1.With its accelerator pedal fully depressed, a car can accelerate at a rate of 3 m s-2. What would the acceleration of the car be if it were towing another car identical to itself?
A 0 m s-2
B 1.5 m s-2
C 3 m s-2
D 4.5 m s-2
E > 6 m s-2
2. A weight lifter of mass 100 kg lifts a bar plus weights of mass 200 kg . The weights are raised from the floor a vertical distance of 1.8 m. In the process of lifting the weights, the weight lifter also raises his own centre of gravity by 0.4 m vertically. What total work has he performed against gravity?
A 2000J
B 3000J
C 4000J
D 5000J
E 6000J
Physics Questions
2012-02-28 8:47 pm
回答 (2)
2012-02-28 9:51 pm
✔ 最佳答案
1) Let m be the mass of the car and F be the motoring force when its accelerator pedal is fully depressed, then:F = 3m
So when it tows another car identical to itself, mass becomes 2m and hence:
F = 2ma
3m = 2ma
a = 1.5 m/s2
(Note: Assumption has been made that the resistive force on car is zero)
2) Gain in gpe of the weights + bar = 200 x 10 x 1.8 = 3600 J
Gain in gpe of the lifter = 100 x 10 x 0.4 = 400 J
Hence total work performed by the lifter = 3600 + 400 = 4000 J
參考: 原創答案
2012-02-28 9:53 pm
1. Use: force = mass x acceleration
F = m.(3)
where F is the force developed by the car engine and m is the mass of the car
hence, F = 3m
When the first car was towing another identical car, we have
F = (2m).a, where a is the new acceleration
i.e. a = F/2m = 3m/2m = 1.5 m/s2
2. Work done in lifting the bar plus weights
= 200g x 1.8 J where g is the acceleration due to gravity, taken to be 10 m/s2
= 3600 J
Work done in raising the lifter's own centre of gravity
= 100g x 0.4 J
= 400 J
Hence, total work done = (3600 + 400) J = 4000 J
F = m.(3)
where F is the force developed by the car engine and m is the mass of the car
hence, F = 3m
When the first car was towing another identical car, we have
F = (2m).a, where a is the new acceleration
i.e. a = F/2m = 3m/2m = 1.5 m/s2
2. Work done in lifting the bar plus weights
= 200g x 1.8 J where g is the acceleration due to gravity, taken to be 10 m/s2
= 3600 J
Work done in raising the lifter's own centre of gravity
= 100g x 0.4 J
= 400 J
Hence, total work done = (3600 + 400) J = 4000 J
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