Test Review Sheet. There will be a TEST on 4/11(Monday). The test will based on Center of Mass, Torque and 1- dimensional Momentum

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Christina Jocelin Scott
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1 Test Review Sheet There will be a TEST on 4/11(Monday). The test will based on Center of Mass, Torque and 1- dimensional Momentum True or False 1. Momentum is an object s mass times its velocity ( True False) 2. If no external force is acting on an isolated system, then the total momentum of the system is conserved ( True False) 3. There is more change in momentum when bouncing takes place ( True False) 4. Perfectly elastic collisions between large objects are uncommon in the everyday world ( True False) 5. Regardless of the shape of an object, if you position your finger under the center of mass, the object will be balanced ( True False) 6. Because our Sun has many planets orbiting it, it actually wobbles as it moves through space (True False) 7. If the object is irregular in shape, the center of mass is always located closer to the more massive end ( True False) Multiple Choice 1. The momentum of an object is defined as the object s a) mass acceleration b) mass velocity c) force acceleration d) force time 2. Which has more momentum, a large truck moving at 30mph or a small truck moving at the same speed? a) the large truck b) the small truck c) the same d) not enough information 3. The change in momentum of an object is equal to the a) force acting on it b) velocity change of the object c) impulse acting on it d) object s mass multiplied the force acting on it 4. The reason padded dashboards are used in cars is that they a) increase the force of impact in a collision b) increase the time of impact in a collision c) decrease the momentum of a collision d) decrease the energy in a collision 5. A car is moving at a certain momentum. If the mass of the car doubles but the speed remains the same, then the momentum of the car will a) remain the same b) double c) quadrupled d) be zero 6. A cannon recoils from launching a cannonball. The speed of the cannon s recoil is small because the a) force against the cannon is relatively small b) cannon has far more mass than the cannonball c) momentum of the cannon is unchanged d) force against the cannon is relatively large 7. When you jump off a step, you usually bend your knees as your reach the ground. By doing this, the time of the impact is about 10 times more what it would be in a still-legged landing and the average force on your body is reduced by a) 5 times b) 10 times c) 15 times d) 20 times

2 8. Which of the following systems is there a decrease in gravitational potential energy? a) A boy is climbing up a tree b) A girl jumps down from a bed c) A crate rests at the bottom of an inclined plane d) A car ascends a steep hill 9. An object will fall over if its center of gravity is a) too high b) too low c) not over its area of support d) outside the object 10. Where is the center of gravity of a softball bat located? a) In the thin handle b) Exactly halfway along the bat c) In the more massive end of the bat d) Outside the bat 11. In which of the following is the center of gravity located at a point where there is no mass? a) boomerang b) dumb-bell c) baseball bat d) cake 12. The center of mass of a rigid object is located at a point a) that is fixed b) that is always at the center c) that changes d) none of the above 13. A ring thrown into the air rotates about its a) midpoint b) center of gravity c) outside the mass d) all of the above 14. Two people sit on a balanced seesaw. When one person leans back away from the center of the seesaw, that person s end of the seesaw will a) rise b) fall d) spins c) stay at the same level 15. To turn a stubborn screw, it is best to use a screwdriver that has a a) short handle b) long handle c) colorful handle d) wide handle 16. Torque is defined as a) mass times velocity b) force times time c) force times lever arm(=distance) d) mass times acceleration Center of Mass x cm = mm1xx1+mm2xx2 +mm3xx3+ +mmnnxxnn mm1+mm2+mm3+ +mmnn, y cm = mm 1yy 1 +mm 2 yy 2 +mm 3 yy 3 + mm nn yy nn mm 1 +mm 2 +mm 3 + mm nn 1. Find the center of mass(com) for the blocks below. All the blocks mass is evenly distributed, so the COM for individual blocks is at the center. Mark the COM as below y(cm) 2 m1 =2kg m2 = 6kg (0,0) x(cm) 3 6 Answer : COM =(, )cm

3 mm1xx1+mm2xx2 +mm3xx3+ +mmnnxxnn x cm = mm1+mm2+mm3+ +mmnn, y cm = mm 1yy 1 +mm 2 yy 2 +mm 3 yy 3 + mm nn yy nn mm 1 +mm 2 +mm 3 + mm nn 2. Find the center of mass(com) for the blocks below. All the blocks mass is evenly distributed, so the COM for individual blocks is at the center. Mark the COM as below y(cm) m3=4kg 4 2 m1=2kg m2=8kg (0,0) x(cm) 3 6 Answer : COM =(, )cm 3. The dots represents the possible location of center of mass of the combined blocks resting on a surface. Find the most likely position of the center of mas of the following objects by circling one of the dots. Justify your answer!! i) m1=2kg m2=6kg ii) All masses are identical iii) All masses are identical

4 4. Two spherical objects are connected by a thin rod as seen above. One spherical object is more massive than the other. If you place the two objects on a frictionless table and spin it, how would the motion look like? The dotted line is the trajectory of the motion. Briefly justify your answer!! a) b) c) d) Torque τ =τ 1 + τ 2 + τ n where τ = r Fsinφ 5. A meter-stick is pivoted at the center. Any unbalance torque applied will result in rotating the meter-stick. If 10N of pushing force are applied as shown, find the net torque acting on the system. (and which direction? clockwise or counter-clockwise?) a) 10N m b) 8N m c) 6N m d) 4N m 0.3m 0.5m F=10N F=10N 6. A meter-stick is pivoted at the center. Any unbalance torque applied will result in rotating the meter-stick. If 15N and 10N of pushing force are applied as shown, find the net torque acting on the system. (and which direction? clockwise or counter-clockwise?) a) 1.0N m b) 1.5N m c) 2.0N m d) 2.5N m F=15N 0.2m 0.5m F=10N

5 Balanced Torque τ left = τ right => m left g r left = m right g r right 7. A meter-stick is placed on a fulcrum at the 50cm mark. The system is in equilibrium. Find the unknown mass. r must be in meters 1meter=100cm, g=9.8m/s 2 10cm 30cm 50cm 80cm 2kg 1kg m=? 8. A meter-stick is placed on a fulcrum at the 25cm mark. If the mass of 1kg is placed at the 0cm so that the whole system is in equilibrium, then the torque on the left and the torque on the right is equal. Find the mass of the meter-stick. 1meter=100cm, g=9.8m/s 2 COM of the meter-stick!! 25cm 50cm 100cm τleft τright m=1kg Fg Fg a) 4kg b) 2kg c) 1kg d) 0.5kg 9. A meter-stick is placed on a fulcrum at the 20cm mark. Find the mass of the meter-stick COM of the meter-stick!! 10cm 20cm 50cm 0.5kg F g a) 0.85kg b) 0.36kg c) 0.17kg d) 0.08kg

6 10. A meter-stick is placed on a fulcrum at the 30cm mark. The system is in equilibrium. Find the mass of the meter-stick 1meter=100cm, g=9.8m/s 2 COM of the meter-stick!! 10cm 30cm 50cm 80cm 8kg F g m=2.67kg **Important! **11. A uniform plank of length 5m and mass 23kg rests horizontally on two supports, with 1.1m of the plank hanging over the right support. To what distance x can a person who weighs 46kg walk on the overhanging part of the plank before it just begins to tip? x 1.1m

Center of Mass / Torque

Center of Mass / Torque Level : Physics I Teacher : Kim 1. Center of Mass( or Center of Gravity or Center of Weight) - The center of mass(com) of an object is the point located at the object s average