Transcript How can you work out the spring constant of an elastic band?

Additional Physics – Forces
Exam Date -
“Work It”
L/O :- To be able to calculate work done
Quick quiz
• What equation would you use to calculate acceleration if
you knew the force applied to an object?
• If the forces on a moving object are balanced how would
you describe it’s speed?
• How can you work out the spring constant of an elastic
band?
• What calculation would you use to work out acceleration
if you knew the change in velocity of an object?
• Sketch out the axis for a velocity time graph.
• What does the steepness of the gradient indicate?
Which of these would require more
work? Why do you think this?
• Lifting a 2kg bar through 1
meter
28/03/2016
• Lifting a 1kg bar through 2
meters
Work done
• When an object is moved by a force we say
work is done on the object.
• The amount of energy transferred to the
object is equal to the work done.
Energy transferred = Work done
How can we calculate work done?
W
F
D
Work done (J) = Force (N) x Distance moved in
direction of force (m)
Calculations:
1.
W
________
F
x D
A 20N weight is raised through a height of 0.4m. Calculate the
work done
How much energy must be transferred?
2. A weightlifter raises a 200N metal bar through a height of 1.5m.
Calculate the gain of gravitational energy (work done).
3. A tractor moves a 2000N trailer 500m. Calculate the work done.
4. A man pushes a 400N shopping trolley 18m to his car. How much
energy must he transfer to do this?
5. A builder transferred 250 J of energy moving a wheelbarrow 5m
across a site. What was the force used to push the wheelbarrow?
6. 80,000 J was used to pull a truck. If the force applied was 2000N
what was the distance the truck was pulled?
POINTS TO REMEMBER: SHOW ALL WORKINGS OUT
INCLUDE THE UNITS
Practical:
• Attach a newton meter to a small box (tie some string around
the box and make a loop for the newton meter)
• Measure out 1 meter across the desk and record the force
needed to drag the box through this distance.
• Wrap an elastic band around the box and repeat. Do this with
2, 3 and 4 elastic bands.
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Think about how you will check the precision of your results
You will need to design a results table
What is the resolution of your measurements?
How can you check the reproducibility of your results?
Power
• It will take less power to
lift 10 shopping bags
onto a table in one go
than to lift 1 bag at a
time for all 10 bags.
• How could you test this
hypothesis?
Hint: think about how you calculate
power and what other information
you would need to know about
lifting the bags
Calculating Power:
• Power is the rate of transfer of energy
Power = Energy transferred (J)
(W)
Time taken (s)
E
P
x
t
Sand bags demo.
Time how long it takes to raise 1 sand bag from
the floor to the desk.
(Use the height of the desk and the weight of
the bag to work out the energy transferred.)
Calculate the power this requires and x by 10.
Time how long it takes to lift all 10 bags at once.
Calculate the power.
Exam Questions:
What have you learnt?
• How do you calculate work done?
• What do you need to include in “calculation”
questions?
• What effects does friction have on work done?
• How do you calculate power?