Additional Physics – Forces L/O :- To be able to calculate work done “Work It” Exam Date -

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 Lifting a 1kg bar through 2 meters 18/12/2015

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 DF Work done (J) = Force (N) x Distance moved in direction of force (m)

Calculations: 1.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 W FDx ________

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. 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?