Module 3.1 - Forces and Motion Flashcards
How do you calculate an object’s average speed?
total distance travelled/time taken
How do you calculate an object’s average velocity?
total displacement/time taken
How do you calculate an object’s acceleration?
change in velocity/time taken
What does the gradient on a velocity time graph show?
acceleration
What does a straight line on a velocity time graph show?
constant acceleration
What does a straight line with a negative gradient on a velocity time graph show?
constant negative acceleration
What does a curve on a velocity time graph show?
acceleration isn’t uniform
When there’s a curve on a velocity time graph, how do you work out the acceleration at a point?
Draw a tangent to the curve at that point and find the gradient of the tangent
What does the area under the graph show on a velocity time graph?
change of displacement
What is the acceleration of objects thrown upwards?
-g
How can a light gate be used to investigate motion?
measures time taken for a card attached to a model vehicle/falling object to move between 2 points
How can a motion sensor be used to investigate motion?
record displacement at regular intervals and can be used to create a displacement time or velocity time graph
What experimental methods can be used to determine acceleration of free fall?
- direct approaches e.g. time a falling ball and work out acceleration due to gravity using equations of motion
- indirect approaches e.g. measuring time taken for pendulum to make a full swing as motion of pendulum is dependent on g
(EX) How can a trap door and electromagnet be used to determine g?
- Apparatus consists of an electromagnet that just supports a steel ball
- When current through the steel ball is switched off, the ball starts to fall & simultaneously an electronic timer is triggered
- Ball falls onto a trap door and timer is stopped
- Distance, s, the balle falls measured w a ruler & time, t for fall is taken from timer
- Experiment should be repeated to find a mean for t. More readings can be taken at different heights
- s=ut+0.5at^2 where u=0, a=g so s=0
5gt^2 so g=(2s)/t2
What are the potential problems with using the trap door and electromagnet method to determine g?
- if electromagnet is too high there’ll be a delay releasing the ball when current switched off and clock triggered. Adjust current so it only just supports ball
- if fall too large/ball too small, air resistance will have a noticeable effect on ball’s speed
- measure height accurately from bottom of ball on electromagnet to top of trap door
(EX) How can light gates be used to determine g?
- Use a light gate and data logger to measure time taken for a piece of card to travel through a light gate as it falls
- Blu tack can be added to lower corner of card to increase stability as it falls
- Average velocity of card while it passes through the light gate by V=L/t where L is length of card & t=transit time recorded by the digital timer for the card to travel through the light gate
- Velocity may be calculated automatically w data logging software
- Ruler measures vertical height of card above light gate, s
- Hold card vertically above light gate then release card
- Calculate acceleration: v^2=u^2+2as, u=o so 2g=v^2/s
- Vary height from which card is released to obtain a set of values for v&s. Plot s (on x axis) against v^2 (on y acis) & calculate the gradient. Gradient = 2g
What are the potential problems with using the light gate method to determine g?
Method assumes card’s velocity is constant as it passes light gate. Effects of error can be minimised by measuring height fallen by card to a line drawn horizontally across the middle of the card, rather than the upper or lower edge of the card
What is meant by the term ‘thinking distance’?
Distance the car travels between driver seeing the hazard and applying the brakes
What is meant by the term ‘braking distance’?
Distance the car travels between applying the brakes and coming to rest
What is total stopping distance?
thinking distance + braking distance
What factors affect thinking distance?
- higher speed
- tiredness
- alcohol and drugs
- distractions
- age of driver
What factors affect braking distance?
- higher speed
- poor road conditions (icy or wet)
- poor condition of brakes
- poor condition of tyres
- mass of car (more people or luggage in the car)
How do you work out the thinking distance?
reaction time of driver (s) x speed of car (ms^-1)
Show that braking distance is proportional to the square of the car’s initial speed.
- when a car brakes, KE transferred to thermal energy in brakes. Work done by friction force = transfer of energy
- decrease in KE = work done = force applied by brakes x distance moved by car
0. 5mv^2=Fd - meaning braking distance is directly proportional to the square of the car’s initial speed
