on an object.
What direction does normal contact force act in?
Perpendicular to the surface.
True or false: An object will take the same time to fall a given distance, regardless of the horizontal component of its velocity.
True.
Define tension.
The force experienced by a cable, rope, or string when pulled or hung. It acts away from the mass of the object.
Define normal contact force.
The reaction force between an object and a surface.
Define upthrust.
The upwards force that a fluid applies on an object. The size of the upthrust force can be found by applying Archimede’s principle.
Define friction.
The resistive force that one surface encounters when moving over another.
Define drag.
The frictional force experienced by an object travelling through a fluid.
What direction do drag forces such as friction act in?
The opposite direction to the movement causing it.
List four factors that would affect the size of the drag force exerted on an object.
- The object’s cross sectional area
- The object’s speed
- The density of the fluid that the object is travelling through (if applicable)
- The object’s shape and texture
Describe the motion of an object falling in a uniform gravitational field in the presence of drag.
- Initially, the object accelerates with an acceleration value of g because weight is the only force acting. Initially, there is no velocity.
- Drag increases as the object falls due to increased speed.
- Eventually the force of drag will be equal to the weight, the object will no longer accelerate as the net force is 0.
For an object falling in a uniform gravitational field in the presence of drag, what is the initial acceleration when it is stationary?
g
For terminal velocity to occur, which two forces must be equal?
Drag and weight.
Name two practical techniques that can be used to determine terminal velocity in fluids.
- Ball bearing in a viscous liquid
- Cupcake case falling through air
What equation can be used to find the moment of a force?
Moment = force x perpendicular distance
What is a couple?
A pair of forces that together cause a net (resultant) moment, but not a net (resultant) force.
State the conditions for a pair of forces acting on the same object to be a couple.
The two forces have to be the same size but acting in opposite directions.
What is a torque?
A force that causes something to turn.
State the equation that can be used to find the torque due to a couple.
Torque = Force x distance
State the principle of moments.
For an object to be in equilibrium, the sum of the clockwise moments must equal the sum of the anti-clockwise moments exerted on it.
What is the centre of gravity of an object?
The point through which the weight appears to act.
What is the centre of mass of an object?
The average position of the object’s mass.
State the two conditions required for an object to be in equilibrium.
The net resultant force on the object must be zero and the net resultant moment on the object must be zero.
Describe the motion of an object that has a net resultant force of zero exerted on it.
Its acceleration is zero, however it may be stationary or moving at a constant velocity.
Explain what is meant by resolving a force in the horizontal and vertical directions.
Finding the horizontal and vertical components of the force.
What does coplanar mean?
In the same plane.
If three coplanar forces add up to zero, what shape will their vector addition diagram make?
A triangle.
State Archimedes principle.
The upward force exerted on an object immersed in a fluid is equal to the weight of the fluid displaced by the object
State the equation which can be used to find pressure of fluids which have a constant density.
p = h ρ g
where p is the pressure, h is the depth of the liquid, ρ is the density of the liquid, and g is the acceleration due to gravity.
What phrase means the same as work done by a force?
Energy transferred by the force.
Define work done.
Work done = force x distance moved in the direction of the force.
What is the formula for work done, if the object moves in the same direction as the direction of the force?
W = Fx
Define power.
The rate of energy transfer or the rate of work done.
Define 1 Joule.
1 Joule is the amount of work when a force of one newton is exerted over a distance of one meter.
State the principle of conservation of energy.
Energy cannot be created or destroyed, only transferred between different stores and objects.
How can you increase an object’s gravitational potential energy store?
Lift the object higher in the gravitational field.
Why are energy transfers in the real world never 100% efficient?
Work done against resistive forces, such as friction and air resistance, means that some energy is always transferred to the thermal energy stores of an object and its surroundings.
State Newton’s first law.
An object will maintain constant velocity, unless acted on by a resultant force.
State Newton’s second law (with the formula and also with words).
F = ma
State Newton’s third law.
If body A exerts a force on body B,then body B exerts the same size force on body A in the opposite direction.
What is impulse?
The change in momentum:
How can impulse be found from a force-time graph?
It is equal to the area underneath the graph.
What is the difference between elastic and inelastic collisions?
In an elastic collision, the kinetic energy before is equal to the kinetic energy afterwards - no energy is lost. However, in an inelastic collision, the kinetic energy at the end is not equal to the kinetic energy at the start - some energy is lost to the surroundings.
