Slides in this set
· The steeper the line on a distance-time graph, the greater the speed it represents.
· Speed (metre/second, m/s) = Distance travelled (metre, m)
Time taken (second, s)
VELOCITY AND ACCERLERATION
· Velocity is a speed in a given direction
· An object moving steadily round in a circle has a constant speed,
its direction of motion changes as it goes round so its velocity is not constant.
· Two moving objects can have the same speed but different velocities, e.g. a car travelling
north at 30 m/s on a motorway has the same speed as a car travelling south at 30 m/s. but
their velocities are not the same because they're moving in opposite directions.
· The acceleration of an object is its change of velocity per second.
· Acceleration (meter/second, m/s2) = Change in velocity (metre/second, m/s)
Time taken for the change (second, s)
· A body travelling at a steady speed is accelerating if
its direction is changing.
· A car decelerates when the drivers brakes, we use the term
DECELERATION or NEGATIVE ACCELERATION for any
situation where an object slows down.…read more
FORCES BETWEEN OBJECTS
Equal & opposite forces:
· Whenever two objects push and pull on each other, they exert equal and opposite forces
on one another, the unit is the newton.
· A boxer who punches an opponent with a force of 100N experiences a reverse force of
100N from his opponent.
· Two roller skaters pull on opposite ends of a rope, the skaters move towards each other,
this is because they pull on each other with equal and opposite forces.
In the mud:
· A tractor can pull a car out of the mud, at any stage the force of the rope on the car is
equal and opposite to the force of the car on the rope.
· To pull the car out of the mud, the force of the ground on the tractor needs to be greater
than the force of the mud on the car. These two forces aren't necessarily equal to one
another because the objects are not the same.
Friction in action:
· When a car moves forwards:
· The force of friction of the ground on the tyre is in the forward direction.
· The force of friction of the tyre on the ground is in the reverse direction.
· The two forces are equal and opposite to one another.…read more
FORCES BETWEEN OBJECTS
· When the resultant force on an object is zero, the object:
· Remains stationary if it was at rest, or
· Continues to move at the same speed and in the same direction if it was already
· A glider on a linear air track floats on a cushion of air, provided the track is level,
the glider moves at constant velocity along the track because friction is absent. The
resultant force on the glider is zero.
· when a heavy crate is pushed across a rough floor at constant velocity, the
resultant force on the crate is zero, the push force is equal in size but acts in the
opposite direction to the force of friction on the floor on the crate.
· When the resultant force is not zero, the movement of the object depends on the
size and direction of the resultant force.
· When a jet plane is taking off, the trust force of its engines is greater than the force
of the air resistance on it, the resultant force on it is the difference between the
thrust force and the force of air resistance on it. The resultant force is therefore
non-zero and the greater resultant force the quicker the take off.
· When a car driver applies the brakes, the braking force is the resultant force on the
can, it acts in the opposite direction to that in which the car is moving, so it slow
the car down.…read more
FORCE AND ACCELERATION
· To find the resultant force by:
Resultant force (N) = mass (KG) x acceleration (m/s2)
· The velocity of the object increases if 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 the object decreases if the resultant force is opposite in direction, we say its
acceleration is negative because it is opposite in direction to its velocity.
ON THE ROAD
· The braking force needed to stop a vehicle in a certain distance depends on:
· The velocity of the vehicle when the brakes are first applied
· The mass of the vehicle
· The greater the velocity, the greater the deceleration needed to stop it in a certain distance,
so the braking force must be greater than at low velocity.
· The greater the mass, the greater the braking force needed for a given deceleration.
· The thinking distance = the distance travelled by the vehicle in the time it takes for the drive
· The braking distance = the distance travelled by the vehicle during the time the braking force
· The stopping distance = the thinking distance + the braking distance
· Factors that affect stopping distances:
· Tiredness, alcohol and drugs all increase reaction times.
· The speed a vehicle is travelling means the further it travels before it stops.
· In wet or icy conditions drivers have to brake with less force.
· Worn brakes or tyres take longer to stop as they are less effective.…read more
How to reduce your weight:
· The weight of an object is the force of gravity on it.
· The mass of an object is the quantity of matter in it.
· The weight of an object:
· Of mass 1kg is 10N
· Of mass 5kg is 50N
· Weight (N) = mass (kg) x gravitational field strength (N/kg)
The forces on falling objects:
· If the abject falls freely, no other forces act on it, so the resultant force on it is its weight.
· If the object falls in a fluid, the fluid drags on the object, the drag force increases with speed.
When an object moves through the air the drag force is called air resistance.
· The acceleration of the object decreases as it falls, this is because the drag force increases as it
speeds up. So the resultant force on it decreases.
· The object reaches a constant velocity when the drag force on it is equal and opposite to its
weight. We call this velocity its terminal velocity. The resultant force is then zero, so
acceleration is zero.…read more