a) Lực động cơ tác dụng vào xe là F" role="presentation" style="margin: 0px; padding: 0px; border: 0px; font-size: 13.696px; vertical-align: baseline; background: transparent; display: inline; line-height: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; color: rgb(0, 0, 0); font-family: Arial, "Liberation Sans", "DejaVu Sans", sans-serif; position: relative;">FF. Ta có:
P=Fv" role="presentation" style="margin: 0px; padding: 0px; border: 0px; font-size: 13.696px; vertical-align: baseline; background: transparent; display: inline; line-height: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; color: rgb(0, 0, 0); font-family: Arial, "Liberation Sans", "DejaVu Sans", sans-serif; position: relative;">P=FvP=Fv (P là công suất động cơ)
⇒Pv=150(N)" role="presentation" style="margin: 0px; padding: 0px; border: 0px; font-size: 13.696px; vertical-align: baseline; background: transparent; display: inline; line-height: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; color: rgb(0, 0, 0); font-family: Arial, "Liberation Sans", "DejaVu Sans", sans-serif; position: relative;">⇒Pv=150(N)⇒Pv=150(N)
Vậy lực ma sát là: Fms=F−Psinα=150−2000×10200=50(N)" role="presentation" style="margin: 0px; padding: 0px; border: 0px; font-size: 13.696px; vertical-align: baseline; background: transparent; display: inline; line-height: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; color: rgb(0, 0, 0); font-family: Arial, "Liberation Sans", "DejaVu Sans", sans-serif; position: relative;">Fms=F−Psinα=150−2000×10200=50(N)Fms=F−Psinα=150−2000×10200=50(N)
b) Từ đỉnh đốc xuống chân dốc.
Biến thiên thế năng: ΔU=−mgh=−2000×10=−20000(J)" role="presentation" style="margin: 0px; padding: 0px; border: 0px; font-size: 13.696px; vertical-align: baseline; background: transparent; display: inline; line-height: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; color: rgb(0, 0, 0); font-family: Arial, "Liberation Sans", "DejaVu Sans", sans-serif; position: relative;">ΔU=−mgh=−2000×10=−20000(J)ΔU=−mgh=−2000×10=−20000(J)
Công của ma sát: Ams=Fms×s=−50×200=−10000(J)" role="presentation" style="margin: 0px; padding: 0px; border: 0px; font-size: 13.696px; vertical-align: baseline; background: transparent; display: inline; line-height: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; color: rgb(0, 0, 0); font-family: Arial, "Liberation Sans", "DejaVu Sans", sans-serif; position: relative;">Ams=Fms×s=−50×200=−10000(J)Ams=Fms×s=−50×200=−10000(J)
Công do xe thực hiện là A thì:
mv022+A+Ams=mv22+ΔU" role="presentation" style="margin: 0px; padding: 0px; border: 0px; font-size: 13.696px; vertical-align: baseline; background: transparent; display: inline; line-height: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; color: rgb(0, 0, 0); font-family: Arial, "Liberation Sans", "DejaVu Sans", sans-serif; position: relative;">mv202+A+Ams=mv22+ΔUmv022+A+Ams=mv22+ΔU
A=ΔU−Ams+mv22−mv022=10000(J)" role="presentation" style="margin: 0px; padding: 0px; border: 0px; font-size: 13.696px; vertical-align: baseline; background: transparent; display: inline; line-height: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; color: rgb(0, 0, 0); font-family: Arial, "Liberation Sans", "DejaVu Sans", sans-serif; position: relative;">A=ΔU−Ams+mv22−mv202=10000(J)A=ΔU−Ams+mv22−mv022=10000(J)
Gọi lực mà xe tác dụng là F.
F=As=10000200=50(N)" role="presentation" style="margin: 0px; padding: 0px; border: 0px; font-size: 13.696px; vertical-align: baseline; background: transparent; display: inline; line-height: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; color: rgb(0, 0, 0); font-family: Arial, "Liberation Sans", "DejaVu Sans", sans-serif; position: relative;">F=As=10000200=50(N)F=As=10000200=50(N)
Gia tốc của xe: a=mgsinα+F−Fmsm=2000×10200+50−50200=0.5(m/s2)" role="presentation" style="margin: 0px; padding: 0px; border: 0px; font-size: 13.696px; vertical-align: baseline; background: transparent; display: inline; line-height: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; color: rgb(0, 0, 0); font-family: Arial, "Liberation Sans", "DejaVu Sans", sans-serif; position: relative;">a=mgsinα+F−Fmsm=2000×10200+50−50200=0.5(m/s2)a=mgsinα+F−Fmsm=2000×10200+50−50200=0.5(m/s2)
Thời gian xe đổ hết dốc: t=v−v0a=20(s)" role="presentation" style="margin: 0px; padding: 0px; border: 0px; font-size: 13.696px; vertical-align: baseline; background: transparent; display: inline; line-height: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; color: rgb(0, 0, 0); font-family: Arial, "Liberation Sans", "DejaVu Sans", sans-serif; position: relative;">t=v−v0a=20(s)t=v−v0a=20(s)
Công suất trung bình: P¯=At=500(W)" role="presentation" style="margin: 0px; padding: 0px; border: 0px; font-size: 13.696px; vertical-align: baseline; background: transparent; display: inline; line-height: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; color: rgb(0, 0, 0); font-family: Arial, "Liberation Sans", "DejaVu Sans", sans-serif; position: relative;">P¯¯¯¯=At=500(W)P¯=At=500(W)
Công suất trung bình ở chân dốc: P=Fv=50×15=750(W)" role="presentation" style="margin: 0px; padding: 0px; border: 0px; font-size: 13.696px; vertical-align: baseline; background: transparent; display: inline; line-height: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; color: rgb(0, 0, 0); font-family: Arial, "Liberation Sans", "DejaVu Sans", sans-serif; position: relative;">P=Fv=50×15=750(W)P=Fv=50×15=750(W)
c) Gọi vận tốc 2 vật sau va chạm là v′" role="presentation" style="margin: 0px; padding: 0px; border: 0px; font-size: 13.696px; vertical-align: baseline; background: transparent; display: inline; line-height: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; color: rgb(0, 0, 0); font-family: Arial, "Liberation Sans", "DejaVu Sans", sans-serif; position: relative;">v′v′.
Động lượng p=mv=200×15=3000(kgm/s)" role="presentation" style="margin: 0px; padding: 0px; border: 0px; font-size: 13.696px; vertical-align: baseline; background: transparent; display: inline; line-height: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; color: rgb(0, 0, 0); font-family: Arial, "Liberation Sans", "DejaVu Sans", sans-serif; position: relative;">p=mv=200×15=3000(kgm/s)p=mv=200×15=3000(kgm/s) được bảo toàn.
Động năng hệ sau va chạm: T=p2m+m2=30002200+300=18000(J)" role="presentation" style="margin: 0px; padding: 0px; border: 0px; font-size: 13.696px; vertical-align: baseline; background: transparent; display: inline; line-height: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; color: rgb(0, 0, 0); font-family: Arial, "Liberation Sans", "DejaVu Sans", sans-serif; position: relative;">T=p2m+m2=30002200+300=18000(J)T=p2m+m2=30002200+300=18000(J)
Nhiệt lượng tỏa ra là độ giảm của động năng:
Q=mv22−T=4500(J)" role="presentation" style="margin: 0px; padding: 0px; border: 0px; font-size: 13.696px; vertical-align: baseline; background: transparent; display: inline-table; line-height: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; color: rgb(0, 0, 0); font-family: Arial, "Liberation Sans", "DejaVu Sans", sans-serif; position: relative;">Q=mv22−T=4500(J)Q=mv22−T=4500(J)