Embedded Files
Forces and Motion
Forces and Motion
Key terms
Key terms
Force: Force is a push or a pull on an object with mass that can cause it to change its velocity (to accelerate)
Motion: Motion can also be defined as a continuous change in the position of an object
Newton: A Newton N is the international unit of measure for force
Balanced force: Balanced forces are two forces acting in opposite directions on an object and are equal in sizes
Unbalanced forces: Unbalanced forces are forces that cause a change in the motion of an object.
Mass: Mass is the actual amount of matter that is contained in an object
Weight: The weight of an object is the force of gravity on the object and may be defined as the mass times the acceleration of gravity. W=mg
Kilogram/gram : A kilogram Kg / gram g is the international unit of measure for mass
Friction is a force that occurs when an object moves against another object or through a fluid.
Objects such as a falling stone or a galloping horse are in motion.
An object is in motion if its position is changing.
The motion of an object can be described using four quantities:
Distance (d): how far the object travels
Time (t): how long the journey takes
Speed (v): how fast the object is going (v stands for velocity)
Acceleration (a): how the object's speed is changing over time.
Forces are either pushes or pulls.
Hitting a ball is an example of a push force. Towing a car is an example of a pull force.
Forces can have different effects:
Speed up an object
Slow down an object
Change the direction of motion
Squash or stretch an object
Types of forces
Types of forces
Doc 15 Aug 2020, 16:27.pdfContact and non-contact forces
Contact and non-contact forces
If the object causing the force needs to touch the object it acts on then the force is called a contact force.
E.g. A hockey stick striking a ball.
Non-contact forces are the forces which act over a distance without requiring contact between the object creating the force and object it acts on.
E.g. A magnet attracting a paper-clip.
A non-contact force creates a force field- an area in which the force has an effect. The strength of the force field decreases with distance away from the source of the force.
Friction
Friction
Friction is a force that occurs when an object moves against another object or through a fluid.
Can act to oppose the movement of an object
Drag is another name for the friction that occurs when an object moves through a fluid such as air or water
Friction can generate heat, it causes wear and tear and can slow moving objects down.
The effects of friction can be reduced if the surfaces of the two materials are lubricated with oil or water etc.
Friction can be increased by making one or more of the surfaces rougher or by pressing them together more strongly
Vectors and Scalars
Vectors and Scalars
Physical quantities can be separated into two categories- vectors that show a size and direction and scalars that only show a size.
The difference between vectors and scalars can be demonstrated by considering the quantities' speed and velocity
Speed is a scalar- it indicates how fast an object is travelling
Velocity is a vector- it indicates how quickly an object is moving and the direction that the object is moving
Think about a car travelling north at 28ms-1 it has a speed of 28ms-1 and a velocity of 28ms-1 north
Speed, Acceleration, Velocity
Speed, Acceleration, Velocity
Speed is a measure of how far an object can travel in a certain amount of time
To calculate speed we need to know two variables:
distance travelled (metres, m) and time taken to travel that distance (seconds, s)
SI units for speed at (ms-1 metres per second)
Acceleration is the rate at which an object changes its speed in a certain amount of time
Acceleration can be a positive value (e.g. an object is speeding up) or a negative value (e.g. an object is slowing down)
Negative acceleration is often called deceleration
Acceleration can be calculated using the following formula
The SI units for acceleration is metres per second per second (ms-2)
Average Speed- Velocity
If you know change in distance or displacement and time taken for that distance you can use the following equation to calculate the average speed or velocity.
Acceleration
Acceleration is a vector quantity that described the rate at which an object's velocity changes
Acceleration can be positive or negative
If velocity and acceleration have the same sign (both +ve and -ve) then the object is speeding up
If velocity and acceleration have opposite signs then the object is slowing down
Acceleration is a vector - it has both size and direction
Distance/ Displacement graphs
Distance/ Displacement graphs
Distance/ Time graphs
Distance is a scalar quantity. Therefore direction is not considered. As the object moves its distance either increases or stays constant (it cannot decrease).
The gradient of a sloped section of the graph gives the speed of the object. The faster the object is travelling the steeper the gradient. A curved line indicates that the object is accelerating (or decelerating).
Displacement/time graphs
Displacement is a vector so direction is important. As the object moves its displacement can increase, stay constant or decrease
The gradient of a diaplacemnt tiem graph gives the velocity that the object is travelling. A positive gradient related to moving forwards and a negative gradient means the object is moving in the opposite direction
Note: steady speed/velocity
Speed/Time graph
The gradient of a speed-time graph gives the object's acceleration
A steep gradient indicates rapid acceleration
The area under the curve gives the total distance travelled
The direction of the object motion cannot be determined so displacement from the starting point in unknown
Velocity/Time graph
The gradient of a velocity-time graph gives the object's acceleration with direction
The area between the graph line and the horizontal axis gives the objects displacement
Kinematics equations
Kinematics equations
vf = final velocity (ms-1), vi = initial velocity (ms-1),
a = acceleration (ms-2), t = time (s), d = displacement (m)
Forces
Forces
Hooke's Law:
Hooke's Law:
Describes how springs will behave when loads are places on them. The load can be either a stretching or a compressing force. When springs are stretched or compressed they produce a force known as restorting force. This force tried to move the spring back to its original position:
F = -k x
Negative sign shows that the force acts in the opposite direction to the extension or compression of the spring
F = force in the spring (N)
k = spring constant (Nm-1)
x = extension (m)
Newton's laws
Newton's laws
1st law: An object maintains constant velocity (including being stationary) unless acted upon by an extenal net force
2nd law: If an object experiences an external net force, it will accelerate such that Fnet = m a
3rd Law: When an object exerts force on another object, the second object will simultaneously exert a force of the same size and opposite direction to the force acting upon it
Free body diagrams
The size of the arrow in a free-body diagram reflects the magnitude of the force. The direction of the arrow shows the direction that the force is acting. Each force arrow in the diagram is labeled to indicate the exact type of force.
Centripetal Acceleration
Centripetal Acceleration
An object moving in a circular motion is constantly changing direction and velocity- therefore the object is undergoing acceleration.
The object accelerates even while maintaining a constant speed
By substituting the centripetal acceleration equation into
Fnet = ma
Fc = mv2 / r
Formula for calculating centripetal acceleration:
ac = v2 / r
ac = Centripetal acceleration
v = Tangential velocity
r = Radius of circle
Torque
Torque
Torque is a turning force. Its size depends on the size of the applied force and the distance that the force is applied from its pivot point.
Equilibrium
Equilibrium
To achieve Equilibrium, all forces that act on that object must be balanced.
Two conditions for equilibrium:
Sum of clockwise torques equals the sum of of the anticlockwise torques
The sum of all translational forces adds to zero. Sum of the upwards forces equals the downwards force. Sum of the left-acting forces equals the sum of the right-acting forces.
Page updated
Report abuse