# Science

[1] A horizontal force of 120 N is required to push a bookcase across the floor at constant velocity. (a) What is the net force acting on the bookcase? (b) How much is the friction force that acts on the sliding bookcase? (c) How much friction acts on the bookcase when it is at rest on a horizontal surface without being pushed?

Answer: (a) 0 N, (b) 120 N, (c) 0 N

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[2] Calculate the acceleration of a hamster if it increases its velocity from rest to 10 m/s in two seconds.

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[3] A school bus slows to a stop with an average acceleration of -2.0 m/s^2. Calculate how much time it takes for the bus to slow down from 10 m/s to a complete stop.

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[4] A small airplane starts from rest at one end of a runway and accelerates at 4.0 m/s^2 for 15 s before takeoff. (a) What is the speed at takeoff? (b) What is the average speed of the plane down the runway? (c) What distance did the plane travel before takeoff?

Answer: 60 m/s, 30 m/s, 450 m

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[5] Madison pushes with a 160 N horizontal force on a 20 kg crate of coffee resting on a warehouse floor. The force of friction on the crate is 80 N. Calculate the acceleration.

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[6] If you stand next to a wall on a frictionless skateboard and push the wall with the force of 40 N, how hard does the wall push back on you? If your mass is 80 kg, calculate your acceleration.

Answer: (a) 40 N, (b) 0.5 m/s^2

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[7] A firefighter of mass 80 kg slides down a vertical pole with an acceleration of 4 m/s^2. Calculate the force of friction that acts on the firefighter.

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[8] An astronaut lands on a planet that has the same mass as Earth but twice the diameter. How does the astronaut’s weight differ from that on Earth?

Answer: weight on other planet is 1/4 the weight on Earth

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[9] What is the gravitational force of attraction between a 150,000,000 kg block of granite and a 200,000,000 kg rock if they are 18.0 meters apart?