Forces & Motion
Speed & Velocity
• Average speed = total distance travelled ÷ time taken
• speed & distance are scalar quantities (magnitude only)
• velocity - the speed in a given direction
• displacement - the distance in a given direction
• both vector quantities (have magnitude and direction)
Distance-Time Graph
• horizontal line - no gradient, stationary (speed = 0)
• uphill line - constant gradient, constant speed (in the positive direction)
• downhill line - constant gradient, constant speed (in the negative direction)
• curved line - gradient increasing, accelerating (speed increasing)
• curved line - gradient decreasing, decelerating (speed decreasing)
• gradient of the graph = speed
Acceleration
• instantaneous speed - the speed of an object at a speci c moment in time
• acceleration - the rate at which velocity is changing
• speeding up/slowing down/changing direction
• acceleration = ( nal velocity - initial velocity) ÷ time taken
• another equation linking acceleration & distance: v^2 = u^2 + 2as
Velocity-Time Graph
• represents how an object’s velocity can change over time
• horizontal line - no gradient, constant velocity
• uphill line - constant gradient, constant acceleration
• downhill line - constant gradient, constant deceleration
• curved line - gradually changing acceleration
• gradient of the graph = acceleration (can be positive or negative)
• area under the graph = distance travelled
Forces
• causes acceleration (speeds up/slows down/changes direction) of an object
• can also change the shape or size of an object
• contact forces - require physical touch
fi fi
,• non-contact - act at a distance through elds, affecting objects without touching
Contact Non-Contact
Friction Gravitational
Push/Pull Magnetism
Air Resistance Electrostatic
Upthrust Strong Nuclear
Tension
Newton’s Laws
Newton’s 1st Law
• a body will remain at rest or move at a constant velocity unless acted upon by an
unbalanced force
Newton’s 2nd Law
• acceleration is directly proportional to the resultant force and is inversely
proportional to its mass
• F = ma
Newton’s 3rd Law
• if body A exerts a force on body B, then body B will exert an equal force on body A
but in the opposite direction
Momentum
• the product of mass and velocity
• vector - has both magnitude and an associated direction
• stationary objects have no momentum - the velocity is zero
• the faster an object moves (greater the velocity), the bigger the momentum will be
• the heavier an object is (bigger the mass), the bigger the momentum will be
• p = mv
The Law of Conservation of Momentum
• when 2 or more objects collide:
• total momentum before collision = total momentum after collision
Vehicle Safety & Momentum
• the force applied to an object = the rate of change of momentum
• f = ∆p/t
fi
, • increasing the amount of time it takes for the vehicle to come to rest reduces the
impact force on the driver
• seat belt & airbag increase the time over which the passenger experiences the
impact force
Terminal Velocity
• if we drop a block…
1. the moment we let go, there is no velocity 7 therefore no air resistance
2. gravitational force causes the object to accelerate, which increases the air
resistance
3. at a certain point, air resistance = weight of the object
4. the object reaches terminal velocity - no resultant force so no acceleration (travels
at a constant speed)
• a parachutist falling from a plane…
1. the moment the parachutist get off the plane, the only force exerted on him is his
own weight
2. gravitational force causes the object to accelerate, which increases the air
resistance
3. air resistance increases until it is equal to the weight of the parachutist
4. parachutist reaches terminal velocity (balanced forced = no acceleration, constant
velocity, and therefore air resistance will stay the same)
5. the parachute is opened - the increase of surface area by the parachute increases
the air resistance
6. the increase in air resistance causes velocity to decrease, which then decreases air
resistance
7. the forces are balanced again as air resistance decrease
8. the parachutist reaches a slower terminal velocity
• the weight of the object is constant throughout the process
• only air resistance and velocity change
Free-fall
• term given to an object falling under its own weight in the absence of any resistive
forces
• the only force exerted on the object is the gravitational force
• the object must move in a vacuum or under conditions where air resistance is really
small
Speed & Velocity
• Average speed = total distance travelled ÷ time taken
• speed & distance are scalar quantities (magnitude only)
• velocity - the speed in a given direction
• displacement - the distance in a given direction
• both vector quantities (have magnitude and direction)
Distance-Time Graph
• horizontal line - no gradient, stationary (speed = 0)
• uphill line - constant gradient, constant speed (in the positive direction)
• downhill line - constant gradient, constant speed (in the negative direction)
• curved line - gradient increasing, accelerating (speed increasing)
• curved line - gradient decreasing, decelerating (speed decreasing)
• gradient of the graph = speed
Acceleration
• instantaneous speed - the speed of an object at a speci c moment in time
• acceleration - the rate at which velocity is changing
• speeding up/slowing down/changing direction
• acceleration = ( nal velocity - initial velocity) ÷ time taken
• another equation linking acceleration & distance: v^2 = u^2 + 2as
Velocity-Time Graph
• represents how an object’s velocity can change over time
• horizontal line - no gradient, constant velocity
• uphill line - constant gradient, constant acceleration
• downhill line - constant gradient, constant deceleration
• curved line - gradually changing acceleration
• gradient of the graph = acceleration (can be positive or negative)
• area under the graph = distance travelled
Forces
• causes acceleration (speeds up/slows down/changes direction) of an object
• can also change the shape or size of an object
• contact forces - require physical touch
fi fi
,• non-contact - act at a distance through elds, affecting objects without touching
Contact Non-Contact
Friction Gravitational
Push/Pull Magnetism
Air Resistance Electrostatic
Upthrust Strong Nuclear
Tension
Newton’s Laws
Newton’s 1st Law
• a body will remain at rest or move at a constant velocity unless acted upon by an
unbalanced force
Newton’s 2nd Law
• acceleration is directly proportional to the resultant force and is inversely
proportional to its mass
• F = ma
Newton’s 3rd Law
• if body A exerts a force on body B, then body B will exert an equal force on body A
but in the opposite direction
Momentum
• the product of mass and velocity
• vector - has both magnitude and an associated direction
• stationary objects have no momentum - the velocity is zero
• the faster an object moves (greater the velocity), the bigger the momentum will be
• the heavier an object is (bigger the mass), the bigger the momentum will be
• p = mv
The Law of Conservation of Momentum
• when 2 or more objects collide:
• total momentum before collision = total momentum after collision
Vehicle Safety & Momentum
• the force applied to an object = the rate of change of momentum
• f = ∆p/t
fi
, • increasing the amount of time it takes for the vehicle to come to rest reduces the
impact force on the driver
• seat belt & airbag increase the time over which the passenger experiences the
impact force
Terminal Velocity
• if we drop a block…
1. the moment we let go, there is no velocity 7 therefore no air resistance
2. gravitational force causes the object to accelerate, which increases the air
resistance
3. at a certain point, air resistance = weight of the object
4. the object reaches terminal velocity - no resultant force so no acceleration (travels
at a constant speed)
• a parachutist falling from a plane…
1. the moment the parachutist get off the plane, the only force exerted on him is his
own weight
2. gravitational force causes the object to accelerate, which increases the air
resistance
3. air resistance increases until it is equal to the weight of the parachutist
4. parachutist reaches terminal velocity (balanced forced = no acceleration, constant
velocity, and therefore air resistance will stay the same)
5. the parachute is opened - the increase of surface area by the parachute increases
the air resistance
6. the increase in air resistance causes velocity to decrease, which then decreases air
resistance
7. the forces are balanced again as air resistance decrease
8. the parachutist reaches a slower terminal velocity
• the weight of the object is constant throughout the process
• only air resistance and velocity change
Free-fall
• term given to an object falling under its own weight in the absence of any resistive
forces
• the only force exerted on the object is the gravitational force
• the object must move in a vacuum or under conditions where air resistance is really
small
