In bench pressing 100kg a weight lifter applies a force of 1040N. How large is the upward acceleration of the weights during the lift?

I know the answer is 0.60m/s^2 but for some reason I get 10.4 m/s^2 on using the relation F = m*a

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The weight lifter applies a force of 1040 N to lift the 100 kg mass upwards. When the mass is lifted upwards there are two forces acting on it, one is the gravitational force of attraction due to the Earth acting downwards and the other is the force applied by the weight lifter.

The net acceleration of the mass of 100 kg is the result of both the forces.

The acceleration due to the gravitational force is a constant 9.8 m/s^2 acting downwards. The acceleration due to the upward force of 1040 N applied on the 100 kg mass by the weight lifter is 1040/100 = 10.4 m/s^2 acting upwards.

The sum of the two gives 10.4 - 9.8 = 0.6 m/s^2 acting upwards. Your answer does not match the 0.6 m/s^2 in your book as you are not taking into consideration the gravitational acceleration.

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